1. Pentaquarks: Challenges and Pitfalls
Moskov Amarian
Old Dominion University
Norfolk, VA 23529
American Physical Society
Spring Meeting, April ,Tampa, FL

2. Outline : Physics Motivation Multiple reports with positive evidences
Single reports with unique results
Multiple reports with null results
Conclusions

3. Physics Motivation Why?: - All known baryons are 3-q states.
- All baryons belong to only octet and decuplet of SU(3) symmetry.
Are there minimal exotic 5-q states?
What are masses and widths of these states?
Do we need to modify CQM or revisit it ?
If 5-q states do not exist then Why?

4. Pentaquark Baryons The anti-decuplet of 5-quark states in the cSM.
Diakonov, Petrov, Polyakov, 1997
The anti-decuplet
of 5-quark states
in the cSM.

5. Evidence for the Q+(1540) LEPS DIANA CLAS-d SAPHIR ITEP SVD/IHEP
HERMES
CLAS-p
ZEUS
pp  S+Q+.
COSY-TOF

6. Other Pentaquark States
CERN/NA49 H1

7. Comparison of experiments
nK+
World Average: ±2.4 MeV
Is a variation in mass really large?
What about
exp. Uncertainties?

8. Typical Criticism to Evidence
It can’t be! A priori, no way… general prejudice
It is a reflection
It is due to “ghost tracks”
It is not significant
It is fake in exclusive reactions
In inclusive case it is not Q+ but S*+
It is not seen in some other experiments

9. The Signal and its Background
Peak at:
M= 1527 ± 2.3 MeV
= 9.2 ± 2 MeV
Significance:
(naïve) (realistic)
mixed event background
excited S* hyperons
(not included in Pythia6)
PYTHIA6
21-July-2004

10. Fake Peaks? remove L(1116) contribution particle miss-assignment
Spectrum of events associated with L(1116)
remove L(1116) contribution
particle miss-assignment
ghost tracks
PID “leaks”
expect peak in M(p-p)
if p+ is p and KS is L(1116)

11. Q+ Mass spectrum with additional p
standard cuts applied
+ K* and L veto
signal/background: :1
signal/background: :3

12. Q+ vs S*+ Is peak a new S*+ or a pentaquark state?
If peak is S*+ ⇒ also see a peak in M(Lp+)
if member of baryon octect: b.r.(Lp+)/(pKs)  3/2
if member of decuplet: ~ 3/2 (M. Polyakov)
S*+
No peak in Lp+ spectrum near 1530 MeV?

13. New data: LEPS deuterium
Minimal cuts: vertex, MMgKK=MN, no f, Eg < 2.35 GeV
L(1520) Q+
Preliminary
Preliminary
MMgK- (GeV)
MMgK+ (GeV)

14. LEPS: Fermi motion corrections
MMgK+ (GeV)
No large difference among the three Fermi motion correction methods
L(1520) resonance

15. Fermi motion corrections: Q+
MMgK- (GeV)
MMgK- (GeV)
No large differences among the three Fermi motion corrections.
MMgK- (GeV)

16. Published Null Experiments
Group
Reaction
Limit
Sensitivity?
BES e+e-
J/Y --> QQ*
<1.1x10-5
No?
Belle e+e-
Y(2S) --> pK0
<0.6x10-5 ??
BaBar e+e-
U(4S) -->pKs0
<1.1x10-4
ALEPH
e+e- ->Z -> pKs0
HERA-B
pA --> pKs0X
<0.02xL*
CDF
pp* --> pKs0X
<0.03xL*
HyperCP
pCu --> pKs0X
<0.3% K0p
PHENIX
AuAu -->n*K-
not given
Belle
K+Si -->pKs0X
Yes?

17. Summary of Null Results

19. Hadron production in e+e-
Slope for p.s.
mesons
Slope:
Pseudoscalar mesons:
~ 10-2/GeV/c2 (need
to generate one qq pair)
Baryons:
~ /GeV/c2
(need to generate two pairs)
Pentaquarks:
~ 10-8 /GeV/c2 (?) (need to
generate 4 pairs)
Slope for
Pentaquark??
Slope for
baryons
Pentaquark production in direct e+e- collisions likely
requires orders of magnitudes higher rates than available.

