Chapter 4 The BESIII Detector Physics Goals and

Design

4.1 Introduction

4.1.1 Major achievements of BES

It has been 12 years since the commission of the Beijing Spectrometer (BES) at Beijing Electron Positron Collider (BEPC). Both machine and detector have undergone the upgrade. The upgraded BES is named BESII and the machine stays the same name as BEPC. So far the peak luminosity of the machine is about 51030/cm2s at the J/ resonance. The major parameters of the detector performance of both BESI and BESII are listed in table 1.

With the data collected, BES collaboration has been studying light hadron spectroscopy, searching for glueball, exotic states, charmed mesons, baryonic excited states, rare decays, and test of QCD in the BEPC energy region. Table 3 lists the entries contributed to the Particle Data Group from the BES. Following are some of the important results:

Table 1. Major parameters of the BES detector performance

Detector Major para. BESI BESII

VC x,y (m) 200 100

MDC xy (m)

p/p (%)dE/dx (%)

200-2501.78 (1+p2)

7.9

~2201.78 (1+p2)

8.4BTOF T (ps) 375 180

1

ETOF T (ps) 720BSC E/E (%)

z (cm)23.84.5

20.32.3

Table 2. Data Collected with BESI and BESII

Ecm (GeV) Physics BES Data

3.1 J/ 7.8106

3.69 (2S) 3.7106

4.03 1.0105

4.03 DS, D 22.3 pb-1

~3.55 (m scan) m 5 pb-1

2-5 (R scan) R value 6+85 points3.1 J/ 2.5107

Table 3. Entries contributed from BES to the Particle Data Group Total entries=84.

Physics J/ (2S) D+DS Entry 34 44 5 2

Figre 1. The J/(left) and (2S)(right)

event sample in the world. BES accumulated the world largest J/ and (2S) events samples. Unit in million.

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The precision measurement of the mass of the , MeV/c2.

This value differs 7.1 MeV/c2 from the previous PGD value, and plays an important role in understanding the lepton universality.

Confirmation of the glueball candidate (2230) in K+K- and KS0KS

0 decay modes, and for the first time found the evidence of (2230) in decay modes +-, 00, .

More than ten first measurements of the (2S) and cj decays. E.g. the first measurement of (2S)+-, baryon+anti-baryon. Confirmed the vector pseudo-scalar suppressions in (2S) in ; Found new vector tensor

suppression modes were in ;and large isospin

violating effect in (2S) decay in

The study of the decays of the

(2S) and its decayed products significantly helps understand NQCD.

Direct measurement of decay constant fDs from pure leptonic decay,

MeV.

The first direct model-independent measurement of Br(DS),

R values in 2-5 GeV energy region. A factor of 2-3 improvement in the uncertainty of the R-value has a great impact on the predicted Higss mass and the interpretation of the g-2 measurement carrying out at BNL.

4.1.2 From BESII to BESIII

Although the detector has achieved its design goal at the time when it was first

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built and then upgraded and great achievement has been made by BES at BEPC, the BESII detector was designed for accepting single bunch and the machine luminosity below 1031/cm2s. It's actually almost a copy of MarkIII, of which the technology utilized is about twenty years old. The following are its shortcomings needed to be overcome.

1. The sampling type barrel electromagentic calorimeter is working with SQS modeand has been running for 12 years. It has never been upgraded. The poor energy resolution significantly limited the detection for the photons. On the other hand, the output signal is very much sensitive to the content of the n-pantane and the BES hall temperature. The pulse height fluctuates about 30% while operating the detector from autumn to the spring of next year. A better detection for the photons is crucial in many important physics topics, particularly in searching for glueballs since there is little doubt that the radiative decay of the J/ is the best place to hunt them.

2. The tracking system does not use low Z gas and wires. The momentum and dE/dx resolution are marginal accepted for conventional physics topics. Both momentum and dE/dx resolution should be improved for better particle identification.

3. The endcaps of the detector cannot be opened without removing all the electronics in front of the endcaps and breaking the vacuum of the beam pipe around the detector. This drawback seriously limits us to repair electronics. e.g. preamplifiers, and the broken wires of the VC, MDC and BSC. On the other hand, the detection information from the endcap detectors, which is consisted of ETOF and ESC, has never been really used for physics analysis due to its insufficient resolution of time-of-flight of ETOF (700 ps) and incomplete track finding and fitting.

4. The muon counter has too small a coverage.

5. The electronics system has never been upgraded. It is suffering aging problem seriously. Many spares are no longer available in the nowadays market, therefore it's extremely difficult to maintain the electronics for normal data taking.

6. There is no slow control system.

It is known that generally speaking, exp = (stat2 + syst

2)1/2, where exp , stat and syst are the total experimental error, statistic error and systematic error respectively.

Since the goal of the BEPC II is to increase the peak luminosity from about 51030/cm2s to 51031/cm2s at the J/ resonance, using multi-bunch train. A factor of ten improvements in statistics needs about a factor of three improvements in systematic error, which requires a better detector.

Figure 2 shows the ratio between the systematic error and the statistic error for the

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results from most of the published paper from BES. One sees that the errors are mostly dominated by systematic errors. This indicates that BESII is actually not good enough to match the present BEPC luminosity.

Figure 2. Ratio between the systematic error and the statistic error for the results from most of the published paper from BES.

4.1.3 Design philosophy for BESIII

Learn the lessons from the BESII, our design philosophy for BESIII is to improve the particle identification; increase the power for the photon detection; be able to open the endcaps fast (e.g. within in two to three days); increase the coverage by building new endcap detector; improve the stability of the long term performance of the detector; replace all the aging electronics and equipments. The detector should be able to adapt the new environment of BEPCII, particularly in the interaction region. Following are the major requests to the BESIII:

Improve the photon detection so that the energy resolution can be better than 10%/E (GeV) and the spatial resolution is comparable to BESII.

Improve the resolution of the time-of-flight measurement to about 120 ps.

Improve the momentum and dE/dx resolutions to the detection of charged particles by using helium based gas and aluminum wires for the drift chamber.

New trigger system using pipeline; new DAQ system which extensively adopt

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data buffer, parallel processing and network techniques; new electronics to adapt about 20 times higher event rate and much more noisy environment.

New vertex chamber and luminosity monitor to adapt the new face of IR of BEPCII.

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4.2 Physics at BEPCII

4.2.1 Physics feature in BEPC energy range

Physics feature in the BEPC energy region (2-5 GeV center-of-mass energy) are (1) rich of resonances, charmonum and charmed mesons; (2) distinct threshold characteristics; (3) transition energy region between smooth and resonances, perturbative and non-perturbative QCD; (4) it's an energy location of glueball, hybrid and exotic states. Significant contribution in the study of physics in this energy region was done by many experiments at VEPP-2M, VEPP2, SPEAR, ADONE, DCI and DM2 in the early seventy's. However, these laboratories soon moved to higher energy for hunting new particles, giving BES at BEPC an unique position in the world in the study of light hadron spectroscopy, charmonia and charmed mesons, search for gluenic matter and test of low energy QCD.

Figure1. Physics feature in the energy of 2-5 GeV range.

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4.2.2 Charmonium decay physics

4.2.2.1 J/ Physics at BESIII/BEPCII

Our knowledge of mesons and in parallel, our understanding of the strong interactions have undergone several major revisions since Yukawa [1] introduced meson as the exchange boson for the strong interaction between nucleons. Our present understanding of the strong interactions is that they are described by a non-Abelian gauge field theory Quantum Chromodynamics (QCD) [2], which describes the interactions of quarks and gluons and thus predicts the existence of other types of hadrons with explicit gluonic degrees of freedom -- glueballs and hybrids. Therefore, the observation of glueballs and hybrids is, to certain extent, a direct test of QCD, and the study on the light hadron spectroscopy, as well as the glueball and hybrid spectroscopy will be a good laboratory for the study of the strong interactions in the strongly coupled non-perturbative regime.

After more than 20 years of theoretical effort, it has not yet been possible to calculate the glueball or hybrid spectrum from first principles, since PQCD cannot be applied at hadronic mass scale. Therefore, many QCD-based phenomenological models and calculations, such as bag models [3], flux-tube models [4], QCD sum rules [5] and lattice QCD [6] are developed to make predictions to the properties of glueballs and hybrids. Of them, lattice QCD is considered as more relevant since it originated from QCD, though it is very computer time consuming and only numerical results can be obtained without any corresponding physical insight.

1. Glueball search

In spite of using different approximations, all the models predict the existence of the lightest glueballs in the 1-2.5 GeV mass range, which is BES/BEPC's running energy region. Naively, one can expect the glueballs have the following signatures:

no place in nonet enhanced production in gluon rich processes such as central production, J/ radiative decay

and annihilation decay branching fractions incompatible with SU(3) predictions for states reduced couplings

However, the glueball may mix with an ordinary meson which has the similar mass and same quantum numbers, and thus it makes the identification of a glueball more complicated. Even so, there are some candidates of glueballs: f0(1500), fJ(1710), (2230), etc.

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a. Experimental status of some glueball candidates f0(1500)The f0(1500) was observed in many experiments, such as pion induced reactions - p , annihilation [7,8], central collisions [9,10] and J/ radiative decays [11,12] , while in glueball suppressed processes collision to KsKs and +-, f0(1500) is absent. All those favor f0(1500) to be a non-

state.

fJ(1710)

The fJ(1710) is a main competitor of f0(1500) for status as the lightest 0++ glueball candidate due to its large production rate in gluon rich processes, such as J/ radiative decays, pp central production etc., and because of the lattice QCD calculation of the lightest 0++ glueball mass . The spin-parity of fJ(1710) in the observed processes is then crucial in determining whether f J(1710) is a or glueball.

(2230)

The (2230) was first observed by MARKIII collaboration in J/ [13]. Later, GAMS [14]

reported a narrow structure at 2220 MeV/c2 decaying into ' in -p 'n interactions at 38 GeV and 100 GeV. With 7.8 106 J/ events, BES[15] measured J/ radiative decays and observed (2230) in , and invariant masses. In addition, stringent limits have been placed on the two-photon coupling of the (2230)by the CLEO collaboration in the reactions KsKs [16] and +- [17]. The copious production of (2230) in J/ radiative decays, the narrow width and a small two-photon width of (2230) suggest it be the lightest tensor glueball candidate. However, (2230) was not seen in the inclusive spectrum by Crystal Ball collaboration and annihilation in flight experiments at CERN .

b. Glueball search at BESIII/BEPCIIThe luminosity of BEPCII will be increased by a factor of 10. Therefore, a large J/ event sample can be obtained in a relatively short time. After the upgrade of BESII to BESIII, a new barrel electromagnet calorimeter with an energy resolution of about 8% will be installed, and hopefully a new TOF with a better time resolution will replace the old one. A better calorimeter will give access to all-neutral and multi-photon final states, and a good particle identification will suppress the background effectively.

Except for some -p reaction and pp central production results, most of the data on f0(1500) is from Crystal Barrel collaboration, who resolved two scalar states in this mass region, and determined its decay branching ratios to a number of final states, including 00, , ', KLKL

and 40, using annihilation to rest. If f0(1500) is a scalar glueball, it should be copiously produced in J/ radiative decays. However, f0(1500) was only observed in J/ +-+- from MARKIII and BESI 7.8106 J/ data. Therefore, searching for more decay modes of f0(1500),

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such as , , ' etc. and studying its spin-parity are important in determining f0(1500)'s nature. With 5107 BESII J/ events, a partial wave analysis can be performed in J/ +- channel to investigate the structure around 1500 MeV. But, this analysis will suffer a big contamination, compared with analyzing J/ 00. However, due to the relatively poor energy resolution of BESII shower counter, it's very difficult to study the neutral channels and multi-photon processes by using BESII J/ data. With BESIII/BEPCII, we can study more decay modes of f0(1500) and then determine its spin-parity, which will be very important to study the nature of f0(1500).

The spin-parity of fJ(1710) is crucial in determining if fJ(1710) is a glueball or a meson. If J=0, then the fJ(1710) and f0(1500) might well represent the glueball and the state, or more likely each is a mixture of both. Nevertheless, if J=2, it will be difficult to assign a glueball status to fJ(1710), since that would be at odds with all current lattice gauge calculations. Recently, BES performed a partial wave analysis on J/ K+K- with BESI J/ data and found a 0++ to be dominant in fJ(1710) mass region of K+ K-. fJ(1710) is also seen in other decay channels, such as J/ 00, , ' etc.. However, we encounter the same problems as we do in studying f0(1500), even with BESII 5107 J/ events. After the upgrades of both BES and BEPC to BESIII and BEPCII, a large J/ data sample could be collected by a high performance detector, consequently, the spin-parity study of fJ(1710) from the above channels becomes possible.

