1. Pentaquark states in High Energy ep Collisions
Tim Greenshaw (given by Stephen Maxfield)
University of Liverpool
IoP HEPP Half Day Meeting, Durham, October 27th 2004

2. The pentaquark: dead or alive?
The strange pentaquark:
Prejudices and predictions.
Strange pentaquark seen by some...
...but not by others
The charmed pentaquark:
A close look at the positive evidence.
The negative evidence.
Some things that can go wrong when looking for pentaquarks.
Further pentaquark results.
What have we learned?

3. Prejudices and predictions
Particle data book in 1984:
In 1997, using chiral soliton model, Dyakanov, Petrov and Polyakov predicted existence of resonance at 1530 MeV with width < 15 MeV.
Triggered new wave of experimental activity.

4. The strange pentaquark – first observation
In the process gn → nK+K-, LEPS observed peak in m(nK+) spectrum.
m = 1540 ± 10 ± 5 MeV.
G < 25 MeV at 90% CL.
No. of events N = 43.
Significance ~ 4.6 s (S/√B).
Minimum quark content
Mass and width consistent with chiral soliton model prediction.
LEPS data:

5. More observations CLAS (1999) gd → npK+K-: m = 1542 ± 5 MeV.
G < 21 MeV.
N = 43, significance ~ 5.2 s.
CLAS, gp → np+K+K-n:
m = 1555 ± 10 MeV.
G < 26 MeV.
N = 41, significance ~ 7.8 s.

6. More observations SAPHIR (1997...1998) gp → nK+KS0:
m = 1540 ± 4 ± 2 MeV.
G < 21 MeV
N = 63 ± 13, significance ~ 4.8 s.
DIANA (1986) K+Xe → pKS0Xe:
m = 1539 ± 4 ± 2 MeV
G < 9 MeV
N = 29, significance ~ 4.4 s.
pKS0 so not necessarily exotic state!

7. More observations HERMES, ed → eKS0pX: m = 1528 ± 3 ± 2 MeV.
G = 17 ± 9 ± 3 MeV.
Significance:
~ s (S/√B)
~ s (N/dN from fit).

8. More observations ZEUS, ep → eKS0pX: G = 8 ± 4 MeV. N = 221 ± 48.
Significance ~ s (fit).
Peak seen in both

9. More observations
Compendium of neutrino interactions in neon and deuterium (Asratyan et al):
m = 1533 ± 5 MeV
G < 20 MeV
N = 27, significance ~ 6.7 s.
SVD-2, pA → pKS0X:
m = 1526 ± 3 ± 3 MeV
G < 24 MeV
N = 50, significance ~ 5.6 s.

10. More observations COSY-ToF, pp → S+pKS0: m = 1530 ± 5 MeV
G < 18 MeV
N = 120, significance ~ s.

11. Strange pentaquark summary – take one
There is apparently overwhelming evidence for a Q+ state with mass about 1540 MeV.
In units of s, significances given as 4.6, 5.2, 7.8, 4.8, 4.4, 3...4, , 6.7, 5.6 and
According to the PDG 2004 the width of this state is ~ 1 MeV (based on re-analysis of DIANA data).
The strange pentaquark seems to be alive and well!

12. Negative searches for the strange pentaquark
ALEPH, e+e-:
CDF, proton antiproton:
L3, gg:

13. Negative searches for the strange pentaquark
HERAb
Belle
BaBar
pK-
pKS0
L(1520)

14. Negative searches for the strange pentaquark
BES
BR < 0.84 x 10-5 at 90% CL.
BES
BR < 1.1 x 10-5 at 90% CL.

15. Revisiting old data
Dalitz plots for K+N → KNp studied in hydrogen and deuterium bubble chambers.
No cuts to enhance possible Q+ signal.
Dominant features due to decays K*(892) → Kp and D(1232) → Np.
Lines show expected positions of Q+ resonance.

