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Searches for pentaquarks on deuterium with CLAS

Searches for pentaquarks on deuterium with CLAS. S. Niccolai, IPN Orsay for the CLAS collaboration. Workshop “New Hadrons: Facts and Fancy” 19-9-2005 Milos, Greece. Searches for pentaquarks on deuterium with CLAS. Published CLAS-d Q + observation G10 : new high-statistic CLAS experiment

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Searches for pentaquarks on deuterium with CLAS

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  1. Searches for pentaquarks on deuterium with CLAS S. Niccolai, IPN Orsayfor the CLAS collaboration Workshop “New Hadrons: Facts and Fancy”19-9-2005 Milos, Greece

  2. Searches for pentaquarks on deuterium with CLAS Published CLAS-d Q+ observation G10: new high-statistic CLAS experiment Preliminary results from G10 (gd→pK+K-(n)) Other reaction channels under analysis Search for X--at CLAS (EG3) Conclusions and outlook S. Niccolai, IPN Orsay

  3. gd →K-pK+(n) Published CLAS-d • Experimental data from 1999 run (G2) • Tagged photons with Eg =0.5-3 GeV • Target: 10 cm long liquid deuterium • Exclusivity: • No Fermi motion corrections • Final state identified with less background Possible reaction mechanism non-spectator proton: pK- rescattering required to detect p in CLAS (pmin(p) = 300 MeV/c) S. Niccolai, IPN Orsay

  4. Published CLAS-d: result PRL 91 (2003) Q+ • Mass = 1.542 GeV • < 21 MeV Significance = (5.2±0.6) s Two different background shapes (gaussian and MC) L(1520) events Reanalysis of these data finds that the statistical significance of the peak may not be as large as quoted High-statistics experiment needed S. Niccolai, IPN Orsay

  5. tagged photons in the energy range 0.8 GeV - 3.59 GeV target: 24 cm long liquid deuterium data taken at 2 settings of CLAS toroidal magnet (2250 A and 3375 A), lower field to increase acceptance for negatives at each setting, the integrated luminosity (25 pb-1) is about 10 times higher than in published deuterium data Reactions channels analyzed gd→pK-K+n Θ+→nK+ gd→ΛK+(0)n(p); Θ+→nK+,pK0 gd→pK-K0 (p) Θ+→pK0 g”n”→K-K+n Θ+→nK+ (with Fermi momentum corrections, to compare with SPring-8) G10: a Q+- dedicated experiment • data taking: March 13 - May 16, 2004 • 10 billion of events collected • 25 TB of data processed S. Niccolai, IPN Orsay

  6. G10 data: gd→pK+K-(n) 1) can we reproduce the published result from G2? • Channel selection: • PID for p, K- and K+ • Cut on missing neutron mass +/-3s, missing momentum Pn>0.08 GeV/c • Cut on f(1020): M(K+K-)>1.07 GeV/c2 • Cut on L(1520) f n L(1520)

  7. gd→pK+K-(n): comparison with G2 • Identical event selection cuts are applied to G10 data, except: • timing cuts, missing neutron mass cut, which are momentum dependent in g10 analysis • fiducial cuts on K- to take into account the difference of acceptance due to the target positions • other cuts are the same • Photon energy scaled to reproduce G2 energy spectrum • G10 has higher photon energy than G2 S. Niccolai, IPN Orsay

  8. G10 vs. G2 • The two distributions are statistically consistent with each other: • 26% c.l. for null hypothesis from the Kolmogorov test (two histograms are compatible). • Reduced 2=1.15for the fit in the mass range from 1.47 to 1.8 GeV/c2 • G10 mass distribution can be used as a background for refitting the published spectrum. Preliminary Analysis by S. Stepanyan S. Niccolai, IPN Orsay

  9. G10 vs. G2: statistical significance • MM(pK-) distributions fitted with 3rd degree polynomial plus Gaussian • The 3rd degree polynomial that fits the G10 spectra is used also as a background for G2 (scaled by x5.9) • For the fit to the G10 distribution Gaussian mean and sigma were fixed (given by CLAS resolution, from MC), fit parameter was the number events in the Gaussian peak. G10 G2 Preliminary S. Niccolai, IPN Orsay

  10. 1) Can we reproduce the published result from G2? • The published result on Q+cannot be reproduced in the analysis of high statistics G10 data • Using G10 distribution as a background, significance of published data goes down to 3.1s • The significance quoted in the published data could be the result of an underestimated backgroundcombined with a statistical fluctuation in the 1.54 GeV/c2 mass region S. Niccolai, IPN Orsay

  11. G10 data: gd→pK+K-(n) B. McKinnon’s analysis 2) Do we see Q+ beyond G2 conditions? • Photon beam energy Eg< 3.6 GeV • Missing neutron momentum Pn>0.2 GeV/c • Invariant mass M(nK+) with constrained • neutron mass or kinematical fit • Various Eg cuts tried, no narrow peak observed • in any of the 4 parallel independent analyses Kin.Fit. D.Carman’s analysis K.Hicks’s analysis T.Mibe’s analysis Preliminary S. Niccolai, IPN Orsay