For (2230), if we combine CERN scan result [18]

Br((2230) ) Br((2230) KsKs) 7.5 10-5 (95% C.L.)

with BES results [15]

Br(J/ )Br( KsKs)=( 0.8) 10-5

and

Br(J/ )Br( ) = ( 0.5) 10-5

we have the lower bound

Br(J/ ) (2.3 0.6) 10-3 .

This is a large branching ratio for a radiative decay. Nevertheless, no (2230) was observed in the inclusive spectrum by Crystal Ball collaboration. This lower bound also implies that all the branches, reported by BES, represent only about 10 % of the total decay width of the (2230). One possibility is that the branching ratio to is over estimated, and another possibility is we haven't found more decay modes or the main decay modes of (2230). According to some theoretical predictions, (2230) can be strongly coupled to ', '', provided it is a glueball. Then, it turns out that (2230) ', '' would probably be the dominant decays. However, the final states of

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these channels have multi-prong and multi-photon. So, we require high statistics, good particle identification and good photon energy resolution to analyze these decays and search for more decay modes of (2230), for instance, J/ , 00, , etc.. On the other hand, the study of the inclusive spectrum directly becomes possible with a good photon energy resolution.

2 Hunting for hibrid states at BESIII/BEPCIIHybrid mesons are color-singlet mixture of constituent quarks and gluons, such as bound states. The evidence of the existence of the hybrid mesons is also a direct proof of the existence of the gluonic degree of freedom and the validity of the QCD theory. The conventional wisdom is that it would be more fruitful to search for low mass hybrid mesons with exotic quantum numbers than to search for glueballs. Hybrids have the additional attraction that, unlike glueballs, they span complete flavour nonets and hence provide many possibilities for experimental detection. In addition, the lightest hybrid multiplet includes at least one JPC exotics.

In searching for hybrids, there are two ways to distinguish them from conventional states. One approach is to look for an access of observed states over the number predicted by the quark model. The drawback to this method is that it depends on a good understanding of hadron spectroscopy in a mass region that is still rather murky, the experimental situation is sufficiently unsettled that the phenomenological models have yet to be tested to the extent that a given state can be reliably ruled out as a conventional meson. The situation is further muddied by expected mixing between conventional states and hybrids with the same JPCquantum numbers. The other approach is to search for quantum numbers which cannot be accommodated in the quark model. The discovery of exotic quantum numbers would be irrefutable evidence of something new.

According to Quantum Field Theory, the JPC of the ordinary mesons can not be: 0+-, 0--, 1-+, 2+-, 3-+, …… .These quantum numbers are called exotic quantum numbers. Hybrid mesons can have exotic quantum numbers. The hybrid state with the exotic quantum numbers is called exotic meson or exotic state. Exotic mesons can not be ordinary states, so they must be hybrids, glueballs or multiquark states. Therefore, it is important to search for the evidence of the existence of exotic states. Recently, E852 experiment at BNL has found some evidences of the existence of exotic states at -p collisions. The JPC’s of these exotic states are 1-+.

According to theoretical estimation, we know that : (J/ MH) > (J/ MM’) > (J/ MG),

where M stands for ordinary mesons, G stands for glueballs and H stands for hybrids. It means that the process of J/ hadronic decays to hybrid states will have relatively large branching ratios. So the J/ hadronic decay is an ideal place for us to study hybrid states and to search for exotic states.

Two exotic states at 1.4 and 1.6 GeV were observed by BNL [19,20]. According to BES preliminary results, there are some hints of the existence of these two states in invariant mass spectrum in J/ channel. It is expected that we could perform better study in this channel to search for exotic states at BESIII/BEPCII, for we will have much larger statistics and much better energy resolution for neutral tracks at BESIII/BEPCII. It should be stated that a good energy resolution

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for neutral tracks is important for the study of the exotic states, and it will help us to reduce background events and to get much better signals of 0 and .

Some phenomenological models predict that the dominant decay channels of exotic mesons are b1(1235) and f1(1285). The dominant decay channel of b1 is and the dominant decay channels of f1 are and 4. So, it seems that these exotic states should appear in the invariant mass spectrum of 5 or 3. If these exotic states are produced through J/X, then we had to study the following decay channels:

J/ X, X ;J/ X, X 5J/ X, X 3

Besides, we can study iso-scalar exotic mesons through the following channels:J/ X, X (1300), (1300) J/ X, X a1(1260), a1(1260) J/ X, X K K1(1400), K1(1400) K*

Since there are lots of neutral and charged tracks in each channels, a large coverage of solid angle is highly necessary to preserve high data selection efficiency. Good energy resolution for neutral and charged tracks is also required to accurately measure the mass and width of these exotic states.

3 Other interesting topics at BESIII/BEPCIIThe 0-+ and 1++ states in 1440 MeV mass region have been controversial for many years. There is a long time argument about whether 0++ f0(980) and a0(980) are molecular states or not. In addition, several extra 2++ states have been observed, and they are inconsistent with quark model predictions. The baryonic decays of J/ and the study of the excited baryonic states are also important topics at BESIII/BEPCII. With the high statistics available it may even be possible to perform a partial wave analysis of cJ decay products generated in ' cJ radiative decays. For example, c1 H is sensitive to the hybrid exotic sector H (JPC=1-+), while c0 f0(980) X would be a source of 0++. References1. Yukawa H., 1935, Proc. Phys-Mass. Soc. (Japan) 17, 48.2. Fritzch, M., M. Gell-mann et. al., Phys. Lett. 47B (1971) 365

Weinberg, S., Phys. Rev. Lett. 31 (1973) 4943. Barnes T., F. E. Close, et al., Nucl. Phys. B224 (1983) 2414. Isgur N., et al., Phys. Rev. D 31 (1985) 29105. Latorre, J.L., et al., Phys. Lett. 147B (1984) 1696. Michael, C., et al., Nucl. Phys. B314 (1989) 347

Bali, et al., Phys. Lett., B309 (1993) 378 Morningstar, C., et al., Phys. Rev. D 56 (1997) 4043

7. Amsler, C., et al., CBR collaboration, Phys. Lett., B340 (1994) 259Amsler, C., et al., CBR collaboration, Phys. Lett., B355 (1995) 425

8. Abele, A., et al., CBR collaboration, Phys. Lett. B380 (1996) 453 Abele, A., et al., CBR collaboration, Phys. Lett. B385 (1996) 425

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Abele, A., et al., CBR collaboration, Nucl. Phys. A609 (1996) 5629. Antinori, F., et al., WA91 collaboration, Phys. Lett., B353 (1995) 58910. Barberis, D., et al., WA102 collaboration, Phys. Lett., B413(1997) 21711. Bugg, D., et al., Phys. Lett., B353 (1995) 37812. Bai, J. Z., et al., BES collaboration, Phys. Lett. B472(2000)20713. Baltrusaitis, R. M., et al., Mark-III collaboration, Phys. Rev. Lett., 56(1986)10714. Alde, D., et al., GAMS collaboration, Phys. Lett., B177(1986) 12015. Bai, J. Z., et al., BES collaboration, Phys. Rev. Lett., 76(1996) 3502 Bai, J. Z., et al., BES collaboration, Phys. Rev. Lett., 81(1998) 117916. Godang, R., et al., CLEO collaboration, Phys. Rev. Lett., 79(1997) 382917. Alam, M.S., et al., CLEO collaboration, Phys. Rev. Lett., 81(1998) 332818. Evangelista, C., et al., JETSET collaboration, Phys. Rev. D56 (1997) 380319. Thompson, D.R., et al., E852 collaboration, Phys. Rev. Lett., 79(1997) 163020. Adams, G., et al., E852 collaboration, Phys. Rev. Lett., 81(1998) 5760

4.2.2.2 ψ(2S) decay

(1) Introduction In charmornium family, ψ(2S) is in a special position. ψ(2S) can decay into J/ψ, ψ, χcJ (J=0,1,2), and possibly into η c, η’ c and 1P1 states, therefore, by collecting sufficient ψ(2S) data sample, one can not only study the property of resonances of ψ(2S), J/ψ, χcJ (J=0,1,2), ηc, but also search for η’c and 1P1

states. The ψ(2S) data sample collected by BEPC/BESI is 3.96 million, by which BES collaboration has made various studies on ψ(2S), J/ψ and χcJ

(J=0,1,2), and copious results have been reported [1-9]. For the BEPCII, the luminosity will be optimized at beam energies of 1.55 GeV (J/ψ) and 1.84 GeV (ψ(2S)), and the designed luminosity at 1.55 GeV is 5x1031cm-2 s-1. It is reasonable to assume that at 1.84 GeV beam energy, the luminosity can reach 7.5x1031cm-2 s-1, namely increase to a factor of 1.5. Based on this value, one year running of BEPCII will produce 1.5x108 ψ(2S) events, which is a factor of 38 higher than the data sample collected by BESI detector, and will improve the statistical error by a factor of 6. Besides, the designed performance of BESIII detector is much better than that of BESI with time resolution of TOF decreasing from 330 ps down to 110 ps, momentum

resolution from 1.7% to 0.5% , and energy

resolution of electromagnetic calorimeter from 0.25/ to 0.08/

,which will increase the detection efficiency (therefore also increase the

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statistics, too), enhance the capability of particle ID and improve the momentum measurement for charged track, the energy measurement for electron and photon, in turn, improve the systematic error by a factor of 2 -3 averagely for all measurements. (2) ψ(2S) decays(a) Hadronic decays BES collaboration has measured branching fractions or upper limits for various hadronic decay channels listed in Table 1. The statistical errors are in the range of 10% to 30 % , and the systematic errors are in similar values. In the BEPCII/BESIII case, the statistical and systematic errors will be down to (2-5)% and (3.5-10)%, respectively, which give the total error of (4-12)%, namely, a factor of 3 improvement compared to the BESI results. For the upper limits, BESIII will set the results of BESI down to a factor of 38 lower level. One expects that the J/ψ and ψ(2S) decays into light hadrons via ggg, or γ*, or γgg. In either case, the partial width for the decay is proportional to |ψ(0)| 2, where ψ(0) is the wave function at the origin in the non-relativistic quark model for . Thus, it is reasonable to expect [10] on the basis of the perturbative QCD that, for any hadronic final state h, the ratio Qh can be estimated by :

(1) where the

leptonic branching fractions are taken from PDG2000[11]. This relation is sometimes refered to as PQCD 15% rule. The Qh values for BES measured ψ(2S) hadronic decays based on PDG’s value for J/ψ branching fractions are also listed in Table 1. A major part of decay channel Table 1 BESI measured ψ(2S) decay branching fractions and PQCD 15% rule test

(# denotes preliminary results; upper limits at C.L.=90%) Channel B(ψ(2S)) (10-4) B(J/ψ) (10-3) Qh (%)

γX 0.53 0.31 0.08 0.86 0.08 6.2 3.81.54 0.31 0.20 4.31 0.30 3.6 0.95.2 0.8 1.0 3.0 0.5 17.3 5.1

AP 10.0 1.8 2.1 < 3.0 >33.3

<3.1 3.8 1.4 < 8.2

<1.7 4.3 0.6 < 4.0

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<2.3 10.9 2.2 < 2.1

VT <4.5 1.23 0.06 0.20 < 3.7

<1.2 6.7 2.6 < 1.8

#0.798 0.528 6.7 2.6 1.20 0.93

VP # <0.29 12.8 1.0 < 0.23

# <0.23 5.0 0.4 < 0.46

# 1.30 0.34 0.16 4.2 0.4 3.1 1.0

VS#

0.63 0.18 0.32 0.09 19.6 7.8

VV 0.392 0.103 0.29 0.04 0.06 13.6 4.9# 1.25 0.56 0.74 0.24 16.9 9.4# 0.64 0.26 1.30 0.25 5.0 2.2# 1.68 0.32 0.80 0.12 21.0 5.1# 0.58 0.22 0.83 0.13 7.0 2.9# 0.082 0.052 0.045 0.015 18.1 12.8# 6.04 0.90# 3.49 0.64 2.3 0.9 15.2 6.6# 2.47 0.96# <1.8 2.09 0.18 < 8.6

