study of excited baryons at besii
DESCRIPTION
Study of excited baryons at BESII. HongXun Yang Representing BES Collaboration IHEP [email protected] January 25-26, 2007,. Outline. Introduction FDC system for PWA N* in the decay of X(2075) and Nx in the decay of Summary. - PowerPoint PPT PresentationTRANSCRIPT
Study of excited baryonsat BESII
HongXun YangRepresenting BES Collaboration
January 25-26, 2007,
Outline
• Introduction
• FDC system for PWA
• N* in the decay of
• X(2075) and Nx in the decay of
• Summary
npJ /
pKJ /
L ~ ~51030 /cm2s at J/ peak
Ecm~2-5 GeV
The Beijing Electron Positron Collider
VC: xy = 100 m TOF: T = 180 ps counter: r= 3 cm MDC: xy = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 % = 7.9 mr B field: 0.4 T p/p=1.78%(1+p2) z = 3.1 cm
BESII Detector
Study of Excited Baryon States
Motivation• Probe the internal structure of light quark baryons
• Search for “missing” baryons predicted by quark model
• Obtain a better understanding of the strong interaction force in the non-perturbative regime
• Examples for FDC application
J/ decays
processes branching ratios(10-3) N* decays
1.10.1
6.00.5
2.00.1
2.10.2
0.90.4
1.30.3
0/ ppJ ppJ /
relatively large branching ratios (PDG2004)
npJ / ppJ /
'/ ppJ ppJ /
NN *
NN *NN *
NN *NN '* NN *
Pure isospin 1/2
Feynman diagram of the production of
For and , N and N systems are limited to be pure
isospin 1/2.
**,*,*, Np
NNJ / NNJ /
FDC system
Physics model
• Feynman rules
• Counterms
• physical parameters
Physical process
• Generate Feynman Diagram
• Manipulate Amplitude and generate fortran source
• Manipulate Kinematics and generate fortran source
• Compile files and link sources
Application of FDC
• Loglikelyhood
• MC integration
• Minimize lnL to get the parameters of each partial wave
2
1
||
iii
N
n MC
data
Acw
w
wL
MCN
kkMC w
Nw
1
1
from BES II data
Events selection
2 good charged tracks Q1+Q2 = 0 |cos| < 0.8 PID: TOF and dE/dx Mp > 1.15 GeV
0.88<Mmiss<1.0 GeV
Background < 8%
npπJ/ψ
N* in npπJ/ψ N*(1440)
N*(1520)
N*(1535)N*(1650)
N*(1675)
N*(1680)
?
L=0 limits it to be
3/2+ or 1/2+
N* in npπJ/ψ
Dalitz Plot:
Acceptance and other reasons make the plot asymmetry.
PWA with FDC
• Try to determine the JPC of the resonance around 2.0 GeV/c2
• Following resonances are consideredN(939), P11(1440), D13(1520), S11(1535),S11(1650), D15(1675), F15(1680), X
• For the background: phase space and sideband
Results of PWA (I)
data fit
Results of PWA (II)
• L = 0 is preferred due to the suppression of the centrifugal barrier factor for L 1
• For L = 0, JP is limited to be ½+ and 3/2+
• S decrease more than 400 if either ½+ or 3/2+ is included in the PWA fit
• S decrease more than 60 if one of them are added while another has been included
Results of PWA (III)
• The peak around 2.0 GeV/c2 cannot be reproduced by reflections of well-established N* resonances
• Jp=1/2+,3/2+ is preferred by PWA
conclusion
Events selection
• 4 good charged tracks
• PID: kaon and at least 1 proton ID2 (4C)<20
• Bg rate:1~2%
../ ccpKJ
• Two clear peaks at 1520, 1690 MeV/c2 in pK mass• N* in K mass
N*(1535)?
N*(1650)?*(1520) *(1690)
pKM KM
N* & * in
(I)
. . /c c pK J
• Bands for excited baryon states in Dalitz plot
X(2075) in
. . /c c pK J
*(1520)
*(1690)
N*(1535)?
N*(1650)?
Phase Space
X(2075)
Possible N* and Λ* in PWA
• mass(JP)
N*: 1535 (1/2-), 1650(1/2-), 1710(1/2+), 1720(3/2+),
1900(3/2+ or 3/2-), 2050(1/2+ or 3/2+)
Λ*: 1405(1/2-), 1520(3/2-), 1600(1/2+), 1670(1/2-),
1690(3/2-), 1800(1/2-), 1810(1/2+), 1890(3/2+)
Mathematical fit
• W/o any constrains, PWA with N* and Λ* can fit data:
(S= -997)
• However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.
