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Yajun Mao ( School of Physics, Peking University)

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  1. Yajun Mao (School of Physics, Peking University) Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

  3. The HERMES Spectrometer Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

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

  6. 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 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

  8. add p to invariant mass Results Peak at: M= 1528 ±2.6 MeV • = 8 ± 2 MeV significance: 3.7 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

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

  11. 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 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  12. Q+Isospin • Well establishedL(1520)pK-with acceptance:1.5% • No peak structure for Q++pK+zero counts at 91% CL Q+not isotensorprobablyisoscalar Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  13. nK+ Comparison of Experiment Results World Average: 1532.5±2.4 MeV Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

  15. Summary of Negative Results Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  16. 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. Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

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

  19. S*+ No peak in Lp+spectrum near 1530 MeV Q+vs S*+ • 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) Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  20. 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? Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  21. Requiring 4th HadronAsp Black requires 4th hadron (all species) only p Red specifies 4th hadron asp K & p Green specifies 4th hadron as notp Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

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

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

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

  25. 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 Yajun Mao, CCAST Workshop, 9-14 Aug, 2004

  26. Direct reconstruction ofQ+invariant mass Mass: Intrinsic Width: Significance:~ 4s 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) HERMES Conclusions and Outlook Yajun Mao, CCAST Workshop, 9-14 Aug, 2004