2.16 0.39 2.12 0.10 10.1 1.9 1.81 0.34 1.30 0.12 13.9 2.9

1.2 0.6 1.27 0.17 9.4 4.6BB 0.94 0.31 0.9 0.2 10.4 4.1

1.28 0.35 1.10 0.29 11.6 4.5 1.1 0.4 1.03 0.13 11 4<0.81<0.73

branching fractions have no large deviation from PQCD expectations, which includes AP ( ), VS ( ), ( )

and multi-hadron final states. The intriguing puzzle, reported in 1983 by Mark II [12]: the Qh for and lower than an order of magnitude of PQCD expectations, is comfirmed by BES results at much higher level of sensitivity [7]. The upper limits on the branching fractions of

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and are found to be more than a factor of 60 and 20 lower than the 15% rule predictions, respectively. The four

decay modes ( ) are suppressed by a factor

of at least 3 [2]. For the AP decay channel, BES collaboration has observed flavor-SU(3)-violating K1(1270)-K1(1400) asymmetries that have opposite character for the ¦×(2S) and J/¦×[5], which cannot be accommodated by adjustments of the singlet-triplet mixing angle [13]. All these suppressions and anomaly will be further studied with higher accuracy and higher statistics in BEPCII/BESIII. (b) Radiative decays By using the Vector Dominance model, a radiative decay might (or might not) be connected with corresponding hadronic decay,it is therefore interesting to examine if suppressions exist for the radiative decays too. BESI has measured the branching fractions for channels (also listed in Table 1)

and calculates the corresponding =6.2% and =3.6%, which are

suppressed by a factor of roughly 2 and 4, respectively. We will further study these suppressions and extend the study to more channels, such as

... at BEPCII/BESIII with higher accuracy and statistics.

BEPCII also pvovide an opportunity to search for some glueball candidates such as η(1440), fJ (1710), ξ(2230), etc. via radiative decays. This might be helpful to distinguish glueballs from radial excitation states for some theoretical models.

(3) J/ψ letponic decay width BESI has determined the most accurate branching fraction for J/ψ leptonic decay via processes of

: =(

) % [3]. In the BEPCII/BESIII case, the tatistical error will be negligible and systematic error will be reduced to 0.05%, as a result, the

relative error of will be lower than 1%.

(4) χcJ physics BESI has measured many channel’s branching fractions for χcJ decays [6], with much improved accuracies, in which many of them are the first measurements (see Table 2). Therefore, from PDG1998 [14] to PDG2000 [11], the whole scenery for χcJ decays are greatly changed. However, the statistical and systematic errors for χcJ decay branching fractions are still too large, ranging (6-20)% and (15-30)% respectively. In the case of BEPCII/BESIII, the statistical errors will be negligible and the errors will be

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governed by the systematic one ranging (5-10)% .

Table 2. BES measured χcJ width and branching fractions(upper limits at C.L.=90%)

Channel BES results PDG98(4.68 0.26 0.65) 10-3 (7.5 2.1) 10-3

(1.49 0.14 0.22) 10-3 (1.9 1.0) 10-3

(5.68 0.35 0.85) 10-3 (7.1 2.4) 10-3

(0.79 0.14 0.22) 10-3 (1.5 1.1) 10-3

(1.59 0.43 0.53) 10-3 <9.0 10-3

(4.2 2.2 2.8 ) 10-3 (8.6 1.2) 10-5

(5.8 3.1 3.2 ) 10-3 (10.0 1.0) 10-5

(15.4 0.5 3.7 ) 10-4 (3.7 0.7) 10-2

(4.9 0.4 1.2 ) 10-5 (1.6 0.5) 10-2

(9.6 0.5 2.4 ) 10-5 (2.2 0.5) 10-2

(14.7 0.7 3.8 ) 10-3 (3.0 0.7) 10-2

(4.5 0.4 1.1) 10-3 (9 4 ) 10-3

(7.9 0.6 2.1 ) 10-3 (1.9 0.5) 10-2

(1.57 0.21 0.54) 10-3 (5.0 2.0) 10-3

(0.49 0.13 0.17) 10-3 (1.4 0.9) 10-3

(1.23 0.20 0.35) 10-3 (3.3 1.3) 10-3

3 (11.7 1.0 2.3 ) 10-3 (1.5 0.5) 10-2

3 (5.8 0.7 1.2 ) 10-3 (2.2 0.8) 10-2

3 (9.0 1.0 2.0 ) 10-3 (1.2 0.8) 10-2

17

(1.96 0.28 0.52) 10-3

(0.61 0.17 0.16) 10-3

(0.92 0.34 0.38) 10-3

(2.00 0.55 0.61) 10-3

(2.14 0.26 0.40) 10-3

(0.42 0.15 0.12) 10-3

(1.48 0.26 0.32) 10-3

< 0.71 10-3

(2.46 0.44 0.65) 10-3

< 1.06 10-3

14.3 2.0 3.0 MeV 13.5 3.3 4.2MeV

The width of χcJ decaying into light hadrons are interesting theoretically, since in addition to the 3PJ color singlet component the contribution of the color octet component in the wave function might also be involved according to the NRQCD theory [15]. The widths of χc1 and χc2 are measured precisely by E760, and the width of χc0

( MeV) by BESI [6]. With BEPCII/BESIII, the error of χc0 can be reduced to 0.3-0.4 MeV. A careful study of the angular distribution of the radiative decay

with high statistics in BESIII will provide more information on

the transition matrix elements, which are closely related to the wave functions and interquark forces.

The radiative decay rates of are also interesting, to which

both QCD radiative correction and relativistic correction may be important. The estimated branching ratio is roughly

. (2)

Due to the limited statistics and bad photon energy resolution in BEPC/BESI, this measurement is not feasible. However, with one year running of

18

BEPCII/BESIII, about 2000 signal events for this channel can be expected (assuming efficiency of 40%) , which will produce (2-3)% statistical uncertainty only.The χcJ decay into light hadrons via gluon intermediate

state,e.g., hadrons. It is plausible that a pair of glueballs could be

favorably produced in its decay process. With BEPCII/BESIII high statistics and better particle ID, it is possible to study hadronic decays like

(3)

and look for some 0++ and 0-+ glueballs, e.g.,

(4)

This study might provide useful information on distinguishing between states, states, and glueballs, if any.

(5) search

Crystal Ball Collaboration reported in 1982 the observation of at 3592

MeV, but its existence has never been confirmed. With 3.8 milion ψ(2S)

events, the search is tried in BESI with invariant mass of all charged

particles, -like signal seems to appear in channels,

but the existence of can not be claimed [16].

Since Crystal Ball gives , and assuming for any

exclusive hadronic channel h based on the same argument as for J/ψ and ψ(2S) decays (see(2)(a))

, (5)

then we can design the event selection criteria by assuming the same decay

channels and same branching fractions for and decays. Since the

radiative photon in decay is of lower energy, this study is

difficult in BESI because of its bad photon energy resolution and lower detection efficiency for low energy photon, and

19

also low statistics. However, all these factors are improved in

BEPCII/BESIII, which makes the search for possible. The final states

which can be studied for this search are listed in Table 3.

Table 3. Final states and branching ratios for search

( branching ratios taken from PDG)

decays (f)

B (f) (10-2) B (γf) (10-5)

1.2 1.2

2.0 2.0

2.1 2.1 ( ) 5.5 0.629 0.629 0.629 0.629 0.629 0.629 1.28 1.28

B (10-5) Nevt (1y)

6.56 9840 1.91 2865

As it can be seen from Table 3 that, for both and topology,

sufficient signal events can be produced for search in BEPCII/BESIII.

However, a severe background from will disturb the

search in the mode because of its large branching fraction (2.48 10-3)

and small mass difference of (3592 MeV) and (3556 MeV). On the

other hand, for the mode, the background comes mainly from

with the branching fraction of 5.35 10-5,

which is only a factor of 2.8 higher than that of the signal. In addition ,

the energy of the radiative photon in these two processes is very different (93

MeV and 639 MeV for and respectively),

therefore the background of can be easily removed.

20

We have made a fast simulation for the search by mode with the

BEPCII/BESIII performances [17]. Based on the character of the

events topology, we abstract following selection criteria to distinguish signal from background:

1. , , PID= , | |<0.75.

2. 3 5, select 3 most energetic ’s. | |<0.75.

Construct 6 ( - ) combinations:

,

, determined

by M.C. are 24.9, 45.7, 16.6 MeV, respectively. The pattern with smallest is chosen ( or is determined).3. <10

4. < 3 GeV

5. If is assigned, (0.35, 0.75) GeV; If is assigned,

(0.35, 0.75) GeV,and <1.3 GeV ( is the lower momentum in two charged

tracks) . The results of this simulation is shown in Table 4. We see that if collecting one year (2S) data, about 456 signal events will be selected with

55 background events from . The ratio of signal to noise is 8.3,

this indicates the possible success for the search with BEPCII/BESIII, if it

exists.

Table 4. Selection of events

Decays

B 1.28 10-5 0.629 10-5 3.59 10-5 1.76 10-5

N0 18690 9180 5241 25690N1 11048 3166 3005 9027

21

N2 3666 1509 1237 4619N3 3626 986 17 371N4 3573 901 17 370N5 3572 870 17 369

0.191 0.095 0.003 0.014N(1y) 367 89 17 38

(6) 1P1 search The R704 [18], E760 [19] and E705 [20] collaborations claim the

existence of 1P1 state in year of 1986,1992 and 1994, respectively. However, the signal statistics is very low (signal events for these three experiments are 5, 59 and 42 respectively). The existence of 1P1 state still needs to be confirmed.

Some theoretical calculation considering the effect of S-D mixing [21] gives

, (5)

in addition 1P1 is expected to be the dominant decay mode. Therefore,

we can search for 1P1 via 1P1 (6)

decay mode. The decay channels of into 4 prong’s and corresponding

branching ratios are listed in Table 3 already, and the total branching ratios of

into 4 prong’s is 6.56 10-2. Therefore, the combined branching ratio for

process (6) is estimated to be 1.312 10-5. Assuming detection efficiency of 0.1, collecting one year events (1.5 108), 197 signal events for process (6) can be expected, which is sufficient to confirm the existence of 1P1 state.

The 1P1 and in process (6) are generated almost at rest, therefore, the signal events of process (6) have following characteristic :

1. (mass of )

2. (mass of )

3. (mass of 1P1)

4.

5.

22

6. cos ,

based on which we can easily distinguish the signal from possible backgrounds, which could be the processes of

2 , (plus one fake photon)

, , (one photon not detected)

, , (plus one fake photon)

, , (plus one fake photon)

, ,(plus one fake photon).

References 1. J.Z.Bai et al., BES Collab., Phys. Rev. Lett. 81 (1998) 3091. 2. J.Z.Bai et al., BES Collab., Phys. Rev. Lett. 81 (1998) 5080. 3. J.Z.Bai et al., BES Collab., Phys. Rev. D58 (1998) 092006. 4. J.Z.Bai et al., BES Collab., Phys. Rev. D58 (1998) 097101. 5. J.Z.Bai et al., BES Collab., Phys. Rev. Lett. 83 (1999) 1918. 6. J.Z.Bai et al., BES Collab., Phys. Rev. D60 (1999) 072001. 7. X.H.Li, representing BES Collab., Nucl. Phys. B (Proc. Suppl.) 75B (1999) 181. 8. F.Liu, representing BES Collab., Nucl. Phys. A675 (2000) 71c. 9. X. Y. Shen, representing BES Collab., Recent results from BES, IV Intern. Conf. on Hyperons, Charm and Beuty Hadrons. June 26-30, 2000, Valencia, Spain. Will be published in Nucl. Phys.. 10. S.J. Brodsky and M.K. Karliner, Phys. Rev. Lett. 78 (1997) 468, Yu-Qi Chen and Eric Braaten, Phys. Rev. Lett. 80 (1998) 5060; and references therein. 11. D.E.Groom et al., Particle Data Group, Eur. Phys. J. C15 (2000) 1. 12. M.E.B. Franklin et al., Mark II Collab., Phys. Rev. Lett. 51 (1983) 963. 13. Preliminary results from this analysis are discussed in the context of SU(3) symmetry breaking, in M. Suzuki, Phys. Rev. D55 (1997) 2840. 14. C.Caso et al., Particle Data Group, Eur. Phys. J. C3(1998) 1. 15. G.T.Bodwin et al., Phys. Rev. D46 (1992) 1914. 16. C.Z.Yuan, BES97(1997)191.