16. Strange pentaquark summary – take two
Many high statistics experiments fail to see the Q+.
Are experiments consistent?
Look at R = N(Q+)/N(L(1520)):
Positive experiments:
SAPHIR, R ~ 0.3
HERMES, R ~
ZEUS, R ~ 0.2 (estimate!)
SVD-2, R > 0.2
Negative experiments:
ALEPH, R < 0.1
BaBar, R < 0.01
Belle, R < 0.02
Is the pattern of the masses seen by the “positive” experiments peculiar?
Maybe the strange pentaquark isn’t so healthy after all?
nK+
pKS0

17. The charmed pentaquark
If why not
A few predictions:
m(Qc0) = 2710 MeV (Jaffe, Wilczek, hep-ph/ ).
m(Qc0) = 2704 MeV (Wu, Ma hep-ph/ ).
Such a Qc0 would be too light to decay to D mesons, but could decay weakly to Θs+π−.
m(Qc0) = 2985 ± 50 MeV, Γ(Qc0) = 21MeV, Karliner, Lipkin (hep-ph/ ).
m(Qc0) = MeV, (Cheung, hep-ph/ ).
Such a Qc0 could decay to D-p.
If m(Qc0) > m(D∗) + m(p) = 2948 MeV, Qc0 can decay to D*p.
This decay mode can be dominant, (Karliner, Lipkin, hep-ph/ ).

18. Charmed pentaquark search – experimental considerations
D+ pseudoscalar meson, without and with lifetime tag:
Have either huge background or low yield.
D* much easier to find due to low Q value of decay D* → D0pS.
Look at mass difference Dm = m(K-p+pS+) – m(K-p+) in chain D*+ → D0pS+ → K-p+pS+:
Background described by “wrong charge D0” sample, Dm = m(K-p-pS+) – m(K-p-).

19. Charmed pentaquark observation
Combine D*+ with particles that have reasonable likelihood of being anti-protons from dE/dx measurements:
Resulting mass spectrum:

20. Charmed pentaquark observation
Significance assessment:
NS + NB = 95 (within 2s of peak).
NB = 51.7 (Bg. only fit).
Prob. signal produced by fluctuation (Poisson statistics) is 4x10-8.
Equivalent to 5.4s.
Signal also observed in independent photoproduction sample:
Summary take one – charmed pentaquark looks healthy!

21. Negative searches for the charmed pentaquark
CDF, proton antiproton:
ZEUS, ep:

22. Negative searches for the charmed pentaquark
ALEPH, e+e-:
Are experiments consistent?
Look at R = N(Qc)/N(D*):
Positive experiment:
H1, R ~ 0.01
Negative experiment:
ZEUS, R < (not same phase space as H1 result?)
Summary take two – the charmed pentaquark also looks sick.

23. Some things that can go wrong and the charmed pentaquark – statistics and fake peaks
How significant is a “5s” signal?
Generate 40 random histograms with 600 events each from the parent distribution:
Three of these histograms shown on right...
...together with the CLAS Q+ signal, S/√B = 5.2 s.
But in case of Qc, BG seems to be well described and shows no evidence of “hump”.

24. More observations JINR, np → npK+K.
Profusion of states seen in m(nK+) spectrum:

25. Reflections E.g. of reflection
Momentum of decay products in D1 rest frame:
Reconstruct mass misidentifying p as p, i.e. using expression:
Result is mass that is too large:
mrec = 3.04 GeV.
mD1 = 2.42 GeV.
Boost to Lab. smears rec. mass.
Max. and min. rec. mass given by extremes of cosq*.
Mass independent of q* only for correct particle assignments
m(D*p) (GeV)
D*p rest frame
Lab frame

26. Reflections Integrating over cosq*:
Expected band seen in m(D*p) at const. m(D*p), no evidence for band in m(D*p) at const. m(D*p) as would be case if p assignment incorrect:
m(D*p) (GeV)
D2 → D*p
D1 → D*p
m(qc)

