07/05/20041 Pentaquark search at HERA-B A. Sbrizzi - NIKHEF Motivation Pentaquark:  + (1540)  pK 0 Conclusion

07/05/20042 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 K 0

07/05/20043 HERA-B detector pA interaction at E cms ~ 41.6 Gev and IR ~ 5 MHz Large acceptance at mid-rapidity (y: mrad, x: mrad)

07/05/20044 Silicon Vertex Detector 7 superlayers of silicon microstrips Good primary vertex resolution (  x ~  y ~ 50  m,  z ~ 450  m)

07/05/20045 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/ )

07/05/20046 Data Sample Data taken in November 2002 – February  10 6 “minimum bias” events ( 12 C, 184 W and 48 Ti) 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  10 6 events ( 12 C, 184 W) using J/  trigger: events with at least 2 muon or electron candidates were written to tape at the speed of ~ 100 Hz (~ J/  )

07/05/20047 Reconstruction of K 0,  ΛpπΛpπ K S  ππ stat.: ~ K s, σ ~ 4,9 MeV stat.: ~ (~ ) Λ, σ ~ 1,8 MeV cuts applied: dist. closest approach < 700  m p t c  > 0.05 cm*Gev/c Backgound rejection: 95% Signal efficiency: 90% Armenteros plot Λ KsKs Λ

07/05/20048  (1520)  p K: similar channel and energy cuts applied: p likelihood > 0.95 K likelihood > 0.95 good proton identification 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 Carbon target

07/05/20049 p-K 0 : Monte Carlo Kinematic distributions: flat x f (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-K 0 and p-K is similar at mid-rapidity (the average in the full acceptance range is ~ 4%).

07/05/ p-K 0 : invariant mass K 0 selection has been shown before likelihood (p) > 0.95 No evidence of resonances in the mass region around GeV. U.L. = 5  b/nucleon (for 12 C) Carbon target Preliminary!

07/05/ Alternative strategies Since no evidence of structures in the invariant mass spectrum of p-K 0 is seen, we tried several different alternative approaches: 1.look at additional strange particles in the event (“strangeness tag”) 2.relax cut on proton likelihood (to accept protons with p < 20 GeV) 3.events with low track multiplicity in order to suppress combinatorial background

07/05/ 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   K0K0 p p wire ++ p ++ ++ -- K-K- K0K0 p p ++ ++ --

07/05/ Strangeness tagging

07/05/ p-K 0 selection 4 long tracks ( >4 VDS hits, >9 OTR hits ) 1 target 1 primary vertex with exactely 3 tracks K 0 : – 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-K 0 : –proton likelihood > 0.30 –mom(p) > mom(K 0 ) –|z  - z wire | < 1 cm

07/05/ p   + -signal p-K 0

07/05/ p   + -signal p-K 0 Is this really a signal ???

07/05/ p   + -signal p-K 0 antip-K 0 anti-p  antisignal p-K 0 antip-K 0

07/05/ Rough cross-section estimate  n (  ) =  n (K 0 ) * N(  )/N(K 0 ) *  (K 0 )/  (  ) N(  ) = 10  (  ) = 4*10 -5 N(K 0 ) = 3*10 6  (K 0 ) = 0.25 HERA-B published cross sections in 2002 for K 0 :  n (K 0 ) = 9  0.3 mb/nucleon (Abt et al., hep-ex/ ) ->  n (  ) = 180  b/nucleon  n (  ) = 0.02  n (K 0 )

07/05/ K* reflections Removing the proton likelihood and replacing the proton with the pion mass, the K * -> K 0  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

07/05/ Track multiplicity in the primary

07/05/ p-K 0 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

07/05/ Conclusions From the analysis of the MB data collected with HERA-B:  (1520)  pK : strong signal   pK 0 : no signal visible Upper limits on the production cross-section U.L. = 5  b/nucleon (for 12 C) Furhter investigations: The excess of events observed in events with only “4 long tracks” is most probably due to statistical fluctuations

07/05/ p-K 0 in low multiplicity events  + K - : 4 long tracks 2 tracks from primary  +  : 5 long tracks 1 track from primary  K0K0 p p wire ++ p ++ ++ -- K-K- K0K0 p p ++ ++ --

07/05/ p-K 0 : fake Carbon target p lik. > 0.95 Black: real, no lambda cont Solid red: fake, lambda cont. Cross red: fake, no lambda cont.