Experimental Review on Light Meson Physics

1. QCHS06 – Ponta Delgada Experimental Review on Light Meson Physics Cesare Bini Universita’ “La Sapienza” and INFN Roma Outline (1) Overview (2) Pseudoscalars (3) Vectors (4) Scalars (5) The 1  2 GeV region

2. (1) Overview: mass spectra of mesons below 1 GeV Pseudoscalar multi-plet: qq states with L=0; S=0  JPC=0-+ Vector multi-plet: qq states with L=0; S=1  JPC=1-- qq states with L=1; S=1  JPC=0++ (??) BUT: provided sand kare there the scalars have an “Inverted Spectrum” Scalar multi-plet: s(500), k(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)

3. (2) Pseudoscalars-I: the h – h’ mixing angle 2 recent results on the mixing angle:  KLOE measures R = BR(f h’g) / BR(fhg) [Phys.Lett.B541(2002)45 + new preliminary]  BES measures R = BR(J/y h’g) / BR(J/yhg) [Phys.Rev.D73,052008(2006)] KLOE extracts the angle in the flavor basis [according to A.Bramon et al. Eur. Phys. J. C7 (1999)] BES extracts the angle in the octet-singlet basis [according to D.Gross,S.Treiman, F.Wilczek, Phys.Rev.D19 (1979)2188] KLOE vs. BES comparison: translate KLOEfP qP[caveat see T.Feldmann hep-ph/9907491]  1.7sdiscrepancy <qP> ~ -14.6o

4. (2) Pseudoscalars-II: the h’ gluonium content Allow theh’ (not theh) to have a gluonium content Zh’(new KLOE analysis preliminary) • Consistency check of the hyp.Zh’=0  X2h’ +Y2h’= 0.93 ± 0.06 • Introduce a further anglefG and extract it using all available data Work is in progress: 3 experimental constraints for 2 angles c2fit  worsefPresolution, estimate offG Space to improve the check ? G(h’)is poorly known, at~8% BR(h’wg), BR(h’r0g)known at 10% and 3% G(h’gg), G(p0gg)known at 3.5% and 7% G(wp0g)known at 3%

5. (2) Pseudoscalars-III: the h mass 3 recent “precision” measurements done with different methods:  NA48 (CERN) high statistics, invariant mass ofh p0p0p0decay [Phys.Lett.B533,196 (2002)]  GEM (Julich)hproduction through: p+d  3He + h [Phys.Lett.B619,281 (2005)]  KLOE (Frascati) decayf  hg  gggusing position photon directions [new preliminary] NA48 NA48 vs. GEM == 8sdiscrepancy: KLOE result (preliminary) is in agreement with NA48 and in disagreement with GEM KLOE NA48 GEM GEM h mass (MeV)

6. (2) Pseudoscalars-IV: planned experiments KLOE@DAFNE: [data taken in 2004-2006 – analysis in progress] e+e- f  hg , h‘g : ~ 3 ×105h/day + 2 × 103h‘/day(simultaneously) rare h, h´ decays, tests of ChPT, C and Isospin invariance + Expression of Interest for KLOE2 with 10 x KLOE ggwidths also CRYSTAL BALL+TAPS@MAMI: [started in 2004 – data taking in progress] gphp , h’p , p+gn, on2H liquid target: ~ 107h/day rare h, h´ decays, tests of ChPT and C-invariance pion polarizabilities, further test of ChPT WASA@COSY: [start in 2007] pppph , pph’ study of production and decays ofhandh’: ~108h/day or 106h’/day isospin simmetry breaking inh(h’) 3p  sinqph

7. (3) Vectors-I: precision measurements Precision measurements done (mostly at Novosibirsk) onr, wandfparameters:  pion form factor (e+e-  p+p-)  r – line shape +r0 – w mixing  e+e-  p+p-p0cross-section + depolarization method  wandfparameters CMD-2 CMD2 (prelim.) SND Summary [see Eidelman, talk Novosibirsk 2006]

8. (3) Vectors-II: modifications in nuclear medium Line-shapes of vector meson produced in dense nuclear medium Mass shift and broadening expected [see the talk by B.Kaempfer] Several experiments: positive evidences reported: • TAPS (Bonn-Elsa) [D.Trnka et al., Phys.Rev.Lett.94(2005) 192303] g+A w+X (wp0+g) on Nb and liquid 2H targets M(w*) = ( 722  4stat (+35/-5)syst ) MeV (~-160 MeV) • KEK PS-E325[R.Muto et al., J.Phys.G30 S1023 (2004)] p (12 GeV) + A  VM + X (VM  e+e-) on C and Cu Excess in ther – wregion  -9%rmass  g4 Jlab preliminary results [see the talk by C.Djalali]

