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Pentaquarks : Challenges and Pitfalls

Moskov Amarian Old Dominion University Norfolk, VA 23529 American Physical Society Spring Meeting, April 16-19 2005 ,Tampa, FL. Pentaquarks : Challenges and Pitfalls. Outline :. Physics Motivation Multiple reports with positive evidences Single reports with unique results

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Pentaquarks : Challenges and Pitfalls

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  1. Moskov Amarian Old Dominion University Norfolk, VA 23529 American Physical Society Spring Meeting, April 16-19 2005 ,Tampa, FL Pentaquarks:Challenges and Pitfalls

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

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

  6. CERN/NA49 H1 Other Pentaquark States

  7. nK+ Comparison of experiments World Average: 1532.5±2.4 MeV

  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 notQ+ butS*+ • It is not seen in some other experiments

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

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

  11. Q+ Mass spectrum with additional p • standard cuts applied • + K* and L veto • signal/background: 2:1 • signal/background:1:3

  12. No peak in Lp+spectrum near 1530 MeV? Q+vsS*+ • Is peak a newS*+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)

  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. MMgK+(GeV) MMgK+(GeV) • No large difference among the three Fermi motion correction methods MMgK+(GeV) LEPS: Fermi motion corrections 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

  17. Summary of Null Results

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

  19. - (1520) - and (1520) of HERA-B - recon: |mp-m| < 3, Cascade topology, z > 2.5cm

  20. pK0s mass, Upper Limit pK0s mass (GeV/c2) for -0.3< rapidity < 0.3 event mixing mass=3.9 MeV/c2 @+ UL(95%): Carbon sensitivity 1.4 1.475 1.55 1.625 1.7 With N  A0.7, UL(95%)Bd/dy|y=0 =3.7 b/N @ 1530GeV/c2

  21. Counts / 3 MeV/c2 1.4 1.6 1.8 2 2.2 Mass GeV/c2  Mass HERA-B (p + C) NA49 -- 20 10 -+ Counts / 7.5 MeV/c2 +- 10 6 ++ 1.4 1.8 2.2 2.6 Mass GeV/c2 C. Alt et al. PRL 92, 042003

  22. NA49  Combined Plot, UL(95%) HERA-B 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

  23. X–- and X0 at HERMES X0 Mixedevent background UL for X--(1860) cross-section: 1.0 - 2.1 nb UL for X0 (1860) cross-section: 1.2 - 2.5 nb Cross-section for X0(1530): 8.8 – 24 nb Cross section very sensitive to assumed pt and pz distributions

  24. K+d X JP = ½- GQ = 0.9 +/-0.3 MeV (K+d X) What do we know about the width of Q+? W. Gibbs, nucl-th/0405024 (2004) Same width is obtained from analysis of DIANA results on K+Xe scattering. (R. Cahn and G. Trilling, PRD69, 11401(2004))

  25. 17cm 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. momentum spectra of K+ and K- 1 / 50MeV momentum, GeV/c

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

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

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

  29. 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

  30. 0 Production of f (980) in e+e- Peak or Not a Peak? There is No Question OPAL CELLO

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

  32. Summary • 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

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