GlueX PAC @ JLAB
23 Aug 2010
Ted Barnes
Physics Div. ORNL
Dept. of Physics and
Astronomy, U.Tenn.
GlueX
+
Exotic Hadron Spectroscopy
1. Hadrons 101
2. Exotica: glueballs, hybrids and
multiquarks/molecules
3. Hybrids: theoretical expectations and
experimental status
4. GlueX@JLAB; g prod of (hybrid) exotics
Hadrons 101
Color singlets and QCD exotica
“confinement happens”.
LGT simulation showing the QCD flux tube
Q
Q
R = 1.2 [fm]
“funnel-shaped” VQQ(R)
Coul. linear conft.
(OGE) (str. tens. = 16 T)
QCD flux tube (LGT, G.Bali et al.;
hep-ph/010032)
Physically allowed hadron states (color singlets) (naïve, valence)
_
Conventional quark model
mesons and baryons.
3
qq
q
100s of e.g.s
3 n
3
(q ) , (qq)(qq), (qq)(q ),…
nuclei / molecules
Basis state mixing may be
very important in some sectors.
6
3 n
ca. 10 e.g.s of (q ) , maybe 1-3 others
X(3872) = DD*!
“exotica” :
2
3
g , g ,…
glueballs
3
qqg, q g,…
22 22
4 4
q
q q,…
(qqq, ),(q
q),…
multiquark clusters
multiquarks
hybrids
maybe 1 e.g.
maybe 1-3 e.g.s
p1 (1600) best
f0 (1500 or 1710)
Incl. exotic J
PC
!
dangerous:fiasco”
the “multiquark
may
not exist as resonances
-N.Isgur
e.g. Q(1540)
Quarkonia:
qq
qq mesons
quantum numbers
Parity Pqq = (-)
(L+1)
C-parity Cqq = (-)
(L+S)
The resulting qq N,L states N2S+1LJ have JPC =
1S: 3S1 1- - ;
1
S0 0 - +
2S: 23S1 1- - ; 21S0 0 - + …
1P: 3P2 2+ + ; 3P1 1+ + ; 3P0 0+ + ; 1P1 1+ -
2P …
1D: 3D3 3- - ; 3D2 2- - ; 3D1 1- - ; 1D2 2- +
2D …
JPC forbidden to qq are called “JPC-exotic quantum numbers” :
0 -- ;
0 +- ; 1 -+ ; 2 +- ; 3 -+ …
x
Plausible JPC-exotic candidates =
hybrids (have all JPC), glueballs (high mass), maybe multiquarks (fall-apart decays).
Quarkonia:
cc e.g.
Fitted and predicted cc spectrum
Coulomb (OGE) + linear scalar conft. potential model
blue = expt, red = theory.
L*S OGE – L*S conft,
T OGE
S*S OGE
as = 0.5538
2
b = 0.1422 [GeV ]
mc = 1.4834 [GeV]
s = 1.0222 [GeV]
Best recent LQCD refs for cc and cc-hybrid (?) spectroscopy:
Results for cc still rather difficult
to distinguish from quark model.
(Summary of JLAB LQCD group results.) For references see:
“Charmonium excited state spectrum in lattice QCD.”
J.J.Dudek, R.G.Edwards, N.Mathur and D.G.Richards,
Phys. Rev. D77, 034501 (2008) and PRD78, 094504 (2008)
n.b. PRD79, 094504 (again) (2009) includes rad. transitions!
Final LQCD predictions fm JLAB:
Exotic cc-H
1
-+
--
Nonexotic cc-H 1
4300(50),
“cc”
4400(60).
J
PC
exotics
(non-qq)
“cc hybrids (?)”
Radiative widths of exotics = ? (The BIG question for GlueX.)
J.J.Dudek, R.G.Edwards and C.E.Thomas,
Exotic and excited-state radiative transitions in charmonium from lattice QCD
PRD79, 094504 (2009), arXiv:0902.2241 [hep-ph].
non-exotic
1-- hybrid
rad. trans.
