Pentaquark search at HERA-B A. Sbrizzi - NIKHEF

1. Pentaquark search at HERA-BA. Sbrizzi - NIKHEF Motivation Pentaquark: +(1540)  pK0 Conclusion

2. Motivation All well established particles fall into the scheme of 2 (qq) or 3 (qqq) quark configuration. On the other hand this is not the only color singlet object which can be built. In fact QCD allows the existence of exotic baryon and meson configurations: hybrids (qqg), four quark states(qqqq), pentaquarks (qqqqq)… At HERA-B we try to verify claims for different exotic states but I will focus on: + (uudds) -> p K0

3. pA interaction at Ecms ~ 41.6 Gev and IR ~ 5 MHz • Large acceptance at mid-rapidity (y: 15-160 mrad, x: 15-220 mrad) HERA-B detector

4. Silicon Vertex Detector • 7 superlayers of silicon microstrips • Good primary vertex resolution (x ~ y ~ 50 m, z ~ 450 m)

5. Ring Imaging CHerenkov detector good Kaon ID for 10< p <60 GeV good proton ID for 20< p <60 GeV (Arino et al., hep-ex/0303012)

6. Data Sample • Data taken in November 2002 – February 2003 • 210106 “minimum bias” events (12C, 184W and 48Ti) • Interaction trigger: events with a minimum number of hits in the RICH or in the ECAL were written to tape at the speed of ~ 1 kHz. • In addition (for B physics analysis): • 150106 events (12C, 184W) using J/ trigger: events with at least 2 muon or electron candidates were written to tape at the speed of ~ 100 Hz (~300.000 J/)

7. KSππ Λpπ Armenteros plot Ks Λ Λ Reconstruction of K0,  • cuts applied: • dist. closest approach < 700 m • ptc > 0.05 cm*Gev/c • Backgound rejection: 95% • Signal efficiency: 90% stat.: ~3.400.000 Ks, σ~ 4,9 MeV stat.: ~940.000 (~450.000) Λ,σ~ 1,8 MeV

8. (1520) p K: similar channel and energy Carbon target cuts applied: p likelihood > 0.95 K likelihood > 0.95 A strong signal visible at 1520 Gev for both particle and antiparticle. Simulation of kinematic reflections of two-body decays are also shown. : ~ 2000, σ ~ 8 MeV : ~ 1000, σ ~ 8 MeV good proton identification

9. p-K0: Monte Carlo Kinematic distributions: flat xf (n=0) and B=2.1 The remaining momentum is assigned to a virtual pion which is fed into FRITIOF 7.02 to simulate further interactions inside the nucleus width at generation ~ 50 keV exp. mass resolution = 3.2  0.2 MeV/c Acceptance for p-K0 and p-K is similar at mid-rapidity (the average in the full acceptance range is ~ 4%) .

10. p-K0: invariant mass Carbon target Preliminary! • K0 selection has been shown before • likelihood (p) > 0.95 No evidence of resonances in the mass region around 1.530 GeV. U.L. = 5 b/nucleon (for 12C)

11. Alternative strategies • Since no evidence of structures in the invariant mass spectrum of p-K0 is seen, we tried several different alternative approaches: • look at additional strange particles in the event (“strangeness tag”) • relax cut on proton likelihood (to accept protons with p < 20 GeV) • events with low track multiplicity in order to suppress combinatorial background

12. p wire wire  K- + + p + K0 p + K0 + - p - p Strangeness tagging The high track capability of the HERA-B detector allows to look for other particles created simultaneously with the + candidate. If + contains an s-bar quark -> the s quark should hadronize with high probability into K- or  correct tag: • + (uudds) and K-(us) or (uds) • - (uudds) and K+(us) or (uds) wrong tag: • +and K+ or  • -and K- or 

13. Strangeness tagging

14. p-K0 selection • 4 long tracks (>4 VDS hits, >9 OTR hits) • 1 target • 1 primary vertex with exactely 3 tracks • K0: • mass gate: [0.487, 0.510] GeV • +- distance of closest approach < 600 m • no  cont.: m(p, ) out of [1.110, 1.125] GeV • Vertex detachment > 5 cm • impact parameter to the target < 1 mm • p-K0: • proton likelihood > 0.30 • mom(p) > mom(K0) • |z - zwire| < 1 cm

15. p  Q+-signal p-K0

16. p  Q+-signal Is this really a signal ??? p-K0

17. p-K0 antip-K0 p  Q+-signal anti-p  antisignal p-K0 antip-K0

18. Rough cross-section estimate n() = n(K0) * N()/N(K0) * (K0)/() N() = 10 () = 4*10-5 N(K0) = 3*106 (K0) = 0.25 HERA-B published cross sections in 2002 for K0 : n(K0) = 9  0.3 mb/nucleon(Abt et al., hep-ex/0212040) -> n() = 180 b/nucleon n() = 0.02n(K0)

19. K* reflections Removing the proton likelihood and replacing the proton with the pion mass, the K* -> K0 resonance becomes evident Events from the K* reflections are not strictky correlated with the mass region around 1.55 GeV -> rejecting those events could only be dangerous

20. Track multiplicity in the primary

21. p-K0 vertex detachment The excess of events at 1.54 GeV seems to be concentrated far from the target at|z| > 0.3 cm, but they are equally distributed upstream and downstream of the target

22. Conclusions • From the analysis of the MB data collected with HERA-B: • (1520)  pK : strong signal •  pK0 : no signal visible • Upper limits on the production cross-section • U.L. = 5 b/nucleon (for 12C) • Furhter investigations: • The excess of events observed in events with only “4 long tracks” is most probably due to statistical fluctuations

23. p wire wire  K- + + p + K0 p + K0 + - p - p p-K0 in low multiplicity events +  : 5 long tracks 1 track from primary + K-: 4 long tracks 2 tracks from primary

24. p-K0: fake Carbon target p lik. > 0.95 Black: real , no lambda cont Solid red: fake, lambda cont. Cross red: fake, no lambda cont.