1. Current Status of Pentaquark States
OUTLINE
Why are pentaquarks interesting?
Status of observations
Outlook for new experiments
Summary
X−− X+

2. Properties of quarks u p d u n d s K− u s u Protons are made of (uud)
Quark Flavor
Charge (Q)
Baryon number
Strangeness (S) u
+2/3
+1/3 d
−1/3 s −1
− 2/3 +1 u d
+2/3
−1/3 p d u
+2/3
−1/3 n s
−1/3 u
−2/3 K− s u
+1/3
+2/3
Protons are made of (uud)
Neutrons are made of (ddu) K+

3. Interactions understood in terms of quarks
Very high energy
“free” quarks not found, only
particles that
contain quarks

4. Asymptotic Freedom
The discovery which is awarded this year's Nobel Prize is of decisive importance for our understanding of how the theory of one of Nature's fundamental forces works, the force that ties together the smallest pieces of matter – the quarks.

5. Strong coupling constant as vs energy
Confinement
Asymptotic Freedom

6. Quarks are confined inside colorless hadrons
Quarks combine to “neutralize” color force q q q
All hadrons could be categorized
as qq mesons and qqq baryons...
…at least until recently.

7. Families of quarks
S=+1
S= 0
I3 = Q ─ ½ (B+S) −½ +½
S=−1

8. Baryons built from meson-baryon, or qqqqq
Hadron multiplets K p
Mesons qq
Baryons qqq N S X W─
Baryons built from meson-baryon, or qqqqq Q+ X+
X−−

9. 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

10. The Anti-decuplet predicted by Diakonov et al.
In the Chiral Soliton Model, nucleons and Deltas are
rotational states of the same soliton field.
Z.Phys. A359, 305 (1997)
Rotational excitations include
G~15 MeV
The mass splittings are
predicted to be equally
spaced
Anchor
JP=½+
G~140 MeV

11. First observation of Q+ at SPring-8
Phys.Rev.Lett. 91 (2003)
= 1.540.01 MeV
< 25 MeV
Gaussian significance 4.6s
background

12. Experimental evidence for Q+
Many experiments
Dedicated experiments starting to take data
Selected examples only
No attempt at completeness

13. Quark lines for production of Q+g K− us us K+ Q+ n
ddu
ddu n
Q+ is composed of (uudds) quarks

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

15. 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

16. gd → p K+K─ (n) in CLAS K+ p K- Sectors 3 & 6 Sectors 1 & 4

17. 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
Further analysis of the deuterium
data find that the significance of
the observed peak may not be as
large as indicated.

18. Evidence for pentaquark states
Spring8
DIANA
JLab-d
ELSA
ITEP
SVD/IHEP
JLab-p
HERMES
ZEUS
pp  S+Q+.
COSY-TOF
CERN/NA49 H1

19. But…

20. 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

21. BaBar null results other channels searched Q+? M(pK0s) (GeV)
BABAR hep-ex/
other
channels
searched
Q+?
M(pK0s) (GeV)
M(pK0s) (GeV)

22. 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)

23. 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+

24. Second Generation Dedicated Experiments

25. LEPS-2/Spring8: deuterium target
Dedicated experiment
Aimed for 4x statistics of 2003 result
Preliminary results announced at N*2004
MX(K−) (GeV)

26. Current activities at Jlab
Pentaquark experiments in Hall B
g10 (completed) gd → Q+ Eg~1 – 3.5 GeV
g11 (completed) gp → Q+ Eg~1 – 3.5 GeV
eg3 (December) 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 (completed), search for excited Q++ and Q0 states.

27. Reaction gd→pK+K−(n) G2 G10
Published final state. Fully exclusive reaction. G2
L(1520)
L(1520)
G10

28. g11 – 6% of data MX(p+p−) (GeV) M2X(p+p) (GeV) M(K+K−) (GeV)L f
MX(p+p−) (GeV)
M2X(p+p) (GeV)
M(K+K−) (GeV)
M(p−p) (GeV)

29. eg3 ─ Search for exotic X5−−
In 20 days expect
46 events nb-1 <s>
s ~ 0.4 –1.5 nb
s µ GX K+ g n S−
X5−− p− X−
W. Liu and C.M. Ko, nucl-th/

30. BEACH2004 Conference Summary (Joel Butler)
The existence of pentaquaks is an experimental issue
Treat each state as a separate entity. Don’t use the non-existence or confusion about higher mass states to cast doubt on Q+ and vice versa
Proponents of signals should listen to all criticisms and address them
Reflections
Justification of cuts, smooth evolution of signal significance with cut values. Be able to demonstrate the stability of signals with respect to cuts
Elimination of ghost tracks, Ks-L ambiguities
Treatment of backgrounds in fits
More data will become available and the truth will eventually emerge Be patient!

31. Summary
Pentaquarks have revived the interest in hadronic physics and spectroscopy.
Jlab is playing and will continue to play a central role in the discovery of the pentaquark or
in the rise and fall of the pentaquark.