17. Y.S.Zhu et al., Search for at BEPCII/BESIII, 中国高能物理

23

发展战略研讨会报告文集，加速器物理分册，82页。 18. C.Baglin et al., Phys. Lett. B171 (1986) 135 19. T.A.Armstrong et al., Phys. Rev. Lett. 69 (1992) 2337 20. L. Antoniazzi et al., Phys.Rev. D50 (1994) 4258 21. Y.P.Kuangg et al., Phys. Rev. D37 (1988) 1210

24

4.2.2.3 Charmed Meson Physics[1]

Charmed mesons , and are the bound states of ( = , , ) quarks.

Since the charm quark is sufficiently massive, some aspects of perturbative QCD are applicable both in their productions and decays. Because the weak couplings of the charm quark are theoretically determined in standard model with three quark generations, charm decays offer a clean laboratory to study strong interaction effects at the boundary between the perturbative and nonperturbative regions. There are three classes of charmed meson decays: Pure Leptonic, Semileptonic and Hadronic Decays.

1. Study of Pure Leptonic Charmed Meson Decays

For the pseudoscalar charmed and mesons, the decay rates for

determine the decay constants and . The decay rates can be

rigorously calculated in the Standard Model. The theoretical prediction for these branching fractions are given by

where is the CKM matrix element and are the so-called decay

constant. The decay constants contain all the nonperturbative QCD information in the

leptonic decays. The term represents the helicity suppression of the leptonic

decays of the pseudoscalar mesons.

The and are two fundamental constants in particle physics. They describe

the overlapping of the meson wave function at origin and play an important role in predicting the branching fractions of meson semileptonic decays, noleptonic decays, and in understanding hadronic wave functions and second order weak processes

including mixing and CP violation. Therefore, the precise measurements of

and are very important. However, since the leptonic decay branching fraction

becomes smaller as becomes larger, it is much more difficult to meaure and

25

via and than and . At present the errors for

and are large [2].

The high precision of data in the BEPCII is expected to give accurate measurements

of the and .

The theoretical evaluation of the decay constants relies on nonperturbative methods of QCD such as QCD sum rules, chiral perturbation theory, Bethe-Salpeter equation and lattice gauge calculations in the full theory. Usually the QCD sum rules calculations [3]—[7], the lattice gauge theory simulations [8]—[10] and the Bethe-

Salpeter equation approach [11] predict that is around the value 200 MeV (take

MeV). The ratio has also been calculated by QCD sum rules [12] and

the lattice gauge theory [9]. This value is about 1.3 in these two approaches.

Different model calculations lead to different values and each of them has its own uncertainties. Therefore, the precise measurements of the decay constants can test those nonperturbative methods and help us to get a deeper understanding about the features of non-perturbative QCD. It can also provide a test on the heavy quark

effective theory. Since is much heavier than and , it is very

difficult to measure . is also a very important constant which plays a fundamental

role in experiments, such as mixing experiment and measurement of CP

violation. The test of nonperturbative QCD methods via can provide a solid

ground for attempts to extrapolate to and better description for mixing.

The ratio of would also be interesting since the measurement of this ratio

can test the heavy quark effective theory[12].

Precise measurements of pure leptonic and decays require singly tagged

event sample and would therefore be accessible at the BEPCII. The tagged event samples are necessary both to suppress backgrounds and to provide a constrained fit for the mass of the missing neutrinos. Monte Carlo simulation[13][14] indicates that the signal can be clearly distinguished from backgroud processes. With one year’s

data, each of the decay constants and can be measured with the error about

7%.

26

2．Semileptonic Decays and CKM Matrix ElementsIn the diverse phenomenology of weak interections, semileptonic and leptonic

decay of hadrons have a special standing. In both types of decays, the final-state particles including a single charged lepton, the clearest experimental signature for a weak process should be mediated by the W boson. Because these decays are relatively simple from a theoretical perspective, they provide a means both to measure fundamental standard-model parameters and to perform detailed studies of the decay dynamics.

The semileptonic decays of Charmed mesons are more complicated than the pure leptonic decays but simpler than the nonleptonic decays. For a process , where

is a final state, the lepton part can be factorized out. What is left is the matrix

element of the weak current between and , where is the weak

current. The decay width of is also related to the CKM matrix element

where is the daughter quark after the transition of c by the emission of W boson and the quark is combined with another quark in D mesons to form the meson . In

general the decay width can be written as where is proportional to

. Hence all the nonperturbative information is included in . It depends

on the initial and final state hadronic wave functions and the hadronization

mechanism. On the ground of Lorentz invariance, the matrix element

can be decomposed as (for is a pseudoscalar meson)

and (for is a vector meson)

where and are the four momenta of D and X respectively and q is the

momentum carried by . The form factors , , , and

27

are governed by nonperturbative hadron dynamics and, therefor, are very

difficult to calculate from the first principles of QCD. At the maximum recoil point

, the condition and must be satisfied. can be

expressed in terms of and

.

At present, there are some phenomenological models to deal with these form factors. Each of these models has its own assumptions and hence limitations. In the quark model approach, the form factors at the maximum recoil point [15] or at the

zero recoil point [16] are calculated and the functional

dependence is obtained through the nearest pole dominance assumption. The form factors at zero or maximum recoil points depend on the hadronic wave function models. Comparison of these models in the semileptonic decays of the Charmed meson are given in [ 15].

Contrary to the quark model approach, QCD sum rules are appropriate to study the dependence of the form factors [17]—[23]. The results from QCD sum rules

indicate that and of the vector current have the nearest pole dominance

behaviour while for and of the axial current such dependence is absent.

However, the lattice QCD calculations [24]—[26] seem to support the pole dominance behaviour of all the form factors. Some form factors are also calculated by the effective QCD Lagrangian[27]

The present precision of experimental data is not enough to give a definite test of the form factor behaviour and the existing hadronic wave function models. The BEPCII data are expected to extract more information about the wave functions of the Charmed mesons and the light mesons and consequently, that of the B meson.

The accurate data from BEPCII will provide the information on the distribution of the final decay products and also the shape of the hadronic form factors.

Consequently, the CKM matrix elements can be measured more accurately than

the present sensitivity (at present the CKM charm matrix elements are poorly measured ( )). Further more, since nonperturbative models are applied both in the Charmed and B mesons, the test of them in Charmed mesons can help us to

28

extrapolate to the B meson and therefore, the CKM matrix element which is very

poorly measured at present can be known much better.

At the BEPCII the semileptonic decay branching fractions of Charmed mesons

can be measured with an accuracy at about 2% and 6% for the decays

and , respectively [14], whereas the present error are about 5% for

and 50% for . This high precision would make the above

statement be possible.

Similar to the pure leptonic decay case, theoretical uncertainties are largely suppressed by taking the ratios such as where the only

uncertainty comes from SU(3) breaking. In this way, the more reliable ratio

can be obtained to accuracy around 8%.

By measuring the inclusive semileptonic decays of the Charmed meson

where is lepton energy the heavy c quark distribution function can be

extracted from the relation

.

At present, only some ansatz for exists [28]. The precise measurement of could give important information about the inner structure of the Charmed mesons.

The analysis of the semileptonic decays of the Charmed mesons based on the high precision data to be obtained at the BEPCII will surely help us to learn about

nonleptonic decay processes where not only the matrix appears but also

the decay mechanism plays an important role. Just as in the case of pure leptonic decays, all the related theoretical techniques such as heavy quark expansion, QCD sum rules, Bethe-Salpeter equation, chiral perturbation theory and lattice gauge simulations will be tested.

3. Non-Leptonic Decays and Weak Decay Mechanisms

Although 25 years have passed since the discovery of Charm, many nonleptonic decay modes of the charm mesons are still not explored. The present experimental

29

precision on Charm decay properties is also not satisfactory [29]. The measured branching fractions of , decays have large errors (more than 20%). Even the

absolute branching fractions for many decay modes of are not known reliably.

Some of them affect the precise measurement of B meson decays. For example, in

order to extract the branching fraction from a measurement of the

bottom decay for a final state X, we need to know the absolute

branching fraction . Unfortunately, we do not have good data on

. The BEPCII will provide the only way to improve the precision of the

absolute branching fraction for mesons to about 2% level, and to estabilish absolute

branching fraction for , , , etc. to about 15% level.

The rich varity of available charm decay modes (meson and baryon decays, Cabbibo allowd, Cabbibo suppressed, and double Cabbibo suppressed decays) offers a possibility to study decay mechanism of the charm hadrons and to test the different theoretical methods. For instance, for meson decay, we have six different quark diagrams as shown in Figure 1. As expected theoretically the decay through diagram (b) should be color suppressed because of the color mismatch. Actually, the present charm data show no color suppression. There are theoretical speculations on the strength of the different diagrams (a)-(f). We definitely need precise data to test these speculations. Another examples is . At the quark level this decay can only go through diagram (c) in Figure 1, i.e., the so-called exchange diagram. But theoretical estimation shows that its branching fraction should be very small. Recent data shows

[2] , which is surprisingly large. There are

theoretical arguements [30] saying that this is because of final state interactions (rescattering). Up to now there is no convincing explanation. Again because of the rich varity, charm decays are ideal places for studying final state interactions. For example, if sufficiently large number of branching ratios are well measured, we can extract the size of contributing isospin amplitudes and their phase shifts.

Because charm hadrons are heavier than light hadrons but lighter than bottom hadrons, charm study will teach us good lessons for applying and testing the different theoretical methods, such as , QCD Sum Rule, Lattice Simulations of QCD, Heavy

Quark Effective Theory, and expansions, etc..

4. Mixing

The process of particle–antiparticle mixing is a sensitive probe of the weak

30

interaction in the neutral K-, D-, and B- mesons. In the standard model, the mixing is expected to be small for . Because mixing is very sensitive to phenomena such as a new quark generation, it is a good place to look for new physics. Similar to the neutral Kaon system, the mass eigen states are different from the CP eigen states. Taking the

phase convention as , we can construct symmetric and

antisymmetric CP eigen states and respectively:

, .

The physical mass eigen states (also weak decay eigen states) can be described by

,

In contrast to , , here , have comparable lifetimes due to large number of

decay channels of the D mesons.

Experimentally we measure the mixing rate defined as

,

where , , and are the mass and width differences of

and respectively.

In the Standard Model, is expected to be very small. A recent estimate [31]

claims that the short and long range contributions are in the same order of magnitude

and . Any measurement means new physics. BEPCII can

provide reconstructed D mesons per year data-taking. A sensitivity of in

measurements is possible for one year data taking.

At the BEPCII, the pair can be produced in the electron positron annihilation at the center-of-mass energy of 3.77 GeV in the physics interaction

, where the is in state and can be described by

31

.

is Cabbibo favoured decay. But can occur through Double

Cabbibo suppressed Decay. So, , have identical final states and can be regarded as identical particles. Therefore their coherent wave function is symmetric and must has charge parity C = +1. But we are considering C = -1 coherent state. So DCSD can not be contribute to . Only mixing contributes. So, we can tag to measure mixing free from DCSD contamination. The observation of the processes

or

would be unambiguious evidence for the existence of mixing. The final states of the above decay modes are very clean because all final particle are observed and D mass peak must be seen. Another method for measuring mixing is to use the semileptonic decay. This methods is free of DCSD contamination.

In summary, mixing can be studied unambiguously in two or more than two independent modes. The reaching sensitivity can approach to the order of .

5. Rare Decays

Studies of the rare decays can be used to test the weak decay mechanisms. There are a number of models, such as extended Technicolor[32], leptonquarks [33], and some supperstring-inspired models[34], in which large flavor-changing neutral current effects would be present in charm. The standard model forbids flavor-changing neutral currents (FCNC) in lowest order. Some decays such as and also require lepton flavor violation (LFV) and are strictly forbidden in the standard model. Some rare decays, such as , … are related to

the interesting weak decay mechanisms. These decays are due to the vertex ,

or , i.e. the so-called penguim diagram, or a box diagram (here l

means e or ) or in ‘a short distance’. It is similar to the case of beauty or , and sensitive to ‘new physics’ at tree or loop levels, since they are forbidden at tree level for standard model.