  12. 1.05 < Eg < 1.15 GeV g10 (3375A) World data T. Mibe’s analysis gd→pK+K-(n): cross section upper limit • Number of “signal” events: number of events into Gaussian peak over a smooth background (3rd degree polynomial) • Acceptance calculation: 4-body phase-space event generator, modified to match kinematics of detected particles with data • Checkednormalization and efficiencies using gd→pp-(p) reaction (0.5% of G10 statistics) ds/dWcm (mb/sr) Preliminary cosθcm(p-) S. Niccolai, IPN Orsay

  13. p L(1520) K- p K+ n n K+ 0.35 GeV/c minimum pp in CLAS Fermi momentum distribution G10 (3375A) Preliminary Cross section upper limit • For gd→Q+pK-around M(nK+)=1.525 GeV/c2, u.l. =450pb (95 % CL) • For the elementary process gn→Q+K-, extraction of c.s. is model dependent. Correction factor for “non-spectator proton” is extracted using L(1520) production as a guide, Y/Y0(0.35) = 0.1±0.01 cross sectionupper limit ≈ 5 nb

  14. L p K+ Q+ n d γd→ΛΘ+ • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  15. L p K+ Q+ n d γd→ΛΘ+ Reaction already studied on 3He-G3 (Niccolai) and d-G2 (Stepanyan, Mirazita) but statistics were too low • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  16. Decay modes under study: • Λ→pπ- Θ+→K+n d γd→ΛΘ+ p p- L p K+ n Q+ n K+ • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  17. Decay modes under study: • Λ→pπ- Θ+→K+n • Λ→pπ-Θ+→K0p K0 → π+π- Five charged particles in the final state d Possible to analyze many different exclusive topologies γd→ΛΘ+ p p- L p K+ p Q+ n p+ K0 p- • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  18. nK+ decay mode: data analysis • Channel ID: • K+, p, p- detected • n identified by missing mass • Lidentified bypp- invariant mass

  19. nK+ decay mode: S- background? HIGH-FIELD G10 DATA gd → pK+S-→ pK+np- pK+ missing mass beforeL cut pK+ missing mass afterL cut No S- background S. Niccolai, IPN Orsay

  20. d pK0 mode, all particles detected p • Channel ID cuts: • -0.01<MM2<0.005 (GeV2/c4) • 1.11<M(pp-)<1.12 GeV/c2 • 0.48<M(p+p-)<0.51GeV/c2 p- L p K+ p Q+ n p+ K0 p-

  21. pK0 mode, all particles detected • Channel ID cuts: • -0.01<MM2<0.005 (GeV2/c4) • 1.11<M(pp-)<1.12 GeV/c2 • 0.48<M(p+p-)<0.51GeV/c2 S. Niccolai, IPN Orsay

  22. γn→Q+K-→pK0K- • Photoproduction on the neutron • Fully exclusive reaction (spectator p undetected) • pK0 strangeness well defined • gn→Y*K0, hyperon background • npa2(1320)pK0K-, mesonic background S. Niccolai, IPN Orsay

  23. γn→Q+K-→pK0K-: data analysis • pK- +- detected • M(+-)→ K0s • MM(pK- +-)→ missing proton • Cut on pmiss<0.15 GeV/c2 • to eliminate non-spectator events • MC studies to understand the • background are underway: • phase-space + gn→Y*K0 Analysis by N. Baltzell & D. Tedeschi S. Niccolai, IPN Orsay

  24. Search for Q+’s decuplet partner X-- NA49 pp Ecm = 17.2 GeV C. Alt, et al., Phys.Rev.Lett.92, 042003 (2004) Sum of 4 combinations • S=-2, Q=-2, ddssu • one positive observation (also X0) • several non-observations M=1.862± 0.002 GeV Statistical significance = 5.6σ G = 18 MeV/c2 S. Niccolai, IPN Orsay

  25. The CLAS EG3 experiment • 5.7 GeV electron beam, 30 nA • Untagged photon beam, 4.4<Eg<5.5 GeV • CLAS torus field optimized to -1980 A, negatives outbending • Target: liquid deuterium, 40 cm long • Luminosity 1034 cm-2 s-1 • 100 pb-1in 40 days of running(12/04-1/05) • Detector calibrations almost finished • First pass data processing underway Goal: identify X-- from its decay reconstruction of 2 detached vertices S. Niccolai, IPN Orsay

  26. X-- Decay Measurement with CLAS (EG3) g n  K+ K+ X5-- X--p-X- X-p- L Lp- p Electron beam S. Niccolai, IPN Orsay

  27. X-- Decay Measurement with CLAS (EG3) g n  K+ K+ X5-- X- -p-X- 4% of the data X- p- L Lp- p S. Niccolai, IPN Orsay

  28. Conclusions and outlook • High statistics G10 data show no Q+ signal in the gd→pK+K-(n) channel, in the region where the lower statistics published data showed a peak • No narrow peak appears in the M(nK+) spectrum, even applying more restrictive cuts than in G2 • The new data allow to set an upper limit on the gd→pK+K-(n) cross section of 450 pb (95% CL) • Present estimate of cross sectionupper limit for the elementary process gn→Q+K-is 5 nb (95% C.L.), further MC studies are in progress • Analysis of other reaction channels is in the final stage • Calibrations of the EG3 data set is underway, its analysis will contribute to solving the issue of the existence of X-- S. Niccolai, IPN Orsay

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