Paper quotes G(hc1 (1-+ exotic) -> J/y g ) ~ 100 keV.
A typical “robust” cc radiative width. GlueX is justified. 
Quarkonia:
nn e.g.
Status of light meson spectroscopy (I=1 e.g.)
I=1 shown, dashed boxes = expected
Approx. status, light (u,d,s) qq spectrum
to ca. 2.1 GeV.
States are well known to ca. 1.5 GeV,
poorly known above (except for larger-J).
n.b. ss is poorly known generally.
Several recent candidates, e.g.
a1(1700), a2(1750).
Strong decays give
M, G, JPC of qq candidates.
Theor. guides for expt qq searches: Extensive strong decay tables
3
Mainly light (u,d,s) hadrons in f.-t. or P0 models.
A few references:
qq meson decays:
S.Godfrey and N.Isgur, PRD32, 189 (1985).
T.Barnes, F.E.Close, P.R.Page and E.S.Swanson, PRD55, 4157 (1997). [u,d mesons]
T.Barnes, N.Black and P.R.Page, PRD68, 054014 (2003).
[strange mesons]
[43 states, all 525 modes, all 891 amps.]
st
T.Barnes, S.Godfrey and E.S.Swanson, PRD72, 054026 (2005). [charmonia: 1 40 cc mesons,
all open-charm strong decay amps, all E1 and many M1 transitions]
F.E.Close and E.S.Swanson, PRD72, 094004 (2005). [open-charm mesons: D and Ds]
qqq baryon decays:
S.Capstick and N.Isgur, PRD34, 2809 (1986).
S.Capstick and W.Roberts, PRD49, 4570 (1994);
PPNP 45, S241-S331 (2000). [BPs, BV modes of u,d baryons]
Exotica:
G/H/M
Glueballs
Theor. masses (LQCD)
The glueball spectrum from an
anisotropic lattice study
Colin Morningstar, Mike Peardon
Phys. Rev. D60 (1999) 034509
G = new I=0 mesons starting with
an “extra” scalar at ca. 1.6 GeV.
Then no new G states until > 2 GeV.
No JPC- exotic G until > 4 GeV.
(= forget it)
PC
No J -exotics until 4 GeV !
3 GeV
2 GeV
1 GeV
Glueballs
G = 1 “extra” I=0 scalar meson
at ca. 1.6 GeV.
“f0(~1600)”
= The worst possible quantum numbers
experimentally! Several broad
overlapping states.
Then no new G states until > 2 GeV.
PC
No J - exotic G until > 4 GeV.
(= forget it)
How to make new (u,d,s,g) hadrons:
Hit things together.
A + B -> final state
You may see evidence for a new resonance in the decay products.
Reactions between hadrons
(traditional approach) are
“rich” but usually poorly
understood.
e.g.s
-
BNL p p -> mesons + baryon
LEAR (CERN) pp annih.
All light-q and g mesons,
incl. qq, glueballs, hybrids, multiquarks.
Glueball discovery? Crystal Barrel expt. (LEAR@CERN, ca. 1995)
pp -> p0 p0 p0
Evidence for a
scalar resonance,
f0(1500) -> p0 p0
n.b.
Some prefer a different scalar,
f0(1710) -> hh, KK.
PROBLEM: Neither f0 decays in a naïve glueball flavor-symmetric way to pp, KK, hh.
qq <-> G mixing?
Molecules
“Extra” hadrons just below
two-hadron thresholds.
S-waves easiest – look for
quantum numbers of an
S-wave pair.
Nuclei are examples…
MANY molecules exist!
Can’t predict molecules
w/o understanding soft
2 -> 2 hadron scattering.
Add X(3872) to the list?
Molecules
Belle Collab. K.Abe et al, hep-ex/0308029;
S.-K.Choi et al, hep-ex/0309032, PRL91 (2003) 262001.
X(3872)
B
+/-
-> K
+/-
+ -
p p J/Y
G < 2.3 MeV
M = 3872.0 +- 0.6 +- 0.5 MeV
M( Do + D*o) = 3871.5 +- 0.5 MeV
n.b. M( D+ + D*-) = 3879.5 +- 0.7 MeV
A DD* molecule???