With the data to be collected by using BES-III at BEPC-II at the resonance, the rare decays of charm quark can be studied. Since it is

a resonance production and just above the threshold of a pair production of the charmed mesons, a great advantage with double tagging techniques for the pair production can be used in investigating the rare decays. The systematic errors can be suppressed greatly.

32

Reference[1] It is mainly based on “Feasibility Study Report on Beijing Tau-Charm Factory”, IHEP-BTCF,

Report-03, October, 1996.[2] Particle Data Group, Review of Partcile Properties, Phys. Rev. D15(2000)543, 573[3] T.M. Aliev and V.L. Eletsky, Yad. Fiz. 38, 1537(1983) [Sov. J. Nucl. Phys. 38, 936(1983)].[4] M. Neubert, Phys. Rev. D45, 2451(1992).[5] E. Bagan, P. Ball, V.M. Braun and H.G. Dosch, Phys. Lett. B278, 457(1992).[6] S. Narison, Phys. Lett. B198, 104(1987)[7] C.A. Domoinguez, Proceedings of the Third Workshop on the Tau-Charm Factory, Marbella,

Spain, 1993, eds, J.Kirkby and R. Kirkby.[8] C.W. Bernard, J.Labrenz and A. Soni, Nucl. Phys. B(Proc. Suppl.)30, 465(1993)[9] A.Abada et al., Nucl. Phys. B376, 172(1992)[10] G. Martinelli, Proceedings of the Third Workshop on the Tau-Charm Factory, Marbella,

Spain, 1993, eds. J. Kirkby and R. Kirkby.[11] Y.Dai, X.Guo, C.Huang and H.Jin, Commum. Theor. Phys. 24, 453(1995)[12] M. Neubert, Phys. Rev. D46, 1076(1992)[13] R.H. Schindler, Proc. Tau-Charm Factory Workshop (SLAC, California, 1998), ed.L. V.

Beers, P.127; P.C. Kim, ibid, P.196.; G. Rong, H.K. He, Z.P. Zheng D.H. Zhang, “Study of D Physics with BES-III at BEPC-II”, May, 2000

[14] G. Rong, H.K. He, Z.P. Zheng, and D.H. Zhang, “Study of D Physics with BES-III at BEPC-II”, 《中国高能物理发展战略研讨会》May, 2000

[15] M.Bauer, B. Stech and M. Wirbel, Z. Phys. C34, 103(1987); ibid., 29, 637(1985); X.H. Guo and T. Huang, Phys. Rev. D43, 2931(1991); B. Grinstein, N. Isgur, and M. Wise, Phys. Rev. Lett. 56, 298(1986); C.W. Luo, T. Huang, X.H. Guo, J, P. Li and G.R. Lu, High Energy and Nucl. Phys. 18,601(1994); J.G. K0rner and G. Schuler, Z. Phys. C38, 511(1988).

[16] N.Isgur, D.Scora, B.Grinstein and M.Wise, Phys.rev. D39, 799(1989); T. Altomari and L. Wolfenstein, Phys. Rev. D37, 681(1988); P. Colangelo, G. Nardulli and L. Tedesco, Phys. Lett. B272, 344(1991)

[17] T.M. Aliev, V.L. Eletskij and Ya.I Kogan, Sov. J. Nucl. Phys. 40, 527(1984).[18] P. Ball, V.M. Braun, H.G. Dosh and M. Neubert, Phys. Lett. B259, 481(1991).[19] P. Ball, V.M. Braun, H.G. Dosh , Phys. Rev. D44, 3567(1991).[20] A.A. Ovchinnikov and V.A. Slobodenyuk, Z.Phys., C44, 433(1989).[21] S.Narison, Phys. Lett. B283, 384(1992).[22] P. Colangelo, G. Nardulli, A.A. Ovchinnikov and N. Paver, Phys. Lett. B269( 201(1991).[23] C.A. Dominguez and N. Paver, Phys. Lett. B207, 499(1988); E B211, 500(1988).[24] M. Crisafulli et al., Phys. Lett. B223, 90(1989)[25] V.Lubicz, G. Martinelli, and C.T. Sachrajda, Nucl. Phys. B356, 310(1991): V.Lubicz,

G.Martinelli, M.McCarthy and C.T. Sachrajda, Phys. Lett. B274, 415(1992)[26] C.Bernard, A.El-Khadra and A.Soni, Phys. Rev. D43, 2140(1992); Phys. Rev. D45,

869(1992)[27] M.Wise, Phys. Rev. D45, 2188(1992); G. Burdman and J. Donoghue, Phys. Lett. B280,

287(1992); T. Yan, H. Cheng, C.Cheung, G.Lin, Y. Lin, and H Yu, Phys. Rev. D46, 1148(1992).

33

[28] C.Peterson, D.Schlatter, J. Schmitt and G. Preparata, Phys. Lett. B172, 447(1986)[29] P.Roudeau, proceedings of Marbella workshop on the Tau-charm Factory, Marbella, Spain,

June 1993; A.J. Weinstein, idid.[30] J.F. Donoghue, Phys. Rev. D33, 1516(1986).[31]G.Gurdman, Talk given at the 2000 workshop Fermilab, June 7-9, 1994, Fermilab-Conf-

94/200(1994).[32] Farhi, E., Susskind, L., Phys. Rep. 74:277(1981); Eichten, E., et al., Phys. Rev.

D34:1547(1986)[33] Buchmuller, W., Wyler, D., Phys. Lett. B177: 377(1986) [34] Campbell, B., et al., Int. J. Mod. Phys. A2:831(1987)

34

4.2.3 The Perspectives of Lepton Experiments at BEPCII

1． Recent status of the lepton physics study

The subject of the lepton physics has been studied for 25 years and remarkable progresses have been reached. There are still quite rooms for its further development, and, no doubt, the will continue to play an important role in the continuing search and exploration for new physics. 1.1 Lepton universality The existence of different families is one of the most important open questions in particle physics.

We do neither understand what causes this triplicity, nor know what generates the different masses. However, we expect the heavier family to be more sensitive to whatever dynamics are related to the generation of mass. This makes the an ideal particle to use to investigate these gaps in our understanding. In the frame of electroweak interaction, is the really identical to electron and muon?

In the Standard Model, the decays in the same way as the muon: through emission of a W boson. However, the heaviness of the makes several extra decay modes kinematically accessible. The can either decay leptonically into its lighter electron and muon brothers, accompanied by appropriate neutrinos, or it can decay into quarks.

Because quarks can appear in three different “colors”, the probability of a hadronic decay is three times greater than leptonic decay. The detailed analysis of the

decays shows an excellent agreement between the measured branching fractions and Standard Model predictions.(1) Comparing the different decays with the weak decays of the muon and charged pion, we can test whether the different leptons couple to the W with same strength. Within the present (and impressive) experimental accuracy of 0.2%, the electron, the muon and the appear to have exactly the same W interactions. (2) The same observation can be made directly from the analysis of W decays at LEPII and the proton-antiproton colliders, although, the present experimental sensitivity is not as good in this case.

Our BES made a key contribution for testing the universality by precise measurement of the lepton mass.(3) The leptonic couplings to the neutral Z particle have been accurately measured at LEP and SLC, through the study of lepton-antilepton production in electron-positron colliders. Again, the experimental data show that the three known leptons have identical interactions with the Z boson, at the present level of experimental sensitivity.(4) Because the decays within the detector – a produced at LEP travels 2.2 mm before decaying (a produced at CLEO travels 0.24 mm) – one can measure its spin

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orientation (polarization) from the distribution of the final decay products. The present data show that there is only left-handed ’s decay. This is in good agreement with the Standard Model. An upper limit of 3% has been set on the probability of a (disallowed) decay from a right-handed .1.2 A lepton with strong interactionsAs the definition, leptons do not couple to the gluonic carriers of the strong interaction. However, an electroweak boson emitted by a lepton can produce quarks, which are strong interacting particles. Electrons and muons only feel this effect indirectly, through tiny quantum corrections. The heavier can decay hadronically, which makes the a unique tool for studying strong interaction dynamics in a clean way.

Theorists showed that the hadronic decay of the can be predicted from first

principles, as a function of the QCD coupling . Summing over all possible hadrons

produced in the decay, avoiding the problems related to messy rearrangement of quarks into hadrons, the decay probability can then be computed at a more fundamental level in terms of quarks and gluons. The result is known up to the third

order in a perturbative expansion in powers of . Comparison of the theoretical

predictions with the experimental measurements gives a precise determination of

at the mass region. The four LEP collaborations and CLEO have all performed their

own measurements of . Moreover, ALEPH and OPAL, through their careful

analysis of the distributions of the hadron final states from decays decay, have been able to separately, the tiny non-perturbative corrections and obtained values in good agreement with theoretical expectations.

The resulting determination, ( ) = 0.345±0.020, shows that the coupling,

measured at the mass scale, is very different from the values obtained at higher energies. The value extracted from the hadronic decays of the Z boson, 0.119±0.003, differs from the decay measurement by eleven standard deviations.

The comparison of these two measurements is of fundamental importance within our present understanding of quantum field theory. Quantum corrections, mainly generated through the virtual production of particle-antiparticle pairs, modify the values of the bare couplings in a way that depends on the energy scale. This is a very important effect, in the context of non-abelian gauge field theories.

Theorists further showed that in non-abelian theories the quantum effects give rise to “asymptotic freedom”, in which the coupling decreases as the energy increases. Asymptotic freedom explains why high-energy experiments feel quarks as nearly free particles, while at low energies they are strongly confined within hadrons. The provides the lowest-energy scale where a very clean measurement of the strong coupling can be performed, which gives an opportunity to test asymptotic freedom in a quantitative way. Using the theoretically predicted dependence on energy, the

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measurement of at the mass scale can be translated into a prediction of at the Z

mass scale: 0.1208±0.0025. This value is in close agreement with the direct measurement from hadronic Z decays, and has a similar accuracy. decays, which result in an even number of pions, have also been used to measure the hadronic vacuum polarization effects that are associated with the photon. It is possible, therefore, to estimate how the electromagnetic fine structure constant is modified at LEP energies. The uncertainty of this parameter is one of the main limitations on the extraction of Higgs mass from LEP/SLD data. From the ALEPH data, the Orsay group is able to reduce the error of the fitted log(MH) value by 30%.

The same data can pin down the hadronic contribution to the anomalous magnetic moment of the muon. Recent ALEPH and CLEO analyses have improved the theoretical prediction by setting a reference value to be compared with the forthcoming measurement of the E821 experiment, which is running at Brookhaven.1.3 Weighing the strange quarkAbout 3% of decays produce a strange quark. The four LEP experiments have investigated these decays. In particular, ALEPH has analyzed kaon production in decay and the associated distribution of the final hadrons. The difference between the dominant decay producing a down quark, and that producing a strange quark is sensitive to the mass difference between the down and strange quarks. Because the former is much lighter, the ALEPH measurement can be translated into a good determination of the S quark mass at the mass scale:119±24 MeV.

Quark masses are also dependent on energy; quarks weigh less at higher energies (and weigh more at lower energies). At 1 GeV, for instance, the strange quark mass becomes 164±33 MeV. These measurements have important implications for the theoretical prediction of CP violation in kaon physics. Future experiments and analyses could provide a more accurate determination of the strange quark mass.1.4 Other challenging subjects in lepton physics(1) decay data has been probed extensively for signatures of new physics beyond the Standard Model framework. Using its huge data sample, CLEO has looked for 40 forbidden decay modes. No positive signal has been found, which puts stringent upper limits (of a few part per million) on the probability of many decays into final states without neutrinos. Anomalous electric and magnetic electroweak dipole couplings of the and possible CP-violating decay amplitudes have also been searched for, with negative results. Within the present experimental accuracy, the appears to be a standard lepton.(2) decays are accompanied by neutrinos, so kinematical analysis of hadronic decays gives an upper limit on neutrino mass:18.2 MeV. Recently, the DONUT experiment at Fermilab observed directly the first experimental evidence of the neutrino, though only 4 events, through the detection of its interaction with a nucleon via the produced .(3) There is an important goal in view about neutrino’s nature, which aims on -μneutrino oscillations. The work could be done by so-called long baseline neutrino experiments.

