Status of Pentaquark Search

1
Status of Pentaquark Search

• Valery Kubarovsky
• Rensselaer Polytechnic Institute / Jlab

• Introduction
• Review of theoretical models
• Current Experimental Status
• Second generation experiments
• Conclusion

2
Introduction

• Several experiments have recently reported the
  observation of the baryon with exotic quantum
  numbers
• Light M1525-1555 MeV
• Narrow Glt 9-25 MeV (possibly 1 MeV)
• Strangeness S1 (opposite to the strangeness
  of the
  usual baryons)
• This new state was identified as the Q
  pentaquark baryon with quark contents
  .
• There is also evidence for two related states
  with the strangeness S-2,

3
The Anti-decuplet in the cSM D. Diakonov, V.
Petrov, M. Polyakov, Z.Phys.A359, 305 (1997)
S1
updated version
D. Diakonov, V. Petrov, arXivhep-ph/0310212
Dm 108 MeV
S0
S-1
S-2
4
Theory Response to the Pentaquark

• Anti-charmed Qoc and anti-beauty Qb
• Q and Q produced in quark-gluon plasma
• Instantons and diquark clustering
• Triquark-diquark cluster model
• Pentaquarks and radially excited baryons
• Peanut-shaped quark-diquark model
• Pentaquarks in the color-flavor-locking
• superconducting phase
• .

• Chiral soliton P
• Kaon-Skyrmion P
• (qq)2-q P or P-
• Kaon-nucleon resonance
• Super radiance resonance
• QCD sum rules
• Lattice QCD P-, or find no signal
• Higher exotic baryon multiplets
• Pentaquarks in string dynamics
• P11(1440) as pentaquark
• P11(1710) as pentaquark
• Q as isotensor pentaquark
• Topological soliton
• Q(1540) as a heptaquark
• Exotic baryons in the large Nc limit

More than 220 papers since July 1, 2003
5
Pentaquarks models
Chiral soliton model (Diakonov, Petrov,
Polyakov)
Predicted the mass and the width of
5Q. Pentaquark comes out naturally from these
models. They represent rotational excitations of
the soliton
JP ½
6
QCD instantons (Shuryak hep-ph/0310270)
DiQuarks are building blocks of multiquark
states pentaquarks and dibaryons
A nucleon is made of a quark and deeply bound
scalar-isoscalar diquarks, absent in the
decuplet. In the instanton liquid model there
are two kind of diquarks, the scalar and the
tensor (with spin1)
7
QCD instantons

• The DiQuark mass is about the constituent mass.
• Two possible scenario
• Scalar DiQuarks are in a P-wave
• M(Q) 1880 MeV
• Scalar and tensor DiQuarks in S-wave
• M(Q) 1550 MeV
• G(Q) 2.6 MeV
• The narrowness of the partial widths follows from
  the small overlap between the three and five
  quarks wave functions due to the different
  internal structure.

8
Current Experimental Status
9
Super Photon ring-8 GeV SPring-8

• Third-generation synchrotron radiation facility
• Circumference 1436 m
• 8 GeV
• 100 mA
• 62 beamlines

10
LEPS detector
g
1m
11
Charged particle identification
s(mass) 30 MeV/c2 for 1 GeV/c Kaon
12
LEPS analysis

• Problems
• no neutron target
• CH start counter
• Fermi motion corrections
• Background
• f?KK-
• (produced from n p)

• K- missing mass gives Q mass
• KK- missing mass gives n

13
Q published mass plot
M 1.54?0.01 MeV G lt 25 MeV Gaussian
significance 4.6s

• Background level is estimated by a fit in a mass
  region above 1.59 GeV.
• Assumption
• Background is from non-resonant KK- production
  off the neutron/nucleus
• is nearly identical to non-resonant KK-
  production off the proton

background
Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020
14
DIANA/ITEP (Moscow)
Charge exchange reaction KNQpK0S K0Spp-

• 750 MeV K beam incident on 700 liters Xe bubble
  chamber.
• Interaction energy is determined by the range of
  the kaon.
• Charged particles are identified by ionization,
  momentum is measured by the range.

15
DIANA

• Selecting forward going protons and kaons - Qk
  and Qp lt 100o.
• p and K0 are emitted back-to-back - cosFpK lt 0.

