1. 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 QCHS06 – Ponta Delgada

2. qq states with L=1; S=1  J PC =0 ++ (??) BUT: provided  and  are there the scalars have an “Inverted Spectrum” Pseudoscalar multi-plet : qq states with L=0; S=0  J PC =0 -+ Vector multi-plet: qq states with L=0; S=1  J PC =1 -- (1) Overview: mass spectra of mesons below 1 GeV Scalar multi-plet:  (500),  (700), f 0 (980), a 0 (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  –  ’ mixing angle 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] KLOE vs. BES comparison: translate KLOE  P   P [caveat see T.Feldmann hep-ph/9907491]  1.7  discrepancy  ~ -14.6 o

4. (2) Pseudoscalars-II: the  ’ gluonium content Allow the  ’ (not the  ) to have a gluonium content Z  ’ (new KLOE analysis preliminary )  Consistency check of the hyp. Z  ’ =0  X 2  ’ +Y 2  ’ = 0.93 ± 0.06  Introduce a further angle  G and extract it using all available data Work is in progress: 3 experimental constraints for 2 angles  2 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%

5. (2) Pseudoscalars-III: the  mass 3 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  3 He +  [Phys.Lett.B619,281 (2005)]  KLOE (Frascati) decay      using position photon directions [new preliminary] GEM NA48 NA48 vs. GEM == 8  discrepancy: KLOE result (preliminary) is in agreement with NA48 and in disagreement with GEM  mass (MeV) KLOE NA48 GEM

6. (2) Pseudoscalars-IV: planned experiments KLOE@DAFNE: [data taken in 2004-2006 – analysis in progress] e + e -    ,  ‘  : ~ 3 ×10 5  /day + 2 × 10 3  ‘/ 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]  p   p,  ’p,  +  n, on 2 H liquid target : ~ 10 7  / day rare ,  ´ decays, tests of ChPT and C-invariance pion polarizabilities, further test of ChPT WASA@COSY: [start in 2007] pp  pp , pp  ’ study of production and decays of  and  ’: ~10 8  / day or 10 6  ’/ day isospin simmetry breaking in  (  ’)  3   sin  

7. (3) Vectors-I: precision measurements Precision 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]

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:  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 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 2 H targets M(  *) = ( 722  4 stat (+35/-5) syst ) MeV (~-160 MeV)

9. Q=0 Q=0Q=1 Q=-1 (the f 0 (980) and a 0 (980)) I 3 =0 Q=0 (the  (500)) Q=0 Q=1 Q=0 Q=-1 (the  (800)) add 1 Quark s add 2 Quarks 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 f 0 and a 0 should be sizeable  f 0 and a 0 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

10. 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

11. (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

12. (4) Scalars-IV: another hint for 4q:   f 0 (980) , a 0 (980)  If are qq states: Mass degeneracy ; very small “coupling” with  large coupling with  and  (OZI rule argument) Expected mass difference; different “couplings” of f 0 and a 0 to  and . If are 4q states: Mass degeneracy; large coupling to  Look at f 0 and a 0 “affinity” to the  == content of quark s in the wavefunction:  radiative decays (CMD-2, SND, KLOE) KLOE observation of f 0 (980):       fit of mass spectrum       Dalitz plot analysis  

13. (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(   f 0 (980)        ) = (1.07 ± 0.07) ×10 -4 (includes a small contribution from  (500)) BR(   f 0 (980)        ) = (2.1  2.4) ×10 -4 BR(   a 0 (980)      ) = (0.70 ± 0.07) ×10 -4 Few remarks:  BR(   f 0 (980)        ) ~ 2 × BR(   f 0 (980)        ) as expected (Isospin)  BR(   f 0 (980)  ) ~ 4  5 × BR(   a 0 (980)  ) (assuming f 0, a 0  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)] ) g  M  (M any meson) Mesong  M  (GeV -1 ) 00 0.12  0.66 ’’ 0.70 f0f0 1.2  2.0 a0a0 > 1.0 (prel.) (c) Coupling to meson pairs: g fKK >> g f  g aKK ~ g a  A Sizeable coupling to KK  is found for both

14. (4) Scalars-VI: results from J/  decays  (500) f 0 (980) J/       J/       f 0 (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, f 0 (980) has large s content

15. (4) Scalars-VII:  widths Another “strong” argument in favour of non qq nature of low mass scalars. f 0 (980) and a 0 (980) have small  compared to f 2 (1270) and a 2 (1320) [PDG 2004 values]:  (f 0 (980)   ) = 0.39 ± 0.13 keV  (a 0 (980)   ) = 0.30 ± 0.10 keV  (f 2 (1270)   ) = 2.60 ± 0.24 keV  (a 2 (1320)   ) = 1.00 ± 0.06 keV Large   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 MeV f 0 (980) peak is observed.   (f 0 (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)]

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: f 0 (980), a 0 (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 -   f 0 (980) vs. √s  quark counting [ S.Pacetti, talk given at QNP06 Madrid ]  4 elementary fields for f 0  need of data at higher √s (2) Heavy ions: elliptic-flow counts the valence quarks [see M.Lisa talk here]

17. 1. again: hint of an inverted spectrum  4-quark structure 2.3 I=0 states: probably one is a glueball (Maiani, Piccinini, Polosa, Riquer hep-ph/0604018) 3.Ratio [f 0 (1370)  KK]/[f 0 (1370)   ] sensitive to the quark structure and to the glueball-tetraquark mixing scheme. (5) 1 ÷ 2 GeV region-I: the second scalar multi-plet

18. (5) 1 ÷ 2 GeV region-II: around the nucleon threshold BES: J/  radiative decays:  Threshold effect on pp  Peak in      ’ (7.7  )  Threshold effect in   Consistent masses and widths  Not 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 2M p Experim.processM(MeV)  (MeV) DM266 ~1930~35 FENICEMh~1870~10 E6873+3-3+3- 1910 ± 10 33 ± 13 BABAR-13+3-3+3- 1880 ± 50130 ± 30 BABAR-22+2-202+2-20 1860 ± 20160 ± 20 BABAR-32+2-2+2- 1880 ± 10180 ± 20 BABAR-4 +-20+-20 1890 ± 20190 ± 20 BABAR-1 BABAR-3 [BABAR coll., Phys.Rev.D73:052003 (2006)]

19. Conclusions Many other things not mentioned:  hybrids, 1 -+ states, BES f 0 (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.