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2. How many amounts of pairs accumulated in the whole world?(1) The largest sample of data belongs to CLEO/CESR, NY of USA, which has accumulated about 10 million pairs at CM energy of 10.6 GeV. Their analysis has a quite high background contamination rate (BCR), which is estimated to be a few of percentage and may be the only dissatisfaction in their great work. (2) The other labs and experiments have contributed their brightnesses for physics, although the samples of lepton pairs they accumulated are less big. Each of four experiments at LEP accumulated 0.2 million of lepton pairs with lower BCR as one order better than a few percentage.

The BES collaboration has collected data at several energies and the sum is about 0.1 million of lepton pairs, which are 2300 from 5000 nb-1 collected at 3.55—3.6 GeV, 74000 from 22.3 pb-1 collected at 4.03 GeV and 24000 from 6000 nb-1

collected at 3.686 GeV, respectively. The BEPC has such a uniqueness for working energy, in the middle of which there is the threshold of pair production, so that a background free condition can be in the area around 3.67 GeV for further studies.(3) ARGUS/DORIS-II and others has stopped their data taking but they published a lot of excellent papers for physics.

3. CP Violation Search on Lepton System3.1 Why do we need to do the search? One of the unsolved fundamental problems in the elementary particle physics is the origin of the CP violation. So far, CP violation has been verified in the K system. Although it can be accommodated in the SM of electroweak interactions by virtue of a phase angle in the Cabibbo-Kobayashi-Maskawa(CKM) matrix, its deep understanding is still missing.There are three reasons to search for possible CP (or Time- Reversal when CPT keeps invariance) violations in the lepton sector:(1) We do not know fundamentally the origin of the CKM matrix. While it is important to continue to refine its parameters and to test its formulation at B-factories and in Kaon decays, the CKM matrix itself is probably not truly basic either physically or mathematically, from the viewpoint of the spontaneously symmetry breakings in the local quantum field theory. (2) Most astrophysics investigations show that additional sources of CP violation, beyond the CKM, are necessary to account for the observed baryon-to-photon ratio of the universe. (3) There is no leptonic CP violation in the minimal Standard Model theory(MSM), but it can be arise in some extensions of the MSM, such as the multi-Higgs doublet, left-right symmetric model, scalar lepto-quark model, and supersymmetry theory. Leptonic CP violation would observe interactions which cannot be described in the framework on the MSM since it arises there only at the multiloop level.3.2 The study status of CP violation on lepton system(1) On theoretical fieldRight now there are many theorists who contributed a lot of papers on this subject for theoretical explanation, numerical calculation and experimental suggestions. Those persons are T.D.Lee, Paul Tsai, C.Nelson, R.D.Kass, Huang Tao/Tao Zhijian, SeongYoulChoi/M.Drees, J.H.Kuhn/E.Mirkes, W.Bernreuther/A.Brandenburg/P.Overmann, A.Hocker,E.Gonzalez, N.Wermes,

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P.Poulose/S.D.Rindani and etc.(2) The CLEO paper is the first one for experimental work. It was published at Physical Review Letters 81:3823—3827, 1998 with the title of

《First Search for CP Violation in Tau Lepton Decay》.

It searches for “a characteristic difference between the - and + decay angular distributions for the semi—leptonic decay modes such as

- K0π-ν.” and finds “no evidence for any CP violation.”(3) The BES group has finished an experimental analysis and is going to submit a

paper of 《A Search for CP Violation in the Pure Leptonic Tau Decays》，which is

from 18 pb-1 BES data sample taken at 4.03 GeV. The amplitude of CP violation has been determined to be

0.0235±0.0315±0.0057 for 427 eμevents.3.3 ProspectFor 20 times of 427 eμevents could be selected at BEPCII, we have a chance to expect a breakthrough on the search, hopefully.4. neutrino mass measurement

decays are accompanied by neutrinos. One of important progresses on neutrino experiments has been heard from Super-Kamiokande group of KEK, Japan. During 22 days of stable data-taking (in June of 1999) the K2K Long Baseline Neutrino Oscillation Experiment observed four neutrino interactions in the inner Super-K detector, which is the first step towards the verification of the neutrino oscillation results given by the Super-Kamiokande in 1998. (CERN Courier 39-8, OCTOBER 1999)

Some physicists, based on the results of Super-K and others, made suggestions of -μneutrino oscillations and of neutrino mass squared differences of around 0.003

eV2. These should be checked by the new-generation long baseline neutrino experiments. The compelling physics of neutrinos was focused by the January conference(Jan. 7-9, 2000, Cracow, Poland). The conference looked forward to new and planned experiments to investigate this new phenomenon further. (CERN Courier 40-3, APRIL2000) Of special interest are the long baseline studies, in which neutrinos produced by an accelerator beam are observed by detectors installed at a distant point, typically several hundred kilometers away, for a direct measurement of neutrino oscillation. These manifest themselves either by the disappearance of the neutrino species produced at the accelerator site or by the appearance of a different neutrino species, depending on the capabilities of the detector installed. About such project, besides Super-K, there are planned and designed ICANOE, OPERA(for CERN beam), MINOS(for Fermilab beam) and etc(maybe one from China).

Why so many experiments are needed for it? Because it is hard to make a conclusion for, even if, the disappearance of the neutrino species, let alone for the appearance of a different neutrino species. And nobody can priorily say whether positive or negative conclusion will come out.

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Before the long waiting for their (maybe negative) results ends, the only knowledge of human about the neutrino mass is come from “kinematical analysis of hadronic decays, which gives an upper limit on neutrino mass: 18.2 MeV.” In fact, some new experiments are setting their physics goal to include neutrino mass study, e.g. SLAC B-factory is planning to determine its limit by using KKπfinal state.5. Possible experiments of physics at BEPCII(general discussions)Before starting the discussion, we should know some basic numbers.

Up to now the maximum peak luminosity of BEPC reached actually, to 4.9E30/cm2/s. The BEPCII should have 10 times higher of the luminosity ability than BEPC. So Lp=4.9E31/cm2/s could be expected for maximum peak luminosity of BEPCII. From the above number, we made some estimations of statistics for further lepton experiments in the following discussions.

The experiences obtained from the lepton work done at BEPC-I told us that good data is the most important factor and compatible data for multi-physics use would not be as good as special data if we want to make very precise measurement. Two energies at BEPCII could be used for lepton experiments: 3.77 GeV for multi-physics goal and 3.67 GeV uniquely for physics. The amounts of lepton pairs produced at 3.67 GeV and 3.77GeV, respectively could be

Ecm Assumed peak luminosity BEPCII reached

0.25Lp 0.5 Lp Lp 2 Lp

3.10 GeV 1.225E31 2.450E31 4.90E31 9.80E313.67 GeV 2.083E31 4.165E31 8.33E31 1.67E32N( + -) 0.58E6 1.16E6 2.32E6 4.64E63.77 GeV 2.205E31 4.41E31 8.82E31 1.76E32N( + -) 0.73E6 1.46E6 2.92E6 5.84E6

N( + -)3670 =86400*180*2.083E31*0.8*2.24 = 0.58E6 N( + -)3770 =86400*180*2.205E31*0.8*2.66 = 0.73E6Therefore, the number of lepton pairs accumulated in one data-taking season could be 0.58 – 4.64 million with BCR of 0.5% or less at 3.67 GeV and 0.73 – 5.84 million with BCR of a few percentage at 3.77 GeV.The event number used for CP violation search could, at least, be

0.0619*0.58E6*0.2 = 7180 (eμevents),and the event number used for neutrino mass study could, at least, be

2*(0.1737+0.1781)*1.61E-3*0.58E6*0.1 = 66 (Kkπevents).Based on the less statistics(compared with CLEO), the BESIII detector at BEPCII could not do better jobs on the study for the forbidden decay channels of lepton, but has good chance for other experiments. They could be (1) CP violation search in lepton decays; (2) measuring the upper limit of the neutrino mass; (3) as some foreign physicists suggested, re-measurement of the lepton mass without data-driven; (4) measurement of S quark mass; (5) non-perturbative QCD study using lepton

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hadronic(semi-leptonic) decays; (6) ( ) study, etc.

Here, we especially expect new suggestions for lepton experiments.So there is a clear reason to set one of our BEPCII physics goals to be on

neutrino mass study. We may have shortcomings on starting time and statistics, but everyone knows that the most important key factor on neutrino mass measurement is the low BCR, which is just BEPCII’s superiority when using KKπfinal state to do it. (Suppose we take data at the CM energy of 3.67 GeV for one year.)If selecting 66 signal events and setting 95% confidence level for fitting work, we will have more than 50% possibility to obtain a upper limit of the neutrino mass which is at one digital MeV level.

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4.2.4 Study , , and Baryons from and Decays

Baryons are the basic building blocks of our world. To understand the internal quark-gluon structure of baryons is one of the most important tasks in nowadays particle and nuclear physics. From theoretical point of view, since baryons represent the simplest system in which the three colors of QCD neutralize into colorless objects and the essential non-Abelian character of QCD is manifest, the systematic study of various baryon spectroscopy will provide us with critical insights into the nature of QCD in the confinement domain.

The main source of information for the baryon internal structure is their mass spectrum, various production and decay rates. Our present knowledge of this aspect came almost entirely from the old generation of experiments of more than twenty years ago. Considering its importance for the understanding of the non-perturbative QCD, a new generation of experiments on u-d quark baryons with electromagnetic probes (real photon and space-like virtual photon) has recently been started at new facilities such as CEBAF at JLAB, ELSA at Bonn, GRAAL at Grenoble and SPRING8 at KEK. On the other hand, baryon spectroscopy with strange quarks is still in its infancy. For example, for the baryons with two strange quarks and one up or down quark, i.e., resonances, there are only two experimentally well established ones, while various QCD-inspired models predict more than thirty such baryons. The theory is totally not challenged due to lack of data.

Figure 1: Feynman graphs for , , and production from collision through or meson.

BEPCII could produce 108 and 107 events. Their decays will provide an excellent place for studying these excited nucleons and hyperons - , , and

resonances. The corresponding Feynman graphs for the production of these

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excited nucleons and hyperons are shown in Fig.1 where represents either or . Since the vector charmonium decays through three gluons and gluons are

flavor blind, the strange s quarks are produced at the same level as the non-strange u-d quarks. Table 1 lists some interested decay branching ratios. The

and are indeed produced at similar branching ratios. The branching ratios for and are expected to be of the same order of magnitude if one ignores the phase space effect. The channels have thresholds above or very close to the mass of and cannot be studied here.

Table 1: decay branching ratios (BR ) for some interested channels

1.3 0.1 1.3 0.2 1.8 0.4 1.1 0.1 0.9 0.2 0.3 0.1

?2.0 0.1 1.1 0.1 6.0 0.5 2.1 0.2 0.9 0.4 1.3 0.3 ?

All channels listed in Table 1 are relative easy to be reconstructed by BES. For example, for , we can select events containing and with , then from missing mass spectrum of we should easily identify the very narrow peak. The channel is a very good place for studying

.

Among three-body channels listed in Table 1, and can be used to study and ; Channels containing p can be used to study

and . Many other channels not listed in Table 1 can also be used to study these baryon resonances.

In fact, the Feynman graphs in Fig.1 are almost identical to those describing the electro-production process if the direction of the time axis is rotated by . The only difference is that the virtual photon here is time-like instead of space-like and couples to through a real vector charmonium meson . So all decay channels which are presently under investigation at CEBAF(JLab, USA), ELSA(Bonn,Germany), GRAAL(Grenoble, France) and Spring8(KEK, Janpan) with real photon or space-like virtual photon can also be studied at BEPC complementary with the time-like virtual photon. In addition, for and , the and

systems are limited to be pure isospin 1/2 due to isospin conservation. This is a big advantage in studying resonances from decays, compared with and experiments which suffer difficulty on the isospin decomposition of 1/2 and 3/2.

On theoretical side, the coupling of provides a new way to probe the internal quark-gluon structure of the resonances. In the simple three-quark picture of baryons, as shown in Fig.1, three quark-antiquark pairs are created independently

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via a symmetric three-gluon intermediate state with no extra interaction other than the recombination process in the final state to form baryons. This is quite different from the mechanism underlying the production from the process where the photon couples to only one quark and asymmetric configuration of quarks is favored. Therefore the processes and should probe different aspects of the quark distributions inside baryons. Since the decay is a glue-rich process, it is also a very good place for looking for hybrid baryons.

