1. Forefront Issues in Meson Spectroscopy
Curtis A. Meyer
Carnegie Mellon University
July 24, 2006
July 24, 2006
National Nuclear Physics Summer School

2. National Nuclear Physics Summer School
Outline of Talk
Introduction
Meson Spectroscopy
Glueballs
Expectations
Experimental Data
Interpretation
Hybrid Mesons
Summary and Future
July 24, 2006
National Nuclear Physics Summer School

3. National Nuclear Physics Summer School
QCD
is the theory of quarks and gluons
white
3 Colors
3 Anti-colors
8 Gluons, each of which
has a color an an anti-color
Charge. u d s c b t
Six Flavors of quarks
July 24, 2006
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4. National Nuclear Physics Summer School
Jets at High Energy
Direct evidence for gluons come from high energy
jets. But this doesn’t tell us anything about the
“static” properties of glue. We learn something about s
2-Jet
3-Jet
gluon bremsstrahlung
July 24, 2006
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5. Deep Inelastic Scattering
As the nucleon is probed to smaller and smaller x,
the gluons become more and more important. Much
of the nucleon momentum and most of its spin is
carried by gluons!
Glue is important to
hadronic structure.
July 24, 2006
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6. National Nuclear Physics Summer School
Strong QCD
See and systems.
Color singlet objects observed in nature:
white
Nominally, glue is
not needed to
describe hadrons.
Focus on “light-quark mesons”
Allowed systems: , ,
July 24, 2006
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7. National Nuclear Physics Summer School
Normal Mesons
Non-quark-antiquark …
quark-antiquark pairs
orbital
r3,w3,f3,K3
r2,w2,f2,K2
r1,w1,f1,K1
p2,h2,h’2,K2
L=2
3--
2--
1--
2-+ u d s u d s
radial
a2,f2,f’2,K2
a1,f1,f’1,K1
a0,f0,f’0,K0
b1,h1,h’1,K1
L=1
2++
1++
0++
1+-
J=L+S
P=(-1) L+1
C=(-1) L+S
G=C (-1) I
(2S+1) L J
1S0 = 0 -+
3S1 = 1--
r,w,f,K*
p,h,h’,K
L=0
1--
0-+
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8. National Nuclear Physics Summer School
Nonet Mixing
The I=0 members of a nonet can mix:
SU(3)
physical
states
Ideal Mixing:
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9. National Nuclear Physics Summer School
Spectrum
1.0
1.5
2.0
2.5
qq Mesons
L = 0 1 2 3 4
Each box corresponds
to 4 nonets (2 for L=0)
exotic
nonets
0 – +
0 + –
1 + +
1 + –
1– +
1 – –
2 – +
2 + –
2 + +
0 + +
Glueballs
Hybrids
Radial
excitations
Lattice
GeV
GeV
(L = qq angular momentum)
July 24, 2006
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10. Glueball Mass Spectrum
QCD is a theory of quarks and gluons
What role do gluons play in the meson
spectrum?
Lattice calculations predict a spectrum
of glueballs. The lightest 3 have JPC
Quantum numbers of 0++ , 2++ and
0-+.
The lightest is about 1.6 GeV/c2
f0(1710)
f0(1500)
a0(1450)
K*0(1430)
f0(1370)
a0(980)
Morningstar et al.
f0(980)
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11. National Nuclear Physics Summer School
Glue-rich channels
Where should you look experimentally for Glueballs? G M
Radiative J/ Decays
0-+ (1440)
0++ f0(1710)
Large signals
Proton-Antiproton Annihilation
Central Production
(double-pomeron exchange)
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12. National Nuclear Physics Summer School
Decays of Glueballs?
Glueballs should decay in a flavor-blind fashion.
’=0 is true for any SU(3) singlet and for any
pseudoscalar mixing angle. Only an SU(3)
“8” can couple to ’.