9. (4) Scalars-I: the inverted spectrum  hint of 4-quark “Building Rule” Mass Q=0 Q=0 Q=1 Q=-1 (the f0(980) and a0(980)) add 2 Quarks s Q=0 Q=1 Q=0 Q=-1 (thek(800)) add 1 Quark s I3=0 Q=0 (the s(500)) 2 important consequences: if 4q hipothesys is correct  thes(500) and thek(800) have to be firmly established  the s-quark content of f0 and a0 should be sizeable  f0 and a0 couplings withf(ss) and with kaons [N.N.Achasov and V.Ivanchenko, Nucl.Phys.B315,465(1989)]

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

11. (4) Scalars-III: are there the s(500) and the k(800) ? • Latest theoretical evaluation:[I.Caprini, G.Colangelo,H.Leutwyler Phys.Rev.Lett.96 (2006) 132001] sas the lowest resonance in QCD Ms = 441+16-8 – i(272+9-12) MeV  Latest experimental “observation” ofs by BES [Phys.Lett.B598 (2004) 149] J/y wp+p-  Ms = 541 ± 39 – i(252 ± 42) MeV ( 472 ± 35 according to a refined analysis including pp scattering data and f gp0p0 KLOE data[D.Bugg hep-ph/0608081]) Evidence of s Evidence of k  Experimental “observation” ofk:BES [Phys.Lett.B633 (2006) 681] J/y  K*K+p-  Mk = 841 ± 30+81-73 – i(309 ± 45+48-72) MeV

12. (4) Scalars-IV: another hint for 4q: f f0(980)g, a0(980)g Mass degeneracy ; very small “coupling” withf large coupling withrandw (OZI rule argument) Expected mass difference; different “couplings” of f0 and a0 tof r and w. If are qq states: If are 4q states: Mass degeneracy; large coupling tof Look at f0 and a0 “affinity” to thef == content of quark s in the wavefunction: f radiative decays (CMD-2, SND, KLOE) p0p0g p+p-g KLOE observation of f0(980): p+p-g  fit of mass spectrum p0p0g  Dalitz plot analysis

13. (4) Scalars-V: results from f 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(f f0(980)g  p0p0g) = (1.07 ± 0.07) ×10-4(includes a small contribution froms(500)) BR(f f0(980)g  p+p-g) = (2.1  2.4) ×10-4 BR(f a0(980)g  hp0g) = (0.70 ± 0.07) ×10-4 Few remarks:  BR(f f0(980)g  p+p-g) ~ 2 × BR(f f0(980)g  p0p0g) as expected (Isospin)  BR(f f0(980)g) ~ 4  5 × BR(f a0(980)g) (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 f ( from the fit [G.Isidori et al. JHEP 0605:049(2006)]) gfMg (M any meson) (c) Coupling to meson pairs: gfKK >> gfpp gaKK ~ gahp A Sizeable coupling to KKis found for both

14. (4) Scalars-VI: results from J/y decays BES data: Phys.Rev. D68 (2003) 52003, Phys.Lett. B607 (2005) 243, Phys.Lett. B603 (2004) 138 s(500) f0(980) f0(980) J/ywK+K- J/ywp+p- J/yfp+p- J/yfK+K- Message: s(500) has a u-d quark structure, f0(980) has large s content

15. (4) Scalars-VII: gg widths Another “strong” argument in favour of non qq nature of low mass scalars. f0(980) and a0(980) have small G(gg) compared to f2(1270) and a2(1320) [PDG 2004 values]: G(f0(980)gg) = 0.39 ± 0.13 keV G(a0(980)gg) = 0.30 ± 0.10 keV G(f2(1270)gg) = 2.60 ± 0.24 keV G(a2(1320)gg) = 1.00 ± 0.06 keV Large G(gg) compact object promptly annihilating in 2 g BUT: experimentally very “poor” measuraments.  Low Energy gg physics still to be done • A recent result by BELLE • (not yet published): • gg p+p- for Wgg>700 MeV • f0(980) peak is observed. • G(f0(980)gg) ~ 0.15 keV [N.N.Achasov and G.N.Shestakov, Phys.Rev.D72,013007 (2005)] A recent estimate of G(s(500)gg) = 4.3 keV [M.R.Pennington Phys.Rev.Lett.97,0011601 (2006)] A complete low energy gg 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]

16. (4) Scalars-VIII: summary and outlook Most 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- fh and e+e-  ff0(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]

17. (5) 1 ÷ 2 GeV region-I: the second scalar multi-plet • again: hint of an inverted spectrum  4-quark structure • 3 I=0 states: probably one is a glueball (Maiani, Piccinini, Polosa, Riquer hep-ph/0604018) • Ratio [f0(1370)KK]/[f0(1370)pp] sensitive to the quark structure and • to the glueball-tetraquark mixing scheme.

18. (5) 1 ÷ 2 GeV region-II: around the nucleon threshold • BES: J/y radiative decays: • Threshold effect on pp • Peak in p+p-h’ (7.7s) • Threshold effect in fw • Consistent masses and widths • Not a vector: (0-+ or 0++) • Properties similar to h’ [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 [BABAR coll., Phys.Rev.D73:052003 (2006)] BABAR-3 BABAR-1

19. 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 s 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.