In summary, the and experiments at BEPCII will play an unique excellent role for studying excited nucleons and hyperons -- , , and resonances. The completion of the light quark (u,d,s) baryon spectroscopy is of crucial importance for us to reveal quark-gluon structure of matter.

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4.2.5 Measurement of R-values in BEPC energy range

Remarkable progress has been made in the precision test of the Standard Model (SM) during the last decade. The copious and precise electroweak data from LEP and SLD has made it possible that the radiative correction effect is sensitive to the output parameters, e.g. the mass of the Higgs mH(Figure 1), of the global fit to the electroweak data.

Figure 1. The result of the SM fit of mt and mH with (MZ2) varying within

one standard deviation.

The QED runing couple constant (MZ2), the Fermi decay constant GF and the

mass of the Z MZ are the three input parameters for the analysis of the electroweak data. The relative uncertainty of (MZ

2) is much worse than that of GF, and MZ. Namely 211 ppm for (MZ

2), 9 and 24 ppm for GF, and MZ respectively. It is crucial for the SM fit to reduce the uncertainty of (MZ

2), which cannot be entirely calculated from QCD because of ambiguities in defining the light quark masses mu and md as well as the inherent non-perturbative nature of the problem at small energy scale since the free quark loops are strongly modified by strong interactions at low energy, but can be related to from quark loop R [2],

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where R(s)(e+e-hadrons)/(e+e-+-)=12Im'(s) and P is the principal value of the integral. (e+e-+-) is the lowest-order QED cross section, (e+e-

+-)=42/3s. On the other hand, the anomalous magnetic moment of the muon a(g-2)/2 is theoretically sensitive to large energy scales and very high order radiative corrections. Any deviation between the SM predicted anomalous magnetic moment of the aSM and that from the experimental measured one aExp may hint new physics. So far the uncertainties of aSM and aExp are 0.7 ppm and 5 ppm respectively. E821, an important experiment carrying out at Brookhaven [3] is aiming at reducing the uncertainty in aExp to about 0.4 ppm. The dominant contribution to the uncertainty in aSM is again the hadronic vacuum polarization ahad, which relates the R values through the dispersion relation [1,3].

where K(s) is a kernel varying from 0.63 at s=4 m2 to 1.0 at s= . figure 2 show the

relative uncertainties in (MZ2) and a from the different energy range.

Figure 2. The relative contribution of the uncertainties in (a) a and (b) (MZ2).

Since the uncertainties in (MZ2) and a are dominated by the errors of the

values of R in the cm energy range below 5 GeV, it is crucial to significantly reduce the uncertainties in the R values measured about 20 years ago with a precision of about (15-20)% in the energy region of 2-5 GeV [1]. The BES collaboration did a R scan in the energy region of 2-5 GeV with BESII. The R values are graphically displayed in Figure 3, together with those measured by MarkI, 2 and Pluto about twenty years ago. The R-values from BESII have an average uncertainty of about 6.6%. The two to three factor improvements in precision of the R-values in 2-5 GeV has a significant impact on the global fit to the electroweak data for the determination of mH. The preliminary fit results show that the predicted mH is significant increased with the preferred value lying just above the LEP2 excluded region, and the new 2

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profile of the fit accommodates the LEP2 bound on the mass more comfortably [4, 5]. The preliminary global fit to the electroweak data , with the preliminary BES new R-value, shows that the mass of Higgs shifted from about 60 GeV to 90 GeV and the up limit from 170 GeV to 210 GeV , s indicated by Figure 4. On the other hand, BESII R-values can also greatly contribute to the interpretation of the E821 g-2 measurement [3].

Figure 3. R-value measured by different experiments below 5 GeV.

Figure 4. The variation of the 2 distribution of the SM fit of mH

A real breakthrough to the electroweak theory physics with regards to the R-values in low energy would be possible only by measuring ( e+e- hadrons) at (1~3)% accuracy. SND and CMD-2 at VEPP-2M in Novosibirsk have significantly improved the measurements of the hadron production cross section via e+e- collision for some of the important exclusive channels in the energy region of 0.36-1.38 GeV [2]. Further

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improvement with the analysis of the existing data is forthcoming. KLOE at DAPNE in Frascati can potentially improve the R-values to a precision of 1% in the energy region from the hadron production threshold to 1.4 GeV. A R scan extending the energy from the threshold to 2 GeV, which links up the scan energy to the lowest done with BESII at BEPC, is highly wished to improve the measurement.

Once the R-values being measured with a precision of 1% in the energies covered by VEPP-2M and DAPNE, the central question will then again how to further decrease the uncertainties of R measured by BESII in the energy region of 2-5 GeV, particularly from 2-3.7 GeV. This will be then again an important and interesting project for the BESIII at the BEPCII.

To further improve the measurement of R values at BEPC, one needs better performance of the detector and a better handle on the uncertainty arising from the hadronic event generator and the calculation of the radiative correction, as well as higher machine luminosity, particularly for the energies below 3.0 GeV. The BEPCII project will increase the peak luminosity about a factor of ten; the detector will have a better particle identification and long term stability of the performance. From the experimental point of view, BESIII has a possibility to measure R in 2-3 GeV with an uncertainty of about 3%.

References 1. Z. G. Zhao, Proc. of LP99, SLAC, USA, July 1999. 2. Z.G. Zhao, J. of Mod. Phys. A, 15(2000)3739-37693. Robert Carey, "New Results from g-2 Experiment", talk given at ICHEP2000,

Osaka, Japan, July 2000.4. B. Pietrzyk, "The Global Fit to the Electroweak Data", talk given at ICHEP2000, Osaka, Japan, July 2000.5. A. Martin, et. al., hep-ph/0008078.

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4.2.6 The study of QCD and Hadron Production at BEPCII

1. IntroductionAs an unique candidate theory of strong interaction, quantum chromodynamics (QCD) has reached great success in some aspects for describing hadron production. However, due to its non-Abelian asymptotic freedom nature, it can describe the evolutions of the quark and gluon phase, but can not give a complete theoretical calculation from the primary quarks to final hadron states. Therefore, QCD can not predict the observables in experiments. It is the ultimate aim to learn strong interaction and the spectrum of hadrons. Due to some historical reasons in the development of high energy physics, such as the energies of the accelerators increase rapidly in order to pursue to discover the so-called new physics and the perturbative calculations are reliable in high energy region, the studies of strong interaction in this range have been made in rather detail. For the processes in high energy reaction, the fraction of energy used for creating the particle masses is very small comparing with the total center of mass energy and the available phase-space, the mass effects is not significant. The special hadronization mechanism for concrete channels is not critical, the hadronic final states keep the main features of perturbative evolution (e.g. jets properties, string effects). On the other hand, the studies of QCD and hadronization in medium-low energy region are rather poor or even blank. BEPC is the unique e+e-

collider currently running in 2-5 GeV energies, the main characteristics of the reactions occurring in BEPC are that the typical reaction energy scales are at the low end of pQCD being adequate, the properties of the final states are mainly governed by the non-perturbative hadronization mechanisms. There are also some particle production thresholds and complex resonant structures in BEPC energy region, the precision of physical analyses are perplexed by the poor understanding to hadronization mechanism. It should be realized that BEPC energy region supplies an unparalleled field for the study of hadronization. No synthetic survey for the hadronization mechanism and final state hadron spectrum were done before. In fact, it is an important subject, almost all accelerators and detectors in the world have done such kinds of studies, which not only furnished many valuable experimental bases and physical outcomes, but also useful for the special topic studies. Data taking for R measurement at BEPC/BES was done in 1998 and 1999. The results of R 6 energy points have been published, the new R values of 85 points have been obtained and the paper was also submitted. The errors of R are about 7%, which are smaller than 10-15% in other experiments. The R measurements at BEPC/BES are highly appraised by international high-energy physics circle. The theoretical uncertainties of calculated anomalous magnetic moment (g-2)/2 and QED running coupling constant s were decreased greatly when R values given by BEPC/BES

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were used as input parameters, and the 2 distribution for Higgs mass fitting is more consistent with the present experiments. The measurement of strong interaction running coupling constant s has being paid more and more attentions in medium-low energy region. s has been measured at the lowest energy scale Q =1.58 GeV in deep inelastic scattering processes and at much higher energy Ecm= 10.52 GeV in e+e-

annihilation at CLEO. To perform the study of QCD and hadronization mechanisms and hadronic spectrums in medium-low energy region is one of the few projects to fill in gaps in these fields. Comparing with other projects, the maximum physical results may be obtained by using the less beam time. It is an opportunity and also a challenge to BEPC/BES.

2. Contents

The main analysis way to the study of hadron production processes in experiments is to measure various final state spectrums, which reflect the reaction mechanisms from different sides. By analysing those spectrums synthetically and comparing them with the hadronic theories one may build up a complete picture of hadron production. The following subjects may be included in this plan:

Hadron production model

The final state hadrons are formed in the processes of small momentum transfer (s is large), the perturbative QCD cannot make complete theoretical descriptions to the transfer processes from the primary quark-antiquark to the observed hadronic states. The better understanding to the hadronization processes come from the widly and detailed experimental analysis to hadronic final states. A reliable hadronization model and the corresponding generator are needed for the precise experimental measurements at BEPC/BES. The famous LUND model is the one of the most successful phenomenological hadronization model. The area law of string color-field derived from LUND model is very similar with the form that in strong coupling lattice gauge theory. The generator based on the area law LUARLW gives consistent predictions with BES data for most final state distributions. This fact shows LUND area law reflects the main features of non-perturbative QCD.

Exclusive cross sections

In order to decrease the systematic errors of R measurements below 3 GeV at BEPCII in the future, an effective way is to measure the exclusive cross sections of some important reaction channels. It requires that the used hadronization model may predict the branching ratios of all channels reliably. e+e- annihilation may produce many different final states, the reaction cross sections are determined by hadronization mechanisms. In generator LUARLW, the matrix element is expressed as the area law, it determines the dynamics of string fragmentation. Therefore, LUARLW may predict

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the exclusive probabilities, in which the parameters are determined by experimental data.

Multiplicity distribution

The multiplicity distribution is an important value to characterize the reaction. The measurement of the average multiplicity as a function of the center of mass energy belongs to absolute measurement. The Monte Carlo is used to determine the efficiency matrix (which reflects the loses of events and tracks) and to convert the observed values to the physical values. The relation between the average multiplicity and the center of mass energy may be expressed as

(a) in parton model: <nch > = a + b ln s,

(b) in QCD : <nch> = a + b exp [c(ln s/2)1/2],

where, a, b, c and are phenomenological parameters. At the same energy, the charged multiplicity measured in experiment and predicted by Monte Carlo have fluctuations, which may be expressed by the variance

Dc = [<nch2> - < nch>2]1/2，

The KNO scaling property of multiplicity is characterized by the moments

Cq = <nchq>/<nch>q，

the strict scaling invariance means that Cq is independent to the energy.

Multiplicity correlation

Both the restrictions of the kinematical conservation law and dynamic mechanism lead to the correlation effects of multiplicity. In experiments, the forward-backward correlation is often measured. The produced particles are classed into two parts by their rapidities. Taking the zero rapidity y=0 as origin, the phase-space is divided into the forward and backward parts. For the selected events with fixed forward multiplicity nf, measure the backward average multiplicity <nb>, the following correlation form was fond in very high energies

<nb> = a + b nf + c nf2,

|b| indicates the strength of the correlation, b>0 for possitive correlation and b<0 for negative one. In medium-low energies, the correlation has not been measured yet.

Inclusive distribution

The spectrum distributions are determined by hadronic dynamics, which imposes

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strong restrictions to the hadron production model. The distributions of inclusive charged particle are easy to measure in experiments. Since the strong interaction is charge-independent, one may infer the corresponding distributions for neutral particles are alike to charged one. In general, the single particle distributions are the functions of (s,p//,p). Two questions are needed to answer for the reaction dynamics and the study of single particle distributions: (a)how do the distributions change with center of mass energy s; (b)how do the distributions change with (p //,p) when s is fixed. The answer for the second question depends on the initial state and the properties of measured particles of final states. Feynman supposed the scaling property for the first question. The single particle distributions are the functions of scaling variable x and p in the limit of s. The scaling assumption is a good approximate behavior in high energy reactions, but it has not been tested precisely in medium-low energies. The usually measured distributions in experiments are Feynman momentum xp, rapidity y, transverse p etc. The running strong interaction constant s may be determined by the scaling deviations.

