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

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Experimental review on light meson physics

QCHS06 – Ponta Delgada

Experimental Review on Light Meson Physics

Cesare Bini

Universita’ “La Sapienza” and INFN Roma


(1) Overview

(2) Pseudoscalars

(3) Vectors

(4) Scalars

(5) The 1  2 GeV region

Experimental review on light meson physics

(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)

Experimental review on light meson physics

(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

Experimental review on light meson physics

(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%

Experimental review on light meson physics

(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 vs. GEM == 8sdiscrepancy:

KLOE result (preliminary) is in agreement

with NA48 and in disagreement with GEM





h mass (MeV)

Experimental review on light meson physics

(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

Experimental review on light meson physics

(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


CMD2 (prelim.)


Summary [see Eidelman, talk Novosibirsk 2006]

Experimental review on light meson physics

(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]

Experimental review on light meson physics

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

“Building Rule”


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)]

Experimental review on light meson physics

(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

Experimental review on light meson physics

(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

Experimental review on light meson physics

(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


If are 4q


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)



KLOE observation of f0(980):

p+p-g  fit of mass spectrum

p0p0g  Dalitz plot analysis

Experimental review on light meson physics

(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]:


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

Experimental review on light meson physics

(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)






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

Experimental review on light meson physics

(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]

Experimental review on light meson physics

(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]

Experimental review on light meson physics

(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.

Experimental review on light meson physics

(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)]



Experimental review on light meson physics


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.


(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.