Experimental Review on Light Meson Physics

Cesare BiniUniversita’ “La Sapienza” and INFN Roma

Outline(1) Overview(2) Pseudoscalars(3) Vectors(4) Scalars (5) The 1 2 GeV region

QCHS06 – Ponta Delgada

qq states with L=1; S=1 JPC=0++ (??) BUT: provided and are there

the scalars have an “Inverted Spectrum”

Pseudoscalar multi-plet:qq states with L=0; S=0 JPC=0-+

Vector multi-plet:qq states with L=0; S=1 JPC=1--

(1) Overview: mass spectra of mesons below 1 GeV

Scalar multi-plet:(500), (700), f0(980), a0(980)

This talk will review: Recent measurements on P and V (“refinement” measurements) Several recent measurements on S(many open questions)

(2) Pseudoscalars-I: the – ’ mixing angle

32

2

2'

sin2cos

sincos2

/

/

p

p

m

m

JBR

JBRR

PP

PP

thsysstatP .... 607030441

KLOE extracts the angle in the flavor basis [according to A.Bramon et al. Eur. Phys. J. C7 (1999)]

2 recent results on the mixing angle: KLOE measures R = BR( ’) / BR() [Phys.Lett.B541(2002)45 + new preliminary] BES measures R = BR(J ’) / BR(J) [Phys.Rev.D73,052008(2006)]

BES extracts the angle in the octet-singlet basis [according to D.Gross,S.Treiman, F.Wilczek, Phys.Rev.D19 (1979)2188]

2.19.15P

32

2

2sin

tan1cot

η

η

P

V

S

NSsP p

p

Z

Z

m

m

ηγφBR

γηφBRR

KLOE vs. BES comparison: translate KLOE P P [caveat see T.Feldmann hep-ph/9907491]

0.13.13)(KLOEP 1.7 discrepancy <P> ~ -14.6o

(2) Pseudoscalars-II: the ’ gluonium contentAllow the ’ (not the ) to have a gluonium content Z’ (new KLOE analysis preliminary)

Consistency check of the hyp. Z’ =0 X2

’ +Y2’ = 0.93 ± 0.06

Introduce a further angle G and extract it using all available data

GGPGP

PP

ZYX

YX

sin;coscos;cossin

sin;cos

'''

Work is in progress:3 experimental constraints for 2 angles2 fit worse P resolution, estimate of G

Space to improve the check ?(’) is poorly known, at~8%BR(’), BR(’0) known at 10% and 3%(’), () known at 3.5% and 7%(0) known at 3%

(2) Pseudoscalars-III: the mass3 recent “precision” measurements done with different methods: NA48 (CERN) high statistics, invariant mass of decay [Phys.Lett.B533,196 (2002)]

GEM (Julich) production through: p+d 3He + [Phys.Lett.B619,281 (2005)]

KLOE (Frascati) decay using position photon directions [new preliminary]

GEM

NA48NA48 vs. GEM == 8 discrepancy:KLOE result (preliminary) is in agreementwith NA48 and in disagreement with GEM

mass (MeV)

KLOE

NA48

GEM

(2) Pseudoscalars-IV: planned experiments

KLOE@DAFNE: [data taken in 2004-2006 – analysis in progress]e+e- , ‘ : ~ 3 ×105 /day + 2 × 103 ‘/day (simultaneously)rare , ´ decays, tests of ChPT, C and Isospin invariance+ Expression of Interest for KLOE2 with 10 x KLOE widths also

CRYSTAL BALL+TAPS@MAMI: [started in 2004 – data taking in progress]pp , ’p , +n, on 2H liquid target: ~ 107 /dayrare , ´ decays, tests of ChPT and C-invariancepion polarizabilities, further test of ChPT

WASA@COSY: [start in 2007]pppp , pp’ study of production and decays of and ’: ~108 /dayor 106 ’/dayisospin simmetry breaking in (’) 3 sin

(3) Vectors-I: precision measurementsPrecision measurements done (mostly at Novosibirsk) on , and parameters:

pion form factor (e+e- ) – line shape + 0 – mixing e+e- cross-section + depolarization method and parameters

CMD2 (prelim.)

