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

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

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?

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.

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

Other Pentaquark States CERN/NA49 H1

Comparison of experiments nK+ World Average: 1532.5±2.4 MeV Is a variation in mass really large? What about exp. Uncertainties?

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

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

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)

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

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?

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

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

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

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?

Summary of Null Results

Hadron production in e+e- Slope for p.s. mesons 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) Slope for Pentaquark?? Slope for baryons Pentaquark production in direct e+e- collisions likely requires orders of magnitudes higher rates than available.

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

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 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

 Mass -- -+ +- ++ HERA-B NA49 (p + C) Counts / 7.5 MeV/c2 1.4 1.6 1.8 2 2.2 Mass GeV/c2 Counts / 3 MeV/c2 -- 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

 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

X–- and X0 at HERMES Mixed event background X0 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

What do we know about the width of Q+? K+d X W. Gibbs, nucl-th/0405024 (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))

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

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?

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

Belle: we see it, we shouldn‘t see it 155fb-1 1 / 5MeV pK- For I=0: nK+: pK0s: pK0L 2 : 1 : 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

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

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

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

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