1. Experimental Status of Pentaquark States
Introduction
Experimental evidence
Production mechanisms
What do we know about the Q+?
Exotic cascades states
Phys.Rev.Lett. 91 (2003)
SPring-8
Special Thanks
CLAS Collaborators
uudds
Mass = 1.54 GeV

2. Quarks are confined inside colorless hadrons
Quarks combine to “neutralize” color force q q q
Mystery remains:
Of the many possibilities for combining quarks with color into colorless hadrons, only two configurations were found, till now…

3. What are pentaquarks? Example: uudss, non-exotic
Minimum quark content is 4 quarks and 1 antiquark
“Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks
Quantum numbers cannot be defined by 3 quarks alone.
Example: uudss, non-exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = − = 0
Example: uudds, exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = = +1

4. Pentaquarks – two approaches
Quark cluster models, e.g.
di-quark description (Jaffe, Wilczek)
Chiral soliton model: (Diakonov,
Petrov, Polyakov)
Pentaquark comes out
naturally from these models
as they represent rotational
excitations of the soliton
[rigid core (q3) surrounded
by meson fields (qq)]
L=1
(ud) s
Soliton:
(simplified)
Meson
fields
L=1, one unit of orbital angular momentum needed to get J=1/2+ as in cSM
Lattice QCD => JP = 1/2-

5. The Anti-decuplet of SU(3)f
Ten observations
Null Results?
X5−−
X50
One experiment
(revised version)
D. Diakonov, V. Petrov, hep-ph/

6. Experimental Evidence
Many experiments
No dedicated experiments to date
but,… dedicated experiments are starting to take data
For new data from SPring-8 see Hicks Session J2 Sun 10:45
Walk through the analysis from CLAS
Selected examples from other experiments

7. Quark lines for the reactiong K− us us K+ Q+ n
ddu
ddu n
Q+ is composed of (uudds) quarks

8. Production mechanismsK+ K− g
pspec Q+ p
(p)
(K0)
Control Reactions n K+ p− g S− K−
pspec p
L*(1520)
nspec

9. JLab accelerator CEBAF
Continuous Electron Beam
Energy 0.8 ─ 5.7 GeV
200 mA, polarization 75%
1499 MHz operation
Simultaneous delivery 3 halls

10. CEBAF Large Acceptance Spectrometer
Torus magnet
6 superconducting coils
Electromagnetic calorimeters
Lead/scintillator, 1296 photomultipliers
Liquid D2 (H2)target +
g start counter; e minitorus
Drift chambers
argon/CO2 gas, 35,000 cells
Gas Cherenkov counters
e/p separation, 256 PMTs
Time-of-flight counters
plastic scintillators, 684 photomultipliers

11. gd → p K+K─ (n) in CLAS K- K+ p

12. Particle identification by time-of-flight

13. Reaction gd→pK+K-(n) Reconstructed Neutrons
Clear peak at neutron mass.
15% non-pKK events within ±3s of the peak.
Almost no background under the neutron peak after event selection with tight timing cut.
Reconstructed Neutrons

14. Deuterium: nK+ invariant mass distribution
Mass = GeV
< 21 MeV
Significance 5.2±0.6 s Q+
NQ = 43 events
Two different
Background shapes
Distribution of
L*(1520) events

15. Searching for Q+ on a proton target
gp→p+K- K+ (n)
Prominent K*0
K*0
M(nK+) [GeV]
no cuts

16. Searching for the Q+ on a proton target
Eg = 3 – 5.5 GeV
gp→p+K-K+(n) n K+ K− p+ g p Q+ N* p−
Cosq*(p+) > 0.8
cut
7.8s Q+
M=1555±10 MeV
< 26 MeV
Cosq*(p+) > 0.8
Cosq*(K+) < 0.6
CLAS Collaboration
PRL 92, (2004).
M(nK+) [GeV]

17. Q+ ─ N* production mechanism?K+ K− p+ g p Q+ N* p−
outside cuts
cuts outside Q+
N* ?
What do p-p scattering
data say?
p-p cross section data in PDG
have a gap in the mass range
2.3–2.43 GeV.
M(nK+K−) [GeV]

18. Diffractive mechanism?K- g
K*0 p+ p Q+
Cos q*(K−p+) > 0.5
Require forward K0*

19. Q+ NOT produced in association with K*0
K*0 events
Non K*0 events

20. HERMES Mass (K0p) [GeV] e+d→(pKs0) X 27.6 GeV positrons M=1528±3.3 MeV
Airapetian et al. Hep-ph/
27.6 GeV positrons
M=1528±3.3 MeV
G ~ 17±9 MeV
Mass (K0p) [GeV]

21. New results from COSY-TOF
COSY-TOF hep-ex/
pp → S+ K0 p
M=1530±5 MeV
< 18 MeV
s ~ 0.4±0.1 mb
2.95 GeV
M(K0p)
GeV/c2
The TOF spectrometer at the COSY facility in Juelich, Germany found evidence for the Q+ in the reaction: p + p → S+ + Q+.

