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Experimental Review on Light Meson Physics

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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

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)

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

2 recent results on the mixing angle:

KLOE measures R = BR(f h’g) / BR(fhg) [Phys.Lett.B541(2002)45 + new preliminary]

BES measures R = BR(J/y h’g) / BR(J/yhg) [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

(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(p0gg)known at 3.5% and 7%

G(wp0g)known at 3%

(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)

(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]

gphp , 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]

pppph , pph’ study of production and decays ofhandh’: ~108h/day

or 106h’/day

isospin simmetry breaking inh(h’) 3p sinqph

(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]

(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 (wp0+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]

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

“Building Rule”

Mass

Q=0 Q=0Q=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)]

(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

(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

(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

(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

(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/ywK+K-

J/ywp+p-

J/yfp+p-

J/yfK+K-

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

(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]

(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]

(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.

(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

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.