Search For Pentaquark Q+ At HERMES Yajun Mao (School of Physics, Peking University) 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Outline Particle Identification Search For pKS Resonance Systematic Studies Summary and Conclusions 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

The HERMES Spectrometer Beam: 27.6 GeV e+/e- from HERA accelerator Reconstruction: Dp/p < 2%, Dq < 0.6 mrad Particle ID: TRD, Preshower, Calorimeter (hadron/lepton sep.) RICH (p, K, p separation) 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Particle Identification hadron/positron separation combining signals from: TRD, calorimeter, preshower, RICH hadron separation Dual radiator RICH for p, K, p 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Event Reconstruction Define appropriate event topology Hadron identification: RICH p: 1 - 15 GeV p: 4 – 9 GeV Lambda is well identified  protons are cleanly identified Reject events from L(1116)  pp- within ±2s of ML invariant mass calibration ± 2MeV direct reconstruction: detection of each decay particle, invariant mass reconstruction 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Detector Mass Calibration HERMES Mass[MeV] 496.8±0.2 1115.70±0.01 1522.7±1.9 1321.5 ±0.3 PDG Mass[MeV] 497.67 1115.68 1519.5±1.0 1321.31±0.13 s width (data)[MeV] 6.2±0.2 2.6±0.1 4.4±3.7 3.1 ±0.3 s width (MC)[MeV] 5.4 2.1 3.5 2.5 Decay Pcm[MeV/c] 206 101 244 139(Lp-) HERMES can precisely measure the above known particles The MC well reproduce the data The systematic error estimated is about 1.9MeV 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Reconstruction after all cuts 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Results add p to invariant mass Peak at: M= 1528 ± 2.6 MeV = 8 ± 2 MeV significance: 3.7 Unbinned fit is used: result independent of bin size and starting point 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

The Signal and Its Background Peak at: 1527 ± 2.3 MeV = 9.2 ± 2 MeV Significance: 4.3 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 The Signal Width Q+ Monte Carlo with complete detector simulation - generated: M=1540 MeV , s=2 MeV - reconstructed: M=1539.5 MeV , s=6.2 MeV 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 A Non-Zero Width For Q+ Observed width FWHM: 19 – 24 MeV Detector resolution (from MC) FWHM: 10 – 14.6 MeV re-fit spectra with Breit-Wigner convolved with a Gaussian (fixed by MC) HERMES width: Γ = 17 ± 9 ± 3 MeV 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Q+ Isospin Well established L(1520)  pK- with acceptance:1.5% No peak structure for Q++  pK+ zero counts at 91% CL Q+ not isotensor probably isoscalar 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Comparison of Experiment Results nK+ World Average: 1532.5±2.4 MeV Large variation in mass not uncommon for new, decaying particles but need to better estimate exp. uncertainties 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Summary of Positive Results hep-ex/0301020, PRL hep-ex/0304040, ? http://na49info.cern.ch hep-ex/0404003 hep-ex/0403051 hep-ex/0403044 hep-ex/0401024 hep-ex/0403011 hep-ex/0312044, PLB Proc. Mong. Acad Sci. hep-ex/0307018, PRL hep-ex/0311046, PRL hep-ex/0307083, PLB hep-ex/0309042, ? 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Summary of Negative Results 0 null results published, only 3 on arXiv so far ⇒ need null results to be published 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 Open Questions How real are positive results? check if peaks are generated by kinematical reflections ghost tracks acceptance or cuts How real are negative results? - need to published scrutinized as hard as positive results Mass? Width? Spin and Parity etc. 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 How Real Is The Peak? check for kinematical reflections detector acceptance and cuts (PYTHIA6 MC / Toy MC) Q+ vs S*+ is Q+ a pentaquark or a previously unobserved S*+ add a forth hadron is the peak still there can we guess the production process for the Q+ can we supress background 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Kinematical Reflection particle miss-identification ghost tracks PID “leaks” expect peak in M(p-p) if p+ is p and KS is L(1116) Spectrum of events associated with L(1116) remove L(1116) contribution 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 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 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Yajun Mao, CCAST Workshop, 9-14 Aug, 2004 The Mass Spectrum relatively large BG, although good PID for proton and KS what if we require one additional hadron? could additional hadron help remove KS from other process? 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Requiring 4th Hadron As p Black requires 4th hadron (all species) only p Red specifies 4th hadron as p K & p Green specifies 4th hadron as not p 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Why Does Additional p Help? remove remove add new cut: |M(KSp)-892|<75 MeV high statistics K* 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

The Mass of (K* removed) |M(pp-) – 1.116|<0.006GeV there is a L(1116) peak from but it only contributes to background add L veto as a new cut 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Mass Spectrum After Removing K* standard cuts applied + K* veto + L veto  background is greatly reduced signal/noise: from 1:3 to 2:1 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

Production Process At HERMES can 4th hadron come from exclusive processes? associated K- or KS from exclusive processes goes backward even decay pions from KS are inaccessible PID threshold requires p(Q+) > 7 GeV tagged pions events can’t come from these exclusive processes  production process has to be at least partially inclusive inclusive processes increase with higher energy exclusive processes decrease with higher energy 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

HERMES Conclusions and Outlook Direct reconstruction of Q+ invariant mass Mass: Intrinsic Width: Significance: ~ 4 s Q+ is probably an isosinglet Requiring additional p improves signal/background, it eliminates KS contamination from various processes Production process is at least partially inclusive Anticipate doubling statistics by end of this summer Will soon report on X-- search and Q+ from TOF (low p) 14-August-2004 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004