22. From ZEUS at DESY… ep →eKs0p X Mass (K0p) [GeV] M=1521.5±1.5 MeV
ZEUS hep-ex/
ep →eKs0p X
M=1521.5±1.5 MeV
~ 8±4 MeV
√s ~ 310 GeV
Q2 > 20 GeV2
Mass (K0p) [GeV]

23. What do we know about this S=+1 state?
LEPS : gC→(nK+) K−X
DIANA : K+Xe→(pK0) X
CLAS-d : gd→(nK+) K−p
CLAS-p : gp→(nK+) p+K−
SAPHIR : gp→(nK+) K0
Mass (GeV)
ITEP : n d,Ne→(pK0) K0
HERMES : e+d→(pK0) X
COSY-TOF: pp→(pK0) S+
ZEUS : ep→e (pK0) X
SVD-2 : pA→(pK0) X
Upper limit or estimate
of G (GeV)
( ) Strangeness undetermined
Decay products in parenthesis

24. Search for pentaquarks in HERA-B
HERA-B hep-ex/
L*(1520)
Q+ ?
M(pK−) (GeV)
M(pK0s) (GeV)
Null result at HERA-B
pA →K0p X
Q+(1540)
L*(1520)
< 0.02
√s ~ 41.6 GeV

25. There is much more to learn
Spin, parity
Chiral soliton model predicts Jp=½+ (p-wave)
Quark model naïve expectation is Jp=½− (s-wave)
Lattice calculations predict Jp=½−
Isospin
Likely I=0, since searches for pK+ partners unsuccessful
Width (lifetime)
Measurements mostly limited by experimental resolution.
Theoretical problem remains why the state is so narrow.
Analysis of existing K+d scattering data indicate that G < 1-2 MeV.
Complete determination of the pentaquark multiplet

26. A new cousin: observation of exotic X5−−
M=1.862± GeV
X50
ssddu Q+
X5−−
X50
X(1530)
NA49 CERN SPS
Phys. Rev. Lett. 92 (2004)

27. But, can it be reproduced??
HERA-B hep-ex/
X(1530)
X−p+ + X+p−
X−p− + X+p+
Mass (GeV/c2)
HERA-B collaboration at DESY (Germany)
Null result with much higher statistics!
They also have a null result for the Q+

28. Predictions depend on dynamicsQ
L S
X X5 Ss S X5
Di-quark
Soliton Ns
Number of states and
mass spectra
Decay modes are sensitive
to dynamical picture
BR(X5 → K− S−)
BR(X5 → p− X−)
BR(X5 → p0 X−)
BR(X5 → p− X0)
Note: Models adjusted to experiment

29. Search for exotic cascades X5−− and X5−
Experiment 04-10, scheduled to run this fall
X −− → p− X−
X − → p− L
L → p− p
Electron
beam
gn→K+K+X5−−
X− ct = 4.9 cm
ct = 7.9 cm
Two decay vertices,
Negatives bend outwards
<gb> ~ 1.5

30. Current activities at Jlab
Pentaquark experiments in Hall B
g10 (currently taking data) gd → Q+ Eg~1 – 3.5 GeV
g11 (starts in mid-May) gp → Q+ Eg~1 – 3.5 GeV
eg3 (November) gvd→ X5−−, X5− Eg> 3.9 GeV
High energy data gp → Q+, X5 Eg~1.5 – 5.4 GeV
Pentaquark experiment in Hall A
E (May), search for excited Q++ and Q0 states.

31. Summary
A key question in non-perturbative QCD is the structure of hadrons.
We have reviewed the evidence for the existence of a new class of colorless hadrons with quantum numbers which cannot be generated from solely three quarks:
There is substantial corroborating evidence for an exotic baryon with
S = +1, which would have a minimal quark content of (uudds).
The observation of a doubly negative S=−2 baryon (ddssu) is consistent with a second corner of the anti-decuplet the family of pentaquarks, but needs additional confirmation.
Dedicated experiments are being mounted which should easily establish (or refute) the observations to date.