Big BRs
Fraction of Ndata Nevent
N*(1900) 3/2+ 108% 5900
N*(2050) 3/2+ 33% 1800
Λ*(1890) 3/2+ 21% 1100
Λ*(1810) 1/2+ 9% 500
Λ*(1800) 1/2- 34% 1900
(1/2- is P-wave decay, which should be suppressed )
Estimated Nevent in Ndata
Nevent/2 (each decay mode, not include c.c.)
N*(1900) 3/2+ ~ 300 -400 N*(2050) 3/2+ ~ 100 -150 Λ*(1890) 3/2+ ~ 100 -150 Λ*(1810) 1/2+ ~ 150 -200 Λ*(1800) 1/2- ~ 20 - 30 (1/2- is P-wave decay, which should be strongly suppressed near threshold)
PWA with tight constrains and w/o X(2075)
• Constrain the Nevent of near threshold states ~ 100-200
• The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900).
• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.
• Significance of X(2075) >> 5 sigma. (S=-952)
PWA with tight constrains and with X(2075)
PWA with looser constrains and w/o X(2075)
Constrain the Nevent of near threshold states ~ 500-600
(already too big at least for Λ*(1810), Λ*(1890) as limited in )
The PWA fit can hardly reproduce the enhancement near pΛ threshold (S=-940).
../ ccJ
PWA with looser constrains and with X(2075)
• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.
• Significance of X(2075) > 5 sigma. (S=-962)
conclusion of pΛ enhancement
• Reproducing the pΛ threshold enhancement with pure N* and Λ* interferences needs many unexpected big BRs and large destructive interferences.
• PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement.
• PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance.
)(GeV/c2
ΛKM
PS, eff. corrected
MMM KΛK
A strong enhancement near the threshold of mass spectrum of ..ccK
(Arbitrary normalization)
BES II pKJ /
NX*
We perform PWA studies on the KΛ mass threshold structure:
The most important we want to study is its production BR
PWA is performed to pKJ /
possible N* and *states listed in PDG are fitted N(1720), N(1900), (1520), (1690), …
many different combinations are tried
different JP of Nx is tried
also tried N(1535) to fit Nx
An example of PWA fit
• Mass and Width scan
• Total fit (S=-952)
• Nevent:
Fraction Nevent
NX 14.7% 799
N(1720) 17.1% 929
N(1900) 13.2% 717
(1520) 4.8% 261
(1570) 21.8% 1184
(1690) 14.4% 782
(1890) 13.8% 750
X(2075) 11.3% 614
Mass scan(GeV/c2)
Width scan(GeV/c2)
• NXN(1535)
• Total fit (S=-932)
• Nevent:
Fraction Nevent
N(1535) 26.0% 1413
N(1720) 9.7% 527
N(1900) 11.4% 619
(1520) 4.8% 261
(1570) 22.2% 1026
(1690) 3.6% 739
(1890) 18.3% 994
X(2075) 11.2% 608
Dalitz plot (data)
Dalitz plot (PWA)
Eve
nts
/10M
eV Crosses: data
Hist.: PWA fit projection
JP check with various combinations
• JP ½- ½+ 3/2- 3/2+ non
• A -940 -848 -848 -930 -813
• B -845 -783 -806 -833 -752
• C -952 -841 -844 -916 -768
• D -880 -768 -752 -822 -650
• E -957 -889 -893 -944 -875
• F -970 -920 -925 -963 -919
• G -954 -925 -919 -944 -909
Fit results
Cases Mass(GeV) Width(MeV) Fraction(%) Nevent Log Likelyhood
a 1.52 ~ 1.62 110 22.3 1210 -940
b 1.56 80 ~110 44.4 2412 -845
c 1.62 70 14.7 799 -952
d 1.6 ~1.64 70 17.1 929 -880
e 1.57 90 20.6 1119 -957
f 1.62 70 ~ 90 19.9 1081 -970
g 1.58 80 15.6 845 -954
1.50 ~1.65 70 ~110 >14.7 >800
• A strong enhancement is observed near the mass threshold of MK at BES II.
• Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has:
Mass 1500~1650MeV
Width 70~110MeV
JP favors 1/2-
consistent with N*(1535)
The most important is:
It has large BR(J/ψ pNX*) BR(NX* KΛ) 2 X 10-4 ,
suggesting NX*has strong coupling to KΛ.
indicating it could be a KΛ molecular state
(5 - quark system).
Summary• FDC applied in the analysis of
and• Clear signals of excited baryons observed• Seemed to be “missing” states observed in
• Possible multi-quark states X(2075) and Nx in
• PWA on Nx, N* and * in
is still going on…
../ ccpKJ ../ ccnpJ
../ ccnpJ
../ ccpKJ
../ ccpKJ
Thank you!