• Peak at 1.539 GeV in the invariant mass of K0p.
• Statistical significance 4.4s.
• Measured width G lt 9 MeV The best limit for width

16
Jefferson Lab (CEBAF)
Continuous Electron Beam Accelerator Facility
17
CLAS - CEBAF Large Acceptance Spectrometer
18
CLAS-gd Exclusive reaction

• Exclusive photoproduction on deuterium

• Experimental data from 1999 run
• Tagged photons with up to 3 GeV energy
• Target 10 cm long liquid deuterium

Possible reaction mechanism

• No correction for Fermi smearing is needed.
• Aids significantly to reduce the background.

19
CLAS

• 15 of non pKK events within 3s of the peak.
• Almost no background under the neutron peak with
  tight timing cut, DtK.

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CLAS Cut out known resonances

• Several known processes can contribute to the
  pKK-(n) final state
• gd?fp(n), f?KK-
• gd?L(1520)K(n), L(1520)?pK-
• Both reactions proceed predominantly on the
  proton (neutron is a spectator).
• Kinematics of both reactions are not a good match
  for Q production.

21
CLAS
Q
M 1.542 GeV G lt 21 MeV (sM9 MeV) Stat.sig.
5.2s (4.6-5.8)
Gaussian background
Simulated background
Distribution of L(1520) events
22
CLAS Photoproduction on hydrogen
After PID
n
23

Possible Q production mechanisms in the reaction
W. Liu, C.M. Ko, V. Kubarovsky, nucl-th/0310087
24
Q Production via N decays
25
CLAS-gp with forward going p

• Fitted mass 1.555 GeV
• G lt 28 MeV consistent with detector resolution
• Estimated significance 7.8s

M(nK)
26
CLAS-gp Indication for a heavy N(2430)?
There are no pN scattering data in the relevant
energy range.
M(K-Q) GeV
27
CERN/NA49

• Proton-proton collisions at 158 GeV.

Similar to CLAS gp
28
What do we know about the width of Q?
Widths seen in experimental analyses are
dominated by resolution effects. More precise
information is obtained in analyses with
theoretical constraints.

• HERMES, PLB585, 213 (2004) GQ 17/-9/-3 MeV
• S. Nussinov et al., hep-ph/0307357 GQ lt 6 MeV
  (non-observation)
• R. Arndt et al., PRC68, 42201 (2003) GQ lt 1
  MeV (non-observation)
• R. Cahn and G. Trilling, PRD69, 11401(2004) GQ
  0.9 /- 0.3MeV (from DIANA results)
• A. Sibirtsev, et al., hep-ph/0405099 (2004) GQ lt
  1 MeV (Kd Kopp)
• First positive identification of Q in Kd,
  including double scattering.
• W. Gibbs, nucl-th/0405024 (2004) GQ 0.9
  /-0.2 MeV (Kd X)

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H1 - A charmed pentaquark Qo(3100)?
c
hep-ex/040305 (2004)
Q2 1 100 GeV2
30
Evidence for Penta-Quark States
31
Summary of Experimental Masses
Shift could be due to different background shapes
and interference effects.
32
Null 5Q Results
HERA-B hep-ex/0403020 pA 920 GeV No Q , X3/2
BES hep-ex/0402012 ee ?J/Y? QQ- BR lt 10-5
PHENIX Nucl-ex/0404001 AuAu -gtnK-X No Q signal
CDF, LEP,E690 FOCUS, BaBar, HyperCP Conference presentations (still unpublished) No Q , Q0c X3/2
-
-
33
Dead lock ?

• The direct comparison between positive and
  negative results is possible but difficult due to
  the lack of theoretical models and experimental
  data.
• Only new high statistics experiments can cut the
  knot.
• And these experiments are coming.

34

• Second Generation Dedicated Experiments

35
LEPS-2/Spring8 deuterium target
Must correct for Fermi motion of target nucleon
bound state in the nucleus
Standard baryon
Exotic
S-1
S1
36
LEPS Default analysis
Preliminary!
Nucleon mass
f meson
M(KK-)
MM(KK-)
Q mass f,N cuts
L(1520) f,N cuts
MM(K-)
MM(K-)
37
LEPS-2/Spring8 deuterium target
Preliminary

• Dedicated experiment
• Aimed for 4x statistics
• of 2003 result
• Announced at N2004

38
Conclusion of LEPS experimental group

• LEPS high statistics experiment has reconfirmed
  the peak, very unlikely to be due to statistical
  fluctuations.
• The preliminary study shows no indication that
  the peak is generated by kinematical reflections,
  detector acceptance, Fermi-motion correction, nor
  cuts.
• existence ranges from very likely to certain,
  but further confirmation is desirable -
  three-star definition by PDG.

