1. 1 Tomoaki Hotta (RCNP, Osaka Univ.) for The LEPS Collaboration Cracow Epiphany Conference, Jan 6, 2005 Introduction LEPS experiment Results from new LD 2 data Summary and outlook LEPS Results on  +

2. 2 Theoretical Prediction of   Baryon M  1890-180*Y] MeV D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305 (Chiral Soliton Model) Exotic: S = +1 – cannot be (qqq) state Low mass: 1530 MeV Narrow width: ~ 15 MeV (30 MeV)* J  =1/2 + *The width corrected by R. Jaffe (hep-ph/0401187)

3. 3 First evidence of   from LEPS  n    K   K  K  n  1.54  0.01 GeV  < 25 MeV Gaussian significance 4.6  Target: neutron in Carbon nucleus Background level is estimated by a fit in a mass region above 1.59 GeV. Assumption: Background is from non-resonant K + K - production off the neutron/nucleus … is nearly identical to non- resonant K + K - production off the proton ++ Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020

4. 4 Evidence for Pentaquark States Spring8 ELSA JLab-p HERMES ITEP pp   +  +. COSY-TOF DIANA SVD/IHEP JLab-d ZEUS CERN/NA49 H1 Nomad a lot of evidence

5. 5 Questions: “Existence of the  + ” is the most important issue – some inconsistencies in the measured mass & width. – negative results (mainly) from high energy experiments. → Can we see the peak again in the new LEPS data ? True mass, width Spin & Parity Production mechanism, cross sections…

6. 6 Data taken from Oct. 2002 to Jun. 2003. ~2×10 12 photons on a 15cm-long LD 2 target. Less Fermi motion effect. LH 2 data were taken in the same period with ~ 1.4×10 12 photons on the target. # of photons: LH2:LD2 ≈ 2:3 we expect # of events from protons: LH2:LD2≈ 2:3 # of events: LH2:LD2≈ 1:3 LEPS New LD 2 and LH 2 runs

7. 7 Super Photon ring-8 GeV SPring-8 Third-generation synchrotron radiation facility Circumference: 1436 m 8 GeV 100 mA 62 beamlines

8. 8  Laser Electron Photon facility at SPring-8 in operation since 2000

9. 9  LEPS detector 1m1m

10. 10 Charged particle identification Mass(GeV) Momentum (GeV) K/  separation (positive charge) K+K+ ++ Mass/Charge (GeV) Events Reconstructed mass d p K+K+ K-K- ++ --  (mass) = 30 MeV(typ.) for 1 GeV/c Kaon

11. 11 Reaction diagrams pK nKnp nK  pK  pn         )1520( )() ()( )()( * *   n  K─K─ K+K+ n ++ pp p  K+K+ K─K─ p  nn  N   (1020) N   K + K - N S=+1 S=-1 “Exotic” “Standard” baryon Meson resonance

12. 12  background N  /N  ratio Invariant mass (K + K - ) (GeV) Events Invariant mass (K + K - ) (GeV) Real data MC(  ) N  : Real data – MC(  ) N  : MC(  )      ratio was almost energy independent.

13. 13 E  dependent  cut point : “N  /N  ratio × Relative acceptance = R”  exclusion cut Invariant mass (K + K - ) (GeV) MM(,K - ) (GeV) 1.8<E  <2.0 GeV2.0<E  <2.2 GeV2.2<E  <2.4 GeV Monte Carlo simulation (K + K - n 3 body phase space) M  =1.019 Expected signal region “Rerative acceptance” = N(1.50<MM( ,K - )<1.55)/N(all)

14. 14 Energy dependent  exclusion cut E  (GeV) R=0.01 R=0.05 R=0.20 KK inv. Mass (GeV)

15. 15 Cut dependence of K + missing mass for the LH2 data MM  (GeV) R=0.010.020.03 0.050.07 0.10 0.20 0.501.00

16. 16 Cut dependence of K + missing mass for LD2 data MM  (GeV) 0.020.03 0.01 0.050.070.10 0.200.501.00

17. 17 Comparison of MM( ,K + ) for the LH2 and LD2 data MM  (GeV) LH2:LD2 ratio of  events is ~2:3  consistent with the expectation. LH2 LD2

18. 18 Comparison of MM d ( ,K + K - ) MM  (GeV) Remove  d  KKd contributions by requiring MM d ( ,KK)>1.89 GeV LH2 LD2 Mising mass for  d  KKX

19. 19 After removing deuterium elastic scattering contributions MM  (GeV) MM  (GeV) Further require 0.89 0.99 GeV S/N at  (1520) peak is better than 1.

20. 20 Cut dependence of K + missing mass for LD2 data after MM d cut MM  (GeV) 0.20

21. 21 Check if the cuts generate the “  + ” peak artificially by analyzing KKN phase space MC sample  MC sample LH2 data

22. 22 KKN phase-space MC data after applying the same selection cuts. MM  (GeV) MM  (GeV) MM  (GeV) No narrow peak in MM( ,K - ). Symmetric between K + and K -.

23. 23  MC data after the cuts. MM  (GeV) MM  (GeV) No narrow peak in MM( ,K - ).  contribution is estimated to be less than 15% of the final sample.

24. 24 MM( ,K - ) for the LH2 data MM  (GeV) 0.20

25. 25  + search in MM( ,K - ) of the LD2 data Apply the same selection cuts. See if the peak is reproduced. See if the peak has reasonable dependence on the  exclusion cut variation.

26. 26 MM( ,K - ) for the LD2 data 0.20 MM  (GeV)

27. 27 Summary of LD2 data analysis MM  (GeV) K + K - from LD2 target MM d ( ,K + K - )>1.89 GeV 0.89< MM Ｎ ( ,K + K - )<0.99 GeV  exclusion cut at R=0.2 Fermi motion correction Reliable background estimation is essential to confirm the existence of the peak. Statistics of LH2 data is small.  increase statistics by mixed event technique (K. Hicks, Ohio univ.)

28. 28 Mixed event analysis with KKN phase space MC data Mix K +, K -,  from different events. Apply the same selection cuts again on the mixed events. Check if the shape of the original distribution is reproduced by the mixed events. MM  (GeV) Mixed event analysis seems to work fine for the exclusive reaction!

29. 29 Can we remove fluctuations? Split to 9 samples. Hist: KKN MC data Black: Mixed (all) Red: Mixed (from smaller # of events)

30. 30 Mixed event analysis MM  (GeV) MM  (GeV) LH2 mixed events are obtained by removing L(1520) contributions. The mixed event spectra are compared with the LD2 missing mass spectra.

31. 31 After removing  (1520) MM  (GeV) Background level around 1.53 GeV in 4 bins is ~220 events IF we take the mixed event BG method. The excess above the BG level is ~90 events. The peak position, width, significance strongly depends on the BG shape. The mixed event BG method may not work if the major BG is due to narrow resonances in K - p or K + K - channels. We need further BG study and it is in progress.

32. 32 Summary and outlook Evidence for an S=+1 baryon (  + ) around 1.54 GeV with a narrow width has been observed. LEPS higher statistics experiment has re- observed the peak. – unlikely to be due to statistical fluctuations. Need to understand the “background” shape. Further analysis in progress. Experiment with larger acceptance detector (TPC) is planned in near future.