27. Reflections
Estimate expected contribution of D1 and D2 reflections from data.
Now label p as p and recalculate mass.
Hence obtain N(D1) + N(D2) in signal region.
Compatible with MC expectation ~ 3.5
Loose D* cuts and p selection
D* cuts as for Qc and p selection
D* cuts as for Qc and p selection
M(D*p) = m(Kppp) - m(Kpp) + m(D*)PDG

28. Split tracks and the Qc
A single track may be found twice, e.g. due to multiple scattering in a tracking chamber that causes “kink” in track not recognised by pattern recognition software.
Perhaps seen opposite in candidate D* event?
If take K from D* decay and re-use track, identifying it as p second time round, obtain m(D*p) ~ 3.1 GeV. K+
ps- p-

29. Split tracks and the Qc
Monte Carlo of D* → Kpp events where K split and identified as p.
Resulting m(D*p) spectrum in DIS:
Mass peak is at m(D*p) = 3.1 GeV, but broader than observations.
However, after applying selection cuts...
m(D*p) [GeV]
Events per 10 MeV
m(D*p) [GeV]
Events per 10 MeV

30. Split tracks and the Qc
No evidence for such effects seen in H1 data: scan all events in signal region.
Check difference of transverse momenta of K and p tracks does not peak at zero. p+ K-
pS+

31. Split tracks and the Q+
Monte Carlo of K0 → p+p- events where p+ split and identified as p second time round.
Resulting m(K0p) spectrum in DIS:
Mass peak is at m(K0p) = 1.54 GeV, but broader than observations.
After applying H1 selection cuts still broader than Q+ observations?
m(K0p) [GeV]
Events per 10 MeV
m(K0p) [GeV]
Events per 10 MeV

32. More observations NA49, pp collisions: Evidence for X- -(1862):
m = 1862 ± 2 MeV.
G < 18 MeV.
N = 69, significance ~ 5.8 s.
Also evidence for X0(1862).

33. More contradictions ALEPH, e+e-, no evidence for X(1862) states:
No evidence for X(1862) states at CDF, proton anti-proton:
Also null result from HERAb and BaBar (see earlier).

34. Coincidences and creativity
Chiral Soliton Model approx. to QCD in limit NC → ∞: for NC = 3 many exotic states, discarded as artefacts.
Couplings used in Dyakanov, Petrov and Polyakov prediction out of date, error of ~ 200 MeV in m(X- -).
With modern values, difficult to get G(Q+) < 10 MeV.
The observation of the Q+ has resulted in new ideas in spectroscopy.
“Diquark-triquark” picture (Karliner Lipkin):

35. Coincidences and creativity
“Diquark-diquark-antiquark” picture (Jaffe and Wilczek):
“Tetrahedron” picture (Yu-xin Liu, Jing-sheng Li, Cheng-guang Bao): u d

36. Coincidences and creativity
Lattice should tell us what QCD is really doing in this non-perturbative region, but so far...
PQ exists?
Parity of lowest state?
Anti-charmed PQ exists?
Operator
Csikor et al
Yes
Negative
Don’t know
Colour variant of KN
Sasaki No
Diquark-diquark-antiquark
Kentucky
Not positive
Simple KN
Chiu-Hsieh
Positive
MIT

37. What have we learned – are pentaquarks dead or alive?
Whether pentaquarks are dead or alive, there is much within QCD that remains to be understood!
Where experiments are poorly understood it is hard to do good theory and where theory is poorly understood it is hard to do good experiments.
The evidence for the existence of pentaquarks is conflicting.
If they exist, it would appear that the production mechanism is exotic: experiments must measure cross-sections and identify kinematic regions in which pentaquarks are observed – further analysis and data needed.
If pentaquarks are real, explaining their width is difficult.
Pentaquarks – dead or alive?
The box has yet to be properly opened!