SND

CMD-2

Summary [see Eidelman, talk Novosibirsk 2006]

(3) Vectors-II: modifications in nuclear mediumLine-shapes of vector meson produced in dense nuclear mediumMass shift and broadening expected [see the talk by B.Kaempfer]Several experiments: positive evidences reported:

KEK PS-E325 [R.Muto et al., J.Phys.G30 S1023 (2004)]

p (12 GeV) + A VM + X (VM e+e-) on C and CuExcess in the – region -9% mass

g4 Jlab preliminary results [see the talk by C.Djalali]

TAPS (Bonn-Elsa) [D.Trnka et al., Phys.Rev.Lett.94(2005) 192303]

+A +X (0+) on Nb and liquid 2H targetsM(*) = ( 722 4stat (+35/-5)syst ) MeV (~-160 MeV)

Q=0 Q=0 Q=1 Q=-1 (the f0(980) and

a0(980))

duud

dusdduusdsudsuud

susddsusdssdsuus

I3=0 Q=0 (the (500))

Q=0 Q=1 Q=0 Q=-1 (the (800))add 1Quark s

add 2Quarks s

“Building Rule”Mass

2 important consequences: if 4q hipothesys is correct the (500) and the (800) have to be firmly established the s-quark content of f0 and a0 should be sizeable f0 and a0 couplings with (ss) and with kaons

[N.N.Achasov and V.Ivanchenko, Nucl.Phys.B315,465(1989)]

(4) Scalars-I: the inverted spectrum hint of 4-quark

Renewed interest after B-factory results: new scalar meson “zoology” above 2.3 GeV

reconsider the low mass spectrum

Assuming 2 quarks interacting by a single gluon exchange. Find other configurations:

Color triplet diquarks and anti-diquarks Attractive interaction between diquark and anti-diquark

giving a color singlet [R.L.Jaffe, Phys.Rev.D15,267(1977)]

it is possible to build up 4-quarks scalar meson

(4) Scalars-II: the 4-quark hipothesys

(4) Scalars-III: are there the (500) and the (800) ?

Latest experimental “observation” of by BES [Phys.Lett.B598 (2004) 149]

J/ M = 541 ± 39 – i(252 ± 42) MeV( 472 ± 35 according to a refined analysis including scattering data and KLOE data [D.Bugg hep-ph/0608081])

Evidence of

Evidence of

Latest theoretical evaluation: [I.Caprini, G.Colangelo,H.Leutwyler Phys.Rev.Lett.96 (2006) 132001]

as the lowest resonance in QCD

M = 441+16-8 – i(272+9

-12) MeV

Experimental “observation” of BES [Phys.Lett.B633 (2006) 681]

J/ K*K+-

M = 841 ± 30+81-73 – i(309 ± 45+48

-72) MeV

(4) Scalars-IV: another hint for 4q: f0(980), a0(980)

ssdduu

dduudduu

00

00

f;2

a

2f;

2aIf are qq

states:

Mass degeneracy ; very small “coupling” with large coupling with and (OZI rule argument)

Expected mass difference; different “couplings” of f0 and a0 to and .

If are 4q states:

ssdduu

ssdduu

2f;

2a 00 Mass degeneracy; large coupling to

Look at f0 and a0 “affinity” to the == content of quark s in the wavefunction:radiative decays (CMD-2, SND, KLOE)

0

000

00

f

f

a

KLOE observation of f0(980): fit of mass spectrum Dalitz plot analysis

(4) Scalars-V: results from radiative decays The signal due to the scalar is “lost” in a large and partly unknown background:

Fit needed to extract the relevant amplitude model dependence

(a) Branching Ratios ( integral of the scalar spectrum) [KLOE analysis – model dependent]: [Phys.Lett.B536,209(2002),Phys.Lett.B537,21(2002),Phys.Lett.B634,148(2006)]

BR( f0(980) ) = (1.07 ± 0.07) ×10-4 (includes a small contribution from (500))

BR( f0(980) ) = (2.1 2.4) ×10-4

BR( a0(980) ) = (0.70 ± 0.07) ×10-4

Few remarks: BR( f0(980) ) ~ 2 × BR( f0(980) ) as expected (Isospin) BR( f0(980)) ~ 4 5 × BR( a0(980)) (assuming f0, a0 KK negligible)

both too large to be compatible to qq states [Achasov, Ivanchenko, Nucl.Phys.B315,465(1989)]

(b) Couplings to the ( from the fit [G.Isidori et al. JHEP 0605:049(2006)]) gM(M any meson)

2/3

2

222

3M

m

mmg MM

Meson gM (GeV-1)

0 0.12

0.66

’ 0.70

f0 1.2 2.0

a0 > 1.0 (prel.)