Charm in nuclear physics???
“Fall-Apart Decay”
The trouble with multiquarks:
(actually not a decay at all: no HI)
Multiquark models found that most channels
showed short distance repulsion:
E(cluster) > M1 + M2.
Thus no bound states. (Remember θ(1540) ! )
n.b. multiquark .ne. molecule
Only 1+2 repulsive scattering (continuum)
in this sector of Hilbert space.
Exceptions:
2)
1)
VNN(R)
-2mN
bag model:
2 2 2
u d s H-dibaryon, MH - MLL = - 80 MeV.
nuclei and hypernuclei
weak int-R attraction allows
“molecules”
If E(cluster) < M1 + M2 …
“VLL(R)”
n.b.
LLhypernuclei
-2mL
R
exist, so this H was wrong.
3)
R
Heavy-light
2 2
Q q (Q = b; c?)
(Light; u,d,s) Hybrids:
Theory and Experiment
Hybrids
[flux-tube model]
New RICH band of meson excitations
expected, starting at ca. 1.9 GeV.
Flavor nonets x 8 J
+-
PC
= 72 states.
+- PC
-+
Includes 0 , 1 and 2 J -exotics.
[bag model]
ca. 1.5 GeV.
-+
Lowest exotic is 1 (TE gluon)
++0 and 2 are at higher mass (TM)
PC
Most recent LQCD results for light exotics, J
-+
=1
+-
, 2
+-
,0
.
J.J.Dudek, R.G.Edwards, M.J.Peardon, D.G.Richards and C.E.Thomas,
([JLAB] Hadron Spectrum Collaboration)
Toward the excited meson spectrum of dynamical QCD
arXiv: 1004.4930v1
n.b.
All 3 of these
PC
exotic J s were
degen. in the
flux-tube model.
2.5 GeV
In the bag model,
-+
1 is lighter.
p1(1600)
2.0 GeV
1.5 GeV
1.0 GeV
u,d
mq incr. ->
Hybrids
Hybrid Meson Decays:
flux-tube model
N.Isgur, R.Kokoski and J.Paton, PRL54, 869 (1985).
Gluonic Excitations of Mesons:
Why They Are Missing and Where to Find Them
HL=1 -> S+P
“S+P” modes
p1 -> b1p, f1p
(poorly studied exptally;
multimeson final states)
-+
1
exotic is observably narrow!
some hybrids are predicted to be
VERY broad
p2(2000)
hybrid;
b1p mode
F.E.Close and P.R.Page,
NPB443, 233 (1995).
Close and Page: some notably narrow nonexotic hybrids in the f-t model
w(2000)
hybrid
Hybrids
(Theory Summary)
Hybrid = qq“g” states (with q=u,d,s) span flavor nonets,
hence there are many experimental possibilities.
flux-tube model
PC
Models agree that the lightest hybrid multiplet contains J -exotics.
PC
f.t. model predicts 8 J x 9 flavors = 72 “extra” resonances at the hybrid threshold.
PC
3/8 J
+-
-+
+-
are exotic, 0 , 1 , 2 .
-+
+-
-+
+-
--
++
The remainder, 0 , 1 , 2 , 0 , 1 , 1
are “overpopulation” rel to the quark model.
-+
Mestm ca. 1.5 - 2.0 GeV. f.t. 1.9 GeV is famous. LGT mass similar to f.t. for 1 .
PC
-+
J = 1 with I=1, “p1”, is especially attractive. It is predicted in the f.t. decay model
to be relatively narrow and to have unusual decay modes.
Hybrid baryons
Non-exotics in a rich N* background spectrum
Spectrum of light (n=u,d) hybrid baryons.
S.Capstick and P.R.Page,
nucl-th/0207027, Phys. Rev. C66 (2002) 065204.
M (MeV)
(flux tube model)
(Light) Hybrids:
Experiment
p1(1600)
PC
The (only) strong J -exotic H candidate signal.
