1. Pentaquark  + search at HERMES Siguang WANG School of Physics, Peking University on behalf of the HERMES Collaboration PRD 91, 057101(2015PRD 91, 057101(2015) NSTAR2015, May 25-28, 2015, Osaka, Japan

2. Outline Significances of  + (if seen) HERMES experiment Previous and present results from deuterium target Results from hydrogen target Summary NSTAR2015 siguang@pku.edu.cn 2

3. Significances of  + ( Groups ever announced positive results ) NSTAR2015 siguang@pku.edu.cn 3

4. Groups with Negative Results (now) NSTAR2015 siguang@pku.edu.cn 4 Reviewed by Tianbo Liu, Yajun Mao, and Bo-Qiang Ma, Int. J. Mod. Phys. A 29, 1430020 (2014)

5. HERA @ DESY NSTAR2015 siguang@pku.edu.cn 5 Hadron-Elektron-Ringanlage @ Deutsches Elektronen-Synchrotron

6. HERMES@HERA NSTAR2015 siguang@pku.edu.cn 6

7. The HERMES Spectrometer NSTAR2015 siguang@pku.edu.cn 7

8. Particle Identification NSTAR2015 siguang@pku.edu.cn 8 Hadron/Positron separation: Combining signals from: TRD(Transition Radiation Detector), Calorimeter, Preshower, RICH

9. Particle Identification NSTAR2015 siguang@pku.edu.cn 9  K Dual radiator RICH for , K, p Improvements in PID reconstruction: Old (track level): Track-by-track reconstructed, separately New(event level): Response pattern of all the tracks present in an event are reconstructed simultaneously, since with multiple tracks Cherenkov rings can overlap and lead to misidentification.

10. HERMES Result in 2004 NSTAR2015 siguang@pku.edu.cn 10 Significance: 3.7  1998-2000 D-target published in Physics Letters B 585 (2004) 213 How about the spectra with new (improved) data?

11. Old and New Topologies for  + Hunting NSTAR2015 siguang@pku.edu.cn 11 Topology in 2004 Topology in 2015 Main Improvements:  Event-level RICH particle ID  Advanced tracking corrections for magnetic fields and detector material based on Kalman-filter algorithm and new alignment Beam Direction

12. NSTAR2015 siguang@pku.edu.cn 12 PDG: M(Ks)=497.614  0.024MeV Analysis PeriodsNumber Ks S (  2  ) Number Background B(  2  ) Purity S/(S+B) New Analysis(98-00+06/07) 3311  6087  1197.4  0.4% Old Analysis(98-00) 963  38180  1584.3  1.3% 1998-2000 D-target With old and New analysis New: 1998-2000 + 06/07 D-target Old: 1998-2000 D-target

13. NSTAR2015 siguang@pku.edu.cn 13 Systematic uncertainty from: ①Different background shapes ②Fit ranges ③Bias from the shape studied with Toy- MC to create and fit with Background+Peak functions Significance: ~2  ①  log-likelihood method gives 1.9  (average value of different fit ranges) ②Toy-MC to create smooth shape and fit with Background+Peak, gives 2.2  Note: “D * -D 0 BG” is a function name of RooFit

14. NSTAR2015 siguang@pku.edu.cn 14 no structure found in hydrogen data an attempt to fit gives "negative peak"

15. Significances of  + ( Groups ever announced positive results ) NSTAR2015 siguang@pku.edu.cn 15

16. Summary NSTAR2015 siguang@pku.edu.cn 16 Thanks! 谢谢！

17. NSTAR2015 siguang@pku.edu.cn 17 Backup

18. NSTAR2015 siguang@pku.edu.cn 18 to fit the D-target and H-target combined data gives a "negative peak“ again

19. CLAS and COSY-TOF Results NSTAR2015 siguang@pku.edu.cn 19 CLAS, PRL 96, 212001 (2006) COSY-TOF, Physics Letters B 649(2007) 252-257

20. LEPS (2009) NSTAR2015 siguang@pku.edu.cn 20

21. DIANA Collaboration (2014) NSTAR2015 siguang@pku.edu.cn 21 arXiv:1307.1653v3 [nucl-ex] 18 Apr 2014 Observation of a narrow baryon resonance with positive strangeness formed in K + Xe collisions