(c) Coupling to meson pairs: gfKK >> gf

gaKK ~ ga

A Sizeable coupling to KKis found for both

(4) Scalars-VI: results from J/ decays

(500) f0(980)

J/

J/

f0(980)

BES data: Phys.Rev. D68 (2003) 52003, Phys.Lett. B607 (2005) 243, Phys.Lett. B603 (2004) 138

J/K+K-

J/K+K-

Message: (500) has a u-d quark structure, f0(980) has large s content

(4) Scalars-VII: widths

Another “strong” argument in favour of non qq nature of low mass scalars.f0(980) and a0(980) have small compared to f2(1270) and a2(1320) [PDG 2004 values]:

(f0(980)) = 0.39 ± 0.13 keV(a0(980)) = 0.30 ± 0.10 keV(f2(1270)) = 2.60 ± 0.24 keV(a2(1320)) = 1.00 ± 0.06 keVLarge compact object promptly annihilating in 2 BUT: experimentally very “poor” measuraments. Low Energy physics still to be done

A recent result by BELLE(not yet published): for W>700 MeVf0(980) peak is observed.(f0(980)) ~ 0.15 keV[N.N.Achasov and G.N.Shestakov, Phys.Rev.D72,013007 (2005)]

A complete low energy physics program can be pursued at DAFNE-2 [see F.Ambrosino et al. hep-ex/0603056, see also F.Nguyen, F.Piccinini, A.Polosa hep-ph/0602205]

A recent estimate of ((500)) = 4.3 keV[M.R.Pennington Phys.Rev.Lett.97,0011601 (2006)]

(4) Scalars-VIII: summary and outlookMost analyses seem to point to a non q-qbar nature of the low mass scalar mesons: Tetraquarks [discussed by many authors...] Extended objects: f0(980), a0(980) as K-Kbar molecules [J.Weinstein,N.Isgur,Phys.Rev.D27(1979)588] They are not elementary particles but are composite objects [V.Baru et al.,Phys.Lett.B586 (2004) 53]

New experimental checks (quark counting): (1) BABAR – ISR measures e+e- and e+e- f0(980) vs. √s quark counting [S.Pacetti, talk given at QNP06 Madrid] 4 elementary fields for f0

need of data at higher √s

(2) Heavy ions: elliptic-flow counts the valence quarks [see M.Lisa talk here]

1. again: hint of an inverted spectrum 4-quark structure2. 3 I=0 states: probably one is a glueball (Maiani, Piccinini, Polosa, Riquer hep-ph/0604018)

3. Ratio [f0(1370)KK]/[f0(1370)] sensitive to the quark structure andto the glueball-tetraquark mixing scheme.

(5) 1 ÷ 2 GeV region-I: the second scalar multi-plet

(5) 1 ÷ 2 GeV region-II: around the nucleon threshold

BES: J/ radiative decays:Threshold effect on ppPeak in ’ (7.7)Threshold effect in Consistent masses and widthsNot a vector: (0-+ or 0++)Properties similar to ’[BES-II coll., Phys.Rev.Lett. 95 (2005) 262001

Phys.Rev.Lett. 96 (2006) 162002]

M = 1830.6 6.7 MeV

= 0 93 MeV

M = 1833.7 7.2 MeV

= 68 22 MeV

BABAR: e+e- hadrons through ISR confirms a vector state around 2Mp

Experim.

process M(MeV) (MeV)

DM2 6 ~1930 ~35

FENICE Mh ~1870 ~10

E687 3+3- 1910 ± 10

33 ± 13

BABAR-1

3+3- 1880 ± 50

130 ± 30

BABAR-2

2+2-

20

1860 ± 20

160 ± 20

BABAR-3

2+2- 1880 ± 10

180 ± 20

BABAR-4

+-20 1890 ± 20

190 ± 20

BABAR-1 BABAR-3[BABAR coll., Phys.Rev.D73:052003 (2006)]

Conclusions

Many other things not mentioned: hybrids, 1-+ states, BES f0(1790) ?, new states above 2 GeV,...

The experimental activities are mostly concentrated on the Scalar sector (the most fundamental and the most elusive) but also on Pseudoscalar and on Vector states.

SCALARS:(1) Convergence of theory and experiments on the as a resonance;(2) There are now many hints of a non standard (non q-qbar) structure for the lowest mass scalar multi-plet and some also for the second scalar multi-plet.

VECTORS and PSEUDOSCALARS: precision measurements are coming. In all cases the main difficulty is to extract “model-independent” conclusions from data: a positive collaboration between theorists and experimentalists is crucial.