Event shapes

The descriptions for the final state particles produced in collision reaction need a set of kinematic parameters (e.g. momentums). Condensing the kinematical properties of all the particles into a group of variables, one can obtain features of the topology of an event. The most important values for the topological properties are sphericity(S), thrust(T), aplanarity(A) and oblateness(O), which reflect directly the geometrical features of final state particles in phase space. The four variables mentioned above are determined by bremsstrahlung of gluons and the non-perturbative properties. S and T parametrise the overall broadness of an event, and A characterize its flatness. Bassed on the measured Sobs, Tobs, Aobs and Oobs, one may get their physical values after the corrections are made by Monte Carlo method which reflect the partly lose of the tracks.

Measurement of s

It is one of the primary important tasks to measure s and to verify the asymptotic freedom property in QCD. In perturbative QCD, s is the expansion parameter. The following observables in e+e- collision may be expressed as the functions with parameter s and they may be used for the measurement or estimation of s. (a) Use the relation between Rexp measured in experiments and the expression RQCD(s, s) derived from QCD, i.e. Rexp = RQCD(s,s), the error is about s Rexp/Rexp which is too large for the present precision of Rexp. (b)Use the scaling deviation property of fragmentation functions, in which only the inclusive distributions for charged particles are needed to measure and it is easy to measure at BES. (c)Use the topological distributions of the events. (d) Use the hadronic energy-currents correlation.

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The string effect of hadronic final state

Almost all distributions predicted by LUND model are consistent with experiments, which make one believing the picture of string fragmentation is an effective phenomenological model in the description of hadronization. The so-called string effect has been observed at LEP, i.e. particles produced between quark and antiquark jets are less than particles produced between quark/antiquark and gluon jets statistically. It is also an interesting question to see that if the string effect is observable in BEPC energy region. The key to this question is to find the different features for hadron bunches fragmentated from quark/antiquark and gluon. No matter whether the string effect may be observed at BEPC, the result will increase greatly the understanding for the dynamics of string fragmentation.

The kinematic and dynamical correclations

The study of correlative effects is more valid way to abstract the dynamical information than single particle spectrum. Comparing with the single particle distributions, the correlation functions are closer relating to hadronization mechanisms. Hence, correlative effects are more rigorous test to hadronization model. In order to separate the pseudo-correlation from the true one, experiments usually measure C(x1,x2) = CL(x1,x2) + CS(y1,y2), where x1 and x2 may be any two kinematical observables, CL(x1,x2) is the long-range correlation and CS(x1,x2) the short-range one.

The Bose-Einstein correlation (BEC)

In quantum mechanics, identical bosons are symmetric for the permutation of their wave functions. This intrinsic property leads to a special statistic correlation, the so-called Bose-Einstein correlation, which exists in a boson’s system even if there is no any interaction existing. The symmetry gives an interferential term which contains the space-time information of hadronic (boson) sources. The manifestation of BEC is that the possibility of finding two identical bosons in a small phase-space is larger than that of two different particles. Bose-Einstein correlation function and the space-time distribution of bosons source is related by Fourior transformation. For this reason, the space-time properties of hadronic source may be inferred by measuring the Bose-Einstein correlation functions. It is expected that the following subjects may be studied for charged which are the most abundant bosons in experiments at BEPCII/BESIII: (a)two-body correlation (with the influences of multi-body correlation and final state electromagnetic and strong interactions); (b)the multiplicity dependence of BEC; (c)the space-time form of hadronic source; (d)BEC in the decay of resonance. The prediction about BEC by LUND model LUARLW may be tested.

The possible fractal structures of final state phase spaceThere was a problem in the study of final particle spectrum, i.e. the attention was only

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paid to averaged distributions and it was thought that the fluctuations are only due to the statistical phenomena for the finite number of particles. The events with abnormal high particle density condensed in small phase-space have been observed in several high energy reactions. The important questions to these discovers are: Do the anomalous fluctuations have their own dynamical origins? Is the phase-space of final state particles isotropic or not? Is the phase-space continuous or fractal? Do the approximate intermittency observed in the very high energies reactions exist also in low energies? Can the intermittency be explained by the known physical laws (such as, cascade evolutions, BEC etc.). The study of this topic has two aspects. (a) Experiment: measurements of fractal moments Fq Hurst indexes. In the one dimension phase-space analysis the valuable such as rapidity y, transverse momentum p, azimuthal angle etc. may be chosen and the combinations of any two variables may be chosen for two-dimension analysis. (b) Mechanisms: if the asymptotic fractal behavior in the parton perturbative evolutions may be kept after the processes of hadronization?

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4.2.7 Searches and new physics

2.7.1. Lepton flavor violating decays

The rare decay processes conserve total lepton

number, but violate the individual lepton numbers. In the standard model the lepton flavor symmetries are conserved, but speculated to be violated in many extensions of the standard model, such as supersymmetric standard models, left-right symmetric models and models where electroweak symmetry is broken dynamically. Recent Super-Kamiokande experiment results indicate that neutrinos have non-vanishing masses, mix with each others and consequently that lepton flavor symmetry and/or lepton number symmetry are broken symmetries. In cosmology mystery of matter and antimatter asymmetry might be understood in terms of the brokendown of the lepton number symmetry together with the non-perturbative effects (Sphaleron) of the standard electroweak theory.

There has been a lot of studies both theoretically and experimentally on testing the lepton flavor conservation law. At present we have various bounds listed in the particle data book from decays, and Z gauge boson decays[1]. With a large sample of , BES will be able to make an additional experimental searching for lepton flavor violation.

To estimate the branch ratio of lepton flavor violating decays allowed by the current experimental data, Peccei, Wang and Zhang [2] took a model-independent approach to new physics and introduced a four-fermion contact interaction

where is the new physics cutoff. This effective operator is forbidden in the standard model, however will be generated in theories where lepton flavor is not conserved, such as the minimal supersymmetric standard model with and/or without R parity, models with large extra dimension [3]. Therefore, any observed signal is a direct evidence for non- standard physics and will improve our understanding of flavor dynamics, especially in the lepton sector.

There is no direct experimental limit on . However, at one-loop, attaching the neutral gauge boson Z to the charm quark loop generates an effective coupling of

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Z to . From the limits given in the particle data book on [1], we

obtained the lower bounds on the branch ratio of the decay into leptons:

Recently Nussinov, Peccei and Zhang [4] have examined “unitarity inspired" relations between two- and three-body lepton flavor violating decays and found that the existing strong bounds on and severely constrain two-body lepton flavor violating decays of vector bosons [ ,, and or pseudoscalars [ ,0 ] into

final states. However the bounds derived in Ref.[4] can be avoided if there is a kinematical suppression or as a result of some cancellations. Searching for lepton flavor violating decays of vector bosons such as remains a worthwhile experimental challenge.

BES started one year ago [5] and has been working on an experiment of searching for . They will publish their result officially in the near future.

2.7.2. Single D meson production in decays

Kinematically can not decay into D meson pairs, however it is able to decay into single D meson. In the standard model, these Cabbibo suppressed and/or

favored weak decays have a typical branch ratio ～ or smaller, which is

unobservable and because of which these processes serve as a probe of new physics. Recently Datta, O'Donnell, Pakvasa and I[6] have studied the possibility of searching for new flavor changing neutral current in the decay of . The purpose of the study is to answer whether new physics can enhance sufficiently for the processes to be observable in the near future experiments. We first perform a model independent analysis, then examine the predictions of the models, such as Top Color models, minimal sypersymmetric standard model with R-parity violation and the two Higgs

doublet model. We found that the branch ratio of could be as large as

[6].

Experimentally with BES-I data ～ , BES found no signal of

single D production in decays, and put limits on decay rates [7](at 90% C.L. ):

, and

. These are preliminary results.

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Next year BES will have collected ～ . The upper limit on

is expected to be reduced to ～ , which is very close to

the theoretical prediction. If there is still no signal found, it will put some constraints on models beyond the standard electroweak theory [6].

2.7.3 CP violation in decays

The origin of CP violation remains one of the outstanding problems in particle physics and cosmology. To pin down the sources and nature of CP violation in or beyond the Cabbibo-Kabayashi-Maskawa model, it would be necessary to consider different observations of CP violation in different channels from the K system, B system, etc. The reaction of interest at BES is [8]

Define CP/T-odd operator,

where are charged particles, for instance in the processes of three or five

pions decay of the , and and are corresponding momentum in the

laboratory frame. Define CP/T-odd operator

If there exists CP violating interaction in decays, one would expect a

non-vanishing expectation value of operator . Theoretically there could be many

sources responsible for the CP violation in the process. One of them is the Chromo-

dipole moment of the charm quark , where has mass

dimension and the the strong interaction coupling constant.

Consider the process of three pion decay of the , its branch ratio is around 1.5%. With ～ at BEPC II there will be around ～ available for the

analysis of CP violation. To estimate the experimental sensitivities to , let us

consider only the statistical uncertainties. Neglecting the systematical uncertainties one expects to be able at BES to probe for as small as

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.

One can easily construct different kind of CP/T-odd operators of three momentum products for the

analysis of CP violation in decays. For instance,

The initial electron and/or positron beams are not polarized at BEPC, otherwise one would be able to construct observables with the initial polarization

vector . There has been proposal to measure CP violation in

. With a large sample of , one expects to probe for and put a strong bound on the electric dipole moment of [9]. 2.7.3. Conclusion and comments

Except for the three kind of processes to probe for new physics in the decay of , there are some other rare decay modes which are interesting, but not reviewed.

For instance, with a large sample of , it is possible and physically interesting to search for Goldstone or Pseudo-Goldstone J, such as Axion in the process

Another example is the invisible decay of investigated by Chang, Lebedev and Ng [10] recently in models with extra Z-bosons, minimal sypersymmetric standard model with R-parity violation and decays into Goldstinos. The third example is provided by Bijnen and Maul [11], who recently have calculated in detail the branch ratio of decay into photon + missing energy in the popular theory these days with large extra dimension. They found that the branch ratio could be as large as 10-5, which is measurable at BES. Before conclusion, we point out that even though we focus our discussions here on decays, it is quite easy to apply the studies here for and system [12].

REFERENCES

[1] Particle Data Group, C. Caso et al., Europ. Phys. J, C3, 1(1998).[2] "Probing for lepton flavor violation in decays of charmonium and bottomonium systems", R.D. Peccei, Jian-Xiong Wang and Xinmin Zhang, May 1998 Note (unpublished); Xinmin Zhang, invited talked given at the national conference on high energy physics, Chengde, China, April (1998).[3] For examples, see, Z.K. Silagadze, hep-ph/9907328, July (1999);T. Huang, Z. Lin and X. Zhang, hep-ph/0009353 (2000); Chan Hong-Mo et al,hep-ph/0006338, hep-

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ph/0007004, hep-ph/0008313/0008324.[4] S. Nussinov, R.D. Peccei and X. Zhang, Hep-ph/0004153, April(2000).[5] G. Tong et al (Private communication).[6] A. Datta, P.J. O'Donnell, S. Pakvasa and X. Zhang, Phys. Rev. D60,014011 (1999).[7] G. Rong et al (in preparation).[8] Xinmin Zhang, Jian-Xiong Wang, Jian-Pin Ma, Dongshen Du and Wu-Jun Huo(in preparation).[9] Xiao-Gang He, Jian-Pin Ma and McKellar, Phys. Rev. D49, 4548 (1994); Ye Yiun-Xiou and Ye Zheng-Yu (unpublished).[10] N. Chang, O. Lebedev and J.N. Ng, hep-ph/9806497, June (1998). [11] J. Bijnens and M. Maul, hep-ph/0006042, July (2000).[12] For example, it has been proposed to probe for CP violation in theprocess $\Psi^\prime \rightarrow J/\Psi + \pi^+ + \pi^- $, [ Xinmin Zhang, Dongshen Du, Pin Wang, A. Datta, Jian-Xiong Wang and Jian-Pin Ma, May 1998 note, unpublished; The experimental analysis started already(Jin Li and Zhi-Jin Guo, private communication)].

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