Flavor-blind decays have always been cited as glueball signals. Not necessarily true – coupling proportional to
daughter mass can distort this.
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13. Crystal Barrel Results
Crystal Barrel Results: antiproton-proton annihilation at rest
Study decays of X
Discovery of the f0(1500)
f0(1500) a pp, hh, hh’, KK, 4p
Solidified the f0(1370)
f0(1370) a 4p
Crystal Barrel Results
Discovery of the a0(1450)
Establishes the scalar nonet
250,000 hhp0 Events
700,000 p0p0p0 Events
f2(1565)+s
f0(1500)
f2(1270)
f0(980)
f0(1500)
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14. National Nuclear Physics Summer School
The f0(1500)
Is it possible to describe
the f0(1500) as a member
of a meson nonet?
Use SU(3) and OZI
suppression to compute
relative decays to pairs
of pseudoscalar mesons
Get an angle of about 143o
90% light-quark
10% strange-quark
Both the f0(1370) and f0(1500) are
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15. National Nuclear Physics Summer School
WA102 Results
CERN experiment colliding p
on a hydrogen target.
Central Production Experiment
Recent comprehensive data set
and a coupled channel analysis.
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16. National Nuclear Physics Summer School
BES Results
Clear Production of f0(1500) and f0(1710), no report
of the f0(1370). f0(1710) has strongest production.
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17. National Nuclear Physics Summer School
Model for Mixing
meson 1
meson
Glueball r2
Glueball
meson r3
flavor blind? r
Solve for mixing scheme
F.Close: hep-ph/
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18. National Nuclear Physics Summer School
Meson Glueball Mixing
Physical Masses
f0(1370),f0(1500),f0(1710)
Bare Masses:
m1,m2,mG
(G) (S) (N)
f0(1370)   0.07
f0(1500)  0.04 –0.700.07
f0(1710)   0.02
~(|1>-|G>)
~(|8>-|G>)
~(|1>+|G>)
m1=137720 m2=167410 mG=144324
Lattice of about 1600
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19. Glueball Expectations
Antiproton-proton: Couples to
Observe: f0(1370),f0(1500)
Central Production: Couples to G and in phase.
Observe: f0(1370),f0(1500), weaker f0(1710).
Radiative J/: Couples to G, |1>, suppressed |8>
Observe strong f0(1710) from constructive |1>+G
Observe f0(1500) from G
Observe weak f0(1370) from destructive |1>+G
Two photon: Couples to the quark content of states,
not to the glueball. Not clear to me that
has been seen.
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20. National Nuclear Physics Summer School
Higher mass glueballs?
Lattice predicts that the 2++ and the 0-+ are the
next two, with masses just above 2GeV/c2.
Radial Excitations of the 2++ ground state
L= States + Radial excitations
f2(1950), f2(2010), f2(2300), f2(2340)…
2’nd Radial Excitations of the  and ’,
perhaps a bit cleaner environment! (I would
Not count on it though….)
I expect this to be very challenging. Evidence from
BES for an (1760)! .
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21. National Nuclear Physics Summer School
Lattice QCD
Flux Tubes Realized
From G. Bali
Color Field: Because of self interaction, confining flux tubes form between static color charges
Confinement arises from flux tubes and their excitation leads to a new spectrum of mesons
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22. National Nuclear Physics Summer School
Flux Tubes
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23. National Nuclear Physics Summer School
Hybrid Mesons
built on quark-model mesons
excited flux-tube
m=1
ground-state
flux-tube
m=0
1-+ or 1+-
normal mesons
S=0,L=0,m=1
J=1 CP=+
JPC=1++,1--
(not exotic)
S=1,L=0,m=1
J=1 CP=-
CP={(-1)L+S}{(-1)L+1}
={(-1)S+1}
JPC=0-+,0+-
1-+,1+-
2-+,2+-
exotic
m=0 CP=(-1) S+1
m=1 CP=(-1) S
Flux-tube Model
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24. National Nuclear Physics Summer School
QCD Potential
linear potential
ground-state
flux-tube
m=0
excited flux-tube
m=1
Gluonic Excitations provide an
experimental measurement of
the excited QCD potential.