39
Pentaquark Searches at JLab
Major program approved by PAC25 (Jan 2004) to
search for pentaquark states
40
G10 performance (deuterium target)

• Exceptional performance of the machine and the
  CLAS detector.
• At each torus current setting accumulated
  electron beam charge is 4 times bigger than G2a
  (target is 2.4 times longer).

41
CLAS - G10 online plots
gd K-pKn
Fully exclusive processes
L(1820)
Ks
gd K-pKs(pp-)psp
42
CLAS - G11
Spectroscopy of Exotic Baryons with CLAS Search
for Ground and Excited States RUNNING NOW !
Ks
43
New Start Conter trigger PID

• Specifications
• Extended target cell coverage (40 cm)
• 2mm thick scintillator light guide
• Higher azimuthal segmentation (4 x 6)
• Time resolution lt 1ns (200 ps in rec)

44
G11 status
e1c

45
CLAS - G11 online plots
p
K0
S-
S0
S0
L(1520)
L
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G10, G11 Analysis efforts

• Common rules for all analyses
• Fully calibrated detector system
• Use only selected golden runs
• (fully functioning equipment)
• Analysis in well defined CLAS fiducial volume
• Energy and momentum corrections
• that use independent well-studied channels
• Analysis without and with kinematical constraints
  
• whenever possible
• Independent analyses by several groups.

47
Jlab Hall A - Search for Q and S5
Run is completed
M1540 - 1620 MeV
s1.3 MeV
Online-analysis
M1560 - 1860 MeV
GPDG 15.6 /- 1.0 MeV MPDG 1519.5 /-1.0 MeV
MM(eK)
48
Hall A Online missing mass distributions
s1.3 MeV
49
CLAS - Search for X5 Pentaquark States

• The search for narrow X5 states can be approached
  
• in at least two ways
• Missing mass technique
• Reconstruction of all decay particles
• We will explore both possibilities

50
CLAS missing mass technique
Data from run at Eg lt 3.8 GeV

• Good missing mass
• resolution and low
• background.
• Needs Eg gt 5 GeV,
• new start counter

51
CLAS X5 Searches (approved experiment)
Electron beam
gn?KKX--

• X0 ct 8.7 cm
• X- ct 4.9 cm
• ct 7.9 cm
• Ks ct 2.7 cm

Measure 3p- and one proton
gb 1.5
52
Summary

• Many labs are involved now in the search for
  pentaquarks,
• using a wide variety of beams, targets and final
  states.
• We have positive and negative results.
• There is also, in parallel much theoretical
  activity.
• The final conclusion is not straightforward. Do
  pentaquarks exist? This is the main question at
  this time. And this question can and must be
  resolved experimentally.
• .Jefferson Lab is in the unique position to
  settle this problem.
• The experimental program which we have begun is
  expected to
• increase the integrated luminosity by more than
  an order of magnitude over the previously
  reported ones.
• We hope to present first results of new high
  statistics dedicated experiments by the end of
  this year.

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Q(1540) as seen with e.m. probes
54
Q(1540) as seen with e.m. probes
55
Q(1540) as seen with e.m. probes
56
Q in non-electromagnetic interactions
57
Q(1540) in hadron-nucleus processes
58
Q(1540) in Kd Scattering
59
NA49 Final results
pp at 16 GeV c.m. energy
Signal for Exotic S -2, Q-2, and Non-exotic
S-2, Q0 at M1.862 0.002 GeV
X--
X0
X(1530)
60
The Q(1540) as a kinematical reflection ?
Can this peak be a kinematical reflection Of high
mass meson decays M KK-? (A. Dzierba et
al., hep-ph/0311125)
Kinematic reflections do not generate narrow nK-
peak
61
Determination of the Q parity in pp-collision

• Consider the process just above threshold (only
  S-wave is allowed).
• The parity of the final state is determined by Q
  parity in this case.
• 0 or 1 (positive Q parity)
• 0- or 1- (negative Q parity)
• L1 P-1
• L0 P1
• If the Q production occurs for spin singlet
  state the Q must have positive parity
• If the Q production occurs for spin triplet
  state the Q must have negative parity