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
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(1) Overview mass spectra of mesons below 1 GeV
Pseudoscalar multi-plet qq states with L0 S0
? JPC0-
Vector multi-plet qq states with L0 S1 ?
JPC1--
qq states with L1 S1 ? JPC0 (??) BUT
provided s and k are 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)
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(2) Pseudoscalars-I the h h mixing angle
2 recent results on the mixing angle ? KLOE
measures R BR(f? hg) / BR(f?hg)
Phys.Lett.B541(2002)45 new preliminary ?
BES measures R BR(J/y? hg) /
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 KLOE fP ? qP
caveat see T.Feldmann hep-ph/9907491
? 1.7 s discrepancy ? ltqPgt -14.6o
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(2) Pseudoscalars-II the h gluonium content
Allow the h (not the h) 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 angle fG
• and extract it using all available data

Work is in progress 3 experimental constraints
for 2 angles c2 fit ? worse fP resolution,
estimate of fG
Space to improve the check ? G(h) is poorly
known, at8 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
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(2) Pseudoscalars-III the h mass
3 recent precision measurements done with
different methods ? NA48 (CERN) high
statistics, invariant mass of h? p0p0p0 decay
Phys.Lett.B533,196 (2002) ? GEM (Julich) h
production through pd ? 3He h
Phys.Lett.B619,281 (2005) ? KLOE (Frascati)
decay f ? hg ? ggg using position photon
directions new preliminary
NA48
NA48 vs. GEM 8s discrepancy KLOE result
(preliminary) is in agreement with NA48 and in
disagreement with GEM
KLOE NA48 GEM
GEM
h mass (MeV)
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(2) Pseudoscalars-IV planned experiments
KLOE_at_DAFNE data taken in 2004-2006 analysis
in progress ee- ? f ? hg , hg 3 105
h/day 2 103 h/day (simultaneously) rare h,
h decays, tests of ChPT, C and Isospin
invariance Expression of Interest for KLOE2
with 10 x KLOE ? gg widths also CRYSTAL
BALLTAPS_at_MAMI started in 2004 data taking in
progress gp?hp , hp , pgn, on 2H liquid
target 107 h/day rare h, h decays, tests of
ChPT and C-invariance pion polarizabilities,
further test of ChPT WASA_at_COSY start in
2007 pp?pph , pph study of production and
decays of h and h 108 h/day or 106
h/day isospin simmetry breaking in h(h)? 3p ?
sinqph
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(3) Vectors-I precision measurements
Precision measurements done (mostly at
Novosibirsk) on r, w and f parameters ? pion
form factor (ee- ? pp-) ? r line shape r0
w mixing ? ee- ? pp-p0 cross-section
depolarization method ? w and f parameters
CMD2 (prelim.) SND
CMD-2
Summary see Eidelman, talk Novosibirsk 2006
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(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
• gA? wX (w?p0g) 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 ? ee-) on C and Cu
• Excess in the r w region ? -9 r mass

? g4 Jlab preliminary results see the talk by
C.Djalali
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(4) Scalars-I the inverted spectrum ? hint of
4-quark
Building Rule
Mass
Q0 Q0 Q1 Q-1 (the
f0(980) and a0(980))
add 2 Quarks s
Q0 Q1 Q0 Q-1 (the k(800))
add 1 Quark s
I30 Q0 (the s(500))
2 important consequences if 4q hipothesys is
correct ? the s(500) and the k(800) have to be
firmly established ? the s-quark content of f0
and a0 should be sizeable ? f0 and a0
couplings with f (ss) and with kaons N.N.Achaso
v and V.Ivanchenko, Nucl.Phys.B315,465(1989)
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(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

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(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
• s as the lowest resonance in QCD
• Ms 44116-8 i(2729-12) MeV

? Latest experimental observation of s by BES
Phys.Lett.B598 (2004) 149 J/y ? wpp- ? 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 of kBES
Phys.Lett.B633 (2006) 681 J/y ? KKp- ? Mk
841 3081-73 i(309 4548-72) MeV
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(4) Scalars-IV another hint for 4q f ?
f0(980)g, a0(980)g
Mass degeneracy very small coupling with
f large coupling with r and w (OZI rule
argument) Expected mass difference different
couplings of f0 and a0 to f r and w.
If are qq states
If are 4q states
Mass degeneracy large coupling to f
Look at f0 and a0 affinity to the f content
of quark s in the wavefunction f radiative
decays (CMD-2, SND, KLOE)
p0p0g
pp-g
KLOE observation of f0(980) pp-g ? fit of mass
spectrum p0p0g ? Dalitz plot analysis
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(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 from s(500)) BR(f ? f0(980)g ?
pp-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 ? pp-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
0605049(2006)) gfMg (M any meson)
Meson gfMg (GeV-1)
p0 0.12
h 0.66
h 0.70
f0 1.2 ? 2.0
a0 gt 1.0 (prel.)
(c) Coupling to meson pairs gfKK gtgt gfpp
gaKK gahp A Sizeable coupling to KK is found
for both
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(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?wKK-
J/y?wpp-
J/y?fpp-
J/y?fKK-
Message s(500) has a u-d quark structure,
f0(980) has large s content
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(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 ? pp- for Wgggt700 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
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(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)5
88 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 ee- ? fh and ee- ?
ff0(980) vs. vs ? quark counting S.Pacetti,
talk given at QNP06 Madrid ? 4 elementary
fields for f0 ? need of data at higher vs
(2) Heavy ions elliptic-flow counts the
valence quarks see M.Lisa talk here
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(5) 1 2 GeV region-I the second scalar
multi-plet

• again hint of an inverted spectrum ? 4-quark
  structure
• 3 I0 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.

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

• BES J/y radiative decays
• Threshold effect on pp
• Peak in pp-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 ee- ? hadrons through ISR confirms a
vector state around 2Mp
BABAR coll., Phys.Rev.D73052003 (2006)
BABAR-3
BABAR-1
Experim. process M(MeV) G(MeV)
DM2 6p 1930 35
FENICE Mh 1870 10
E687 3p3p- 1910 10 33 13
BABAR-1 3p3p- 1880 50 130 30
BABAR-2 2p2p-2p0 1860 20 160 20
BABAR-3 2p2p- 1880 10 180 20
BABAR-4 pp-2p0 1890 20 190 20
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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.