Observations of exotic quantum number nonets are the
best experimental signal of gluonic excitations.
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25. National Nuclear Physics Summer School
Hybrid Predictions
Flux-tube model: 8 degenerate nonets
1++, ,0+-,1-+,1+-,2-+,2+- ~1.9 GeV/c2
S=0
S=1
Lattice calculations nonet is the lightest
UKQCD (97) 0.20
MILC (97) 0.30
MILC (99) 0.10
Lacock(99) 0.20
Mei(02) 0.10
Bernard(04) §0.139
§ 0.2
§ 0.11
§ 0.6
In the charmonium sector:
0.08
0.11
Splitting = 0.20
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26. National Nuclear Physics Summer School
Decays of Hybrids
Decay calculations are model dependent, but the 3P0
model does a good job of describing normal meson
decays.
0++ quantum numbers (3P0)
The angular momentum in the flux tube stays in one of
the daughter mesons (L=1) and (L=0) meson.
L=0: ,,,,…
L=1: a,b,h,f,…
,, … not preferred.
1b1,f1,,a ,21,11,9 MeV (partial widths)
first lattice prediction ~400MeV
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27. National Nuclear Physics Summer School
E852 Results
p-p -> hp- p
(18 GeV)
Mass = MeV/c2
Width= MeV/c2
p1(1400)
The a2(1320) is the dominant
signal. There is a small (few %)
exotic wave. a2 p1
Interference effects show
a resonant structure in 1-+ .
(Assumption of flat background
phase as shown as 3.)
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28. National Nuclear Physics Summer School
CBAR Exotic
Crystal Barrel Results: antiproton-neutron annihilation
Mass = MeV/c2
Width= MeV/c2
Same strength as the a2.
p1(1400)
Produced from states with one unit
of angular momentum.
Without p1 c2/ndf = 3, with = 1.29
hp0p-
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29. National Nuclear Physics Summer School
Controversy
In analysis of E852 ± data, so evidence of the 1(1400)
In CBAR data, the 0 channel is not conclusive.
Analysis by Szczepaniak shows that the exotic wave is
not resonant – a rescattering effect.
The signal is far too light to be a hybrid by any model.
This is not a hybrid and may well not be a state.
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30. National Nuclear Physics Summer School
E852 Results
At 18 GeV/c
to partial wave analysis
suggests
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31. understood acceptance
An Exotic Signal
Correlation of
Phase
&
Intensity
Exotic
Signal
p1(1600)
Leakage
From
Non-exotic Wave
due to imperfectly
understood acceptance
3p m= G=
ph’ m= G=
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32. National Nuclear Physics Summer School
In Other Channels
E852 Results
1-+ in ’
p-p  ’-p at 18 GeV/c
The 1(1600) is the
Dominant signal in ’.
Mass = 1.597 GeV
Width = 0.3400.040 GeV
1(1600)  ’
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33. National Nuclear Physics Summer School
In Other Channels
E852 Results
1-+ in f1 and b1
p-p  +--p
p-p  w0-p
1(1600)  b1
1(1600)  f1
Mass=1.709 GeV
Width=0.4030.08 GeV
In both b1 and f1, observe
Excess intensity at about
2GeV/c2.
Mass ~ 2.00 GeV,
Width ~ 0.2 to 0.3 GeV
Mass = 1.687 GeV
Width = 0.2060.03 GeV
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34. National Nuclear Physics Summer School
1(1600) Consistency
3 m= =168
0 m= =340
f1 m= =403
b1 m= =206
Not Outrageous, but not great agreement.
Mass is slightly low, but not crazy.
Szczepaniak: Explains much of the 0 signal
as a background rescattering similar to the .