-
E.I.Ivanov et al.
(BNL E852)
PRL86, 3977 (2001).
-
p p -> p h ‘ p
-+
1
exotic reported in
p p -> p h ‘ p
-
ph’ is a nice channel because nn couplings
are weak for once (e.g. the a2(1320) noted here).
The reported exotic P-wave is dominant!
A.Alekseev et al. (COMPASS Collab.) Observation of a J
PC
-+
=1 exotic
resonance in diffractive dissociation of 190 GeV pi- into pi- pi- pi+
ArXiv:0910.5842v3 (Sept. 2009)
M = 1660 +-10 +0 -64 MeV , Gamma = 269 +-21+42 -64 MeV.
n.b.
resonant phase
motion (confirmed
but not shown here)
is of course the
crucial test
p1(1600)
PC
J
exotic
(confirmed)
Summary regarding meson spectroscopy and exotics:
Theorists expect new types of mesons (glueballs and hybrids) starting at ca. 1.5 - 2 GeV.
PC
A few candidates exist. Looking for J -exotics is a good strategy.
Also overpopulation - need to better establish the qq sector above 1.5 GeV and ss!
Charm mesons (cs and cc sectors) have surprised people recently –
cs low masses hence tiny widths; also perhaps new molecular states and hybrids.
Data on the spectrum is needed to compare with models and LGT.
Strong and EM widths are also useful information.
Strong decays are poorly understood in terms of QCD.
n.b. Exciting discoveries in meson spectroscopy are often serendipitous:
J/Y
Ds0*(2317)
Ds1(2460)
X(3872)
Y(4260)
Hybrids:
JLAB and elsewhere
(final comments)
Existing and planned facilities (the competition?): *
BES-III (Beijing) [now]
+ -
e e to sqrt(s) ca. 4.2 GeV.
Mainly very large J/y and y’ event samples.
Charmonium decays, in future unusual cc sector states? Y(4260)?
Application to light and exotic spectroscopy? Could do e.g. rad decays.
COMPASS (CERN) [now]
-
High energy p beam available, former E852 people (S.U.Chung) revisiting p p
but at higher E (more diffractive). Have seen evidence for the p1(1600), will
proceed to other interesting final states.
PANDA (GSI) [2017+]
Dedicated spectroscopy experiment. Official goal is cc hybrids.
pp annihilation to sqrt(s) ca. 5 GeV (higher energy LEAR).
Light spectroscopy “for free”.
n.b. Lower energy pp produced the f0(1500) glueball candidate,
and has a large p1(1400) signal.
*
Not really. Discovery of unusual hadrons requires confirmation by other experiments.
GlueX at JLAB:
PC
Photoproduction (~new expt approach) accesses exotic-J easily (S=1 beam)
“plucking the string” - Isgur.
[or is it vec dom?]
Several production mechanisms, 2 are:
• t-channel CEX, e.g. g (-> r0) + p+ - -> p1+ • diffr., e.g. g + P -> 2 +(Also s- and u-channel baryon resonances.)
n.b. You get the poorly explored ss sector for free.
Theorists can contribute by
1. LGT spectroscopy and decays,
2. modeling photoproduction of both exotic and ordinary (qq) resonances
(CLAS data?).
Recent e.g. of 3pi CEX photoproduction (CLAS),
showing a2(1320) and p2(1670) qq states.
a2
p2
The importance of GlueX at JLAB:
(Summary. C.Meyer presentation follows)
The light (u,d,s,g) meson spectrum is poorly known above ca. 1.5 GeV.
Theorists expect a rich new spectroscopy of hybrids, including exotics,
starting not far above this mass. Widths and decay mechanisms are obscure
[models], will be explored by LQCD in the near future.
GlueX’s goal is to establish the meson spectrum to ca. 2.5 GeV, and
“see what’s out there”. [serendipity in spectroscopy]
Past lessons (4pi detector, hermiticity, neutral and final modes, studies of
all decay modes, robust PWA) have been learned (LEAR, E852) and are
being implemented.
It will be very exciting!
END