20. - and (1520) of HERA-B - (1520)
- recon: |mp-m| < 3, Cascade topology, z > 2.5cm

21. pK0s mass (GeV/c2) for -0.3< rapidity < 0.3
pK0s mass, Upper Limit
pK0s mass (GeV/c2) for -0.3< rapidity < 0.3
event mixing
mass=3.9
UL(95%): Carbon
sensitivity
With N  A0.7, UL(95%)Bd/dy|y=0 = GeV/c2

22.  Mass -- -+ +- ++ HERA-B NA49 (p + C) Counts / 7.5 MeV/c2
Mass GeV/c2
Counts / 3 MeV/c2 -- 20 -+ 10
Counts / 7.5 MeV/c2 +- 10 6 ++
Mass GeV/c2
C. Alt et al. PRL 92,

23.  Combined Plot, UL(95%) HERA-B NA49 Rapidity: -0.7 < y < 0.7
Counts / 3 MeV/c2
Rapidity: -0.7 < y < 0.7
mass = 6.6 GeV/c2 @ 1862 MeV/c2
--
B d/dy b/C
Assuming N  A0.7,
UL(95%) Bd/dy|y=0
= 2.5 b/N
@ m(--) =1862 GeV/c2

24. X–- and X0 at HERMES Mixed event background X0
UL for X-- (1860) cross-section: nb
UL for X0 (1860) cross-section: nb
Cross-section for X0(1530): – 24 nb
Cross section very sensitive to assumed pt and pz distributions

25. What do we know about the width of Q+?
K+d X
W. Gibbs, nucl-th/ (2004)
JP = ½-
GQ = 0.9 +/-0.3 MeV (K+d X)
Same width is obtained from analysis of DIANA results
on K+Xe scattering. (R. Cahn and G. Trilling, PRD69, 11401(2004))

26. Belle: The basic idea
Small fraction of kaons interacts in the detector material. Select secondary pK pairs to search for the pentaquarks.
Momentum spectrum of the projectile is soft.  low energy regime.
17cm
momentum spectra
of K+ and K-
Slide1
1 / 50MeV
momentum, GeV/c

27. Belle: we see it, we don‘t see it
155fb-1
1 / 5MeV
pK-
(1520)
Slide1
pKS
m, GeV
What should we have
expected here?

28. stot: K+d Q+ width: 0.9+/-0.3 MeV K+ Q+ n 1 / 50MeV momentum, GeV/c
momentum spectra
of K+ and K-
only narrow momentum bin
can contribute to Q+ production
if only 1 MeV wide and smeared
by Fermi motion. K+ n Q+
Momentum range possibly contributing to Q+ production.
Q+ width: 0.9+/-0.3 MeV

29. Belle: we see it, we shouldn‘t see it
155fb-1
1 / 5MeV
pK-
For I=0:
nK+: pK0s: pK0L
2 : : 1
(1520)
Slide1
< 80 events
This is approx. what we
should have expected here!
Assume that background
events have same isospin
structure as Q+ events.
pKS
m, GeV

30. Criticism to Null Results
Production mechanisms are different
Background conditions are different
Cross Section upper limits are not conclusive
Per se NULL result IS NOT NEGATIVE

34. Production of f (980) in e+e- Peak or Not a Peak? There is No Question
OPAL
CELLO

35. Production of Q+(1540) in ep scattering Peak or Not a Peak
Production of Q+(1540) in ep scattering Peak or Not a Peak? This is a Question
CLAS

36. Summary But still there is no consensus
It is already two years since first pentaquark reports
PDG adopted Q+ as a 3-star resonance
But still there is no consensus
We need to be careful claiming both: evidence and non evidence
We need to develop and apply different tests except of peak search in mass spectra
Current factual status doesn’t allow to draw ultimate conclusion