Still room for a narrower exotic state.
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35. National Nuclear Physics Summer School
New Analysis
Dzierba et. al. PRD 73 (2006)
Add 2(1670)! (L=3)
Add 2(1670)!3
Add 2(1670)! ()S
Add a3 decays
Add a4(2040)
10 times statistics in
each of two channels.
Get a better description
of the data via moments
comparison
No Evidence for the 1(1670)
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36. National Nuclear Physics Summer School
p1 IG(JPC)=1-(1-+)
h’1 IG(JPC)=0+(1-+)
h1 IG(JPC)=0+(1-+)
K1 IG(JPC)= ½ (1-)
Exotic Signals
1(1400) Width ~ 0.3 GeV, Decays: only 
weak signal in p production (scattering??)
strong signal in antiproton-deuterium.
NOT A
HYBRID
Does
this
exist?
1(1600) Width ~ 0.16 GeV, Decays ,’,(b1)
Only seen in p production, (E852 + VES)
1(2000) Weak evidence in preferred hybrid
modes f1 and b1
The right
place. Needs
confirmation.
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37. National Nuclear Physics Summer School
Exotics and QCD
In order to establish the existence of gluonic excitations,
We need to establish the existence and nonet nature of
the 1-+ state.
We need to establish at other exotic QN nonets – the
0+- and 2+-.
In the scalar glueball sector,
the decay patterns have
provided the most sensitive
information. I expect the
same will be true in the
hybrid sector as well.
DECAY PATTERS ARE CRUCIAL
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38. Photoproduction
More likely to find exotic hybrid mesons using beams of photons
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39. National Nuclear Physics Summer School
The GlueX Experiment
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40. National Nuclear Physics Summer School
Exotics
in Photoproduction
1-+ nonet
p1 IG(JPC)=1-(1-+)
h’1 IG(JPC)=0+(1-+)
h1 IG(JPC)=0+(1-+)
K1 IG(JPC)= ½ (1-) N g e X
g  ,,
Need to establish nonet nature
of exotics:   0
Need to establish more than one
nonet:
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41. National Nuclear Physics Summer School
0+- and 2+- Exotics N g e X
In photoproduction, couple to r, w or f?
b0 IG(JPC)=1+(0+-)
h0 IG(JPC)=0-(0+-)
h’0 IG(JPC)=0-(0+-)
K0 I(JP)=½(0+)
a1,f0,f1
wf0,wf1,ra1
ff0,ff1,ra1
wp, a1,f0,f1
b2 IG(JPC)=1+(2+-)
h2 IG(JPC)=0-(2+-)
h’2 IG(JPC)=0-(2+-)
K2 I(JP)= ½(2+)
“Similar to p1 ”
wh,rp,wf0,wf1,ra1
fh,rp,ff0,ff1,ra1
Kaons do not have exotic QN’s
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42. National Nuclear Physics Summer School
Summary
The first round of J/ experiments opened the door
to exotic spectroscopy, but the results were confused.
LEAR at CERN opened the door to precision, high-statistics
spectroscopy experiments and significantly improved both
our understanding of the scalar mesons and the scalar
glueball.
Pion production experiments at BNL (E852) and VES
opened the door to states with non-quark-anti-quark
quantum numbers. Recent analysis adds to controversy.
CERN central production (WA102) provided solid new data
on the scalar sector, and a deeper insight into the scalar
glueball.
BES is collecting new J/ data. CLEO-c can hopefully
add with the 0 program.
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43. National Nuclear Physics Summer School
The Future
The GlueX experiment at JLab will be able to do for
hybrids what Crystal Barrel and WA102 (together)
did for glueballs. What are the properties of static
glue in light-quark hadrons and how is this connected
to confinement.
The antiproton facility at GSI (HESR) will look for hybrids
in the charmonium system – PANDA. May also be able to
shed more light on the Glueball spectrum.
July 24, 2006
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