Recent results and future prospects of the Laser Electron Photon experiment at SPring-8

1. 第六届中日核物理研讨会,上海, 2006/05/18 Recent results and future prospects of the Laser Electron Photon experiment at SPring-8 T. Hotta (RCNP Osaka University) the LEPS collaboration • LEPS experiment • Search for the Q+ pentaquark • Upgrade of the LEPS facilty  LEPS2

2. Laser Electron Photon beamline at SPring-8 • SPring-8: world’s largest synchrotron radiation facility (8 GeV) • LEPS beamline: ~3 GeV polarized photon beam produced by backward Compton scattering of Laser photon.

3. LEPS Experiment Magnetic spectrometer in the forward direction (p±, K±, p, d) Beam: GeV photon Target: p, d, A g 1m

4. The LEPS Collaboration Research Center for Nuclear Physics, Osaka University：T. Nakano, D.S. Ahn, M. Fujiwara, K. Horie, T. Hotta, Y. Kato, K. Kino, H. Kohri, N. Muramatsu, T. Sawada, T. Tsunemi, M. Uchida, M. Yosoi, R.G.T. Zegers Department of Physics, Pusan National University：J.K. Ahn, J.Y. Park School of Physics, Seoul National University：H.C. Bhang, K.H. Tshoo Department of Physics, Konan University：H. Akimune Japan Atomic Energy Research Institute / SPring-8：Y. Asano, A. Titov Institute of Physics, Academia Sinica：W.C. Chang, D.S. Oshuev, Japan Synchrotron Radiation Research Institute (JASRI) / SPring-8：H. Ejiri, S. Date', N. Kumagai, Y. Ohashi, H. Ohkuma, H. Toyokawa, T. Yorita Department of Physics and Astronomy, Ohio University：K. Hicks, T. Mibe Department of Physics, Kyoto University：K. Imai, H. Fujimura, M. Miyabe, Y. Nakatsugawa, M. Niiyama Department of Physics, Chiba University：H. Kawai, T. Ooba, Y. Shiino Wakayama Medical University：S. Makino Department of Physics and Astrophysics, Nagoya University：S. Fukui Department of Physics, Yamagata University：T. Iwata Department of Physics, Osaka University：S. Ajimura, M. Nomachi, A. Sakaguchi, S. Shimizu, Y. Sugaya Department of Physics and Engineering Physics, University of Saskatchewan：C. Rangacharyulu Department of Physics, Tohoku University：M. Sumihama Laboratory of Nuclear Science, Tohoku University：T. Ishikawa, H. Shimizu Department of Applied Physics, Miyazaki University：T. Matsuda, Y. Toi Institute for Protein Research, Osaka University：M. Yoshimura National Defense Academy in Japan：T. Matsumura

5. Pentaquark Q+ The antiquark has a different flavor than the other 4 quarks. D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305. • Exotic: S=+1 • Low mass: 1530 MeV • Narrow width: ~ 15 MeV • Jp=1/2+ M = [1890-180*Y] MeV

6. : Positive result : Negative result Time dependent experimental status ofQ+ LEPS-d2 CLAS-d2 CLAS-d1 LEPS-d LEPS-C CLAS g11 SAPHIR CLAS-p BELLE DIANA BaBar ZEUS nBC Hermes SPHINX JINR SVD2 HyperCP SVD2 ALEPH, Z COSY-TOF HERA-B BES J,Y CDF FOCUS WA89 2002 20032004 2005 • Both positive & negative results exist. • Confirmation of the existence is still most important. • Theoretical attempt to explain observation vs. non-observation. •  Hosaka: May 20, Session 15 in this symposium.

7. First evidence from LEPS Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020 g nK+K-n Low statistics: but Tight cut: 85% of events are rejected by the f exclusion cut. Unknown background: BG shape is not well understood. Events from a LH2 target were used to estimate it. Possible kinematical reflections. Correction: Fermi motion correction is necessary. Q+

8. LEPS LD2 runs • Collected Data (LH2 and LD2 runs) Dec.2000 – June 2001LH2 50 mm~5×1012 photons published data (target: 5mm plastic scint.) May 2002 – Apr 2003LH2 150 mm~1.4×1012 photons Oct. 2002 – June 2003LD2 150 mm~2×1012 photons • #neutrons × #photons in K＋K－detection mode LD2 runs ~ ×5 statistics expected # of Q+ : 17  85

9. Search forQ+ing nK+K-n • A proton is a spectator (undetected). • Fermi motion is corrected to get the missing mass spectra. • Tight f exclusion cut is essential. • Background is estimated by mixed events. g pK+K-p g nK+K-n Q+ L(1520) Counts/12.5 MeV Counts/12.5 MeV preliminary preliminary MMgK+(GeV) MMgK-(GeV)

10. Q+ search in g d  L(1520) KN reaction • Θ＋is identified by K－p missing mass from deuteron. ⇒ No Fermi correction is needed. • K- n and pn final state interactions are suppressed. • Isospin selection: some reaction mechanism may favor specific isospin state. • CLAS(d)@JLAB: s<450 pb. • acceptance coverage is different from LEPS (forward). γ Θ＋ ? L(1520) p K－ n detected p

11. A possible reaction mechanism • Q+ can be produced by re-scattering of K+. • K momentum spectrum is soft for forward going L(1520). missing momentum γ L(1520) LD2 K+/K0 p/n Q+ n/p Formation momentum • Exchanged kaon can be on-shell. • KN  Q+ formation cross section. Pmiss GeV/c

12. Background process • Quasi-free L(1520) production must be the major background. • The effect can be estimated from the LH2 data. γ L(1520) p n n K+ The other background processes which do not have a strong pK- invariant mass dependence can be removed by sideband subtraction.

13. Event selections in K－p mode K＋mass : 0.40 – 0.62 GeV/c2 Λ(1520) : 1.50 – 1.54 GeV/c2 Non-resonant KKp + p + … γp→K－pKπ π－mis-ID as K－ MMp(γ,K－p) GeV/c2 M(K－p) GeV/c2 Events with Λ(1520) production were selected. E > 1.75 GeV was also applied.

14. Sideband BG estimation L(1520) LD2 Counts/5 MeV correction for f contribution M(K－p) GeV/c2 MMd(γ,K－p) GeV/c2 • 10-MeV Eg smearing has been applied to remove fine structure due to fluctuation. • No visible signal. • Validity of the sideband method with Eg smearing was checked by using two independent regions of the sideband.

15. K－p missing mass spectrum Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level. LD2 Q+ Counts/5 MeVC 1.53-GeV peak: (in the 5 bin = 25 MeV) preliminary No visible signal in sidebands. preliminary Counts/5 MeV MMd(γ,K－p) GeV/c2 sideband Quasi-free L* MMd(γ,K－p) GeV/c2 sum Normalization of L* is obtained by fit in the region of MMd < 1.52 GeV.

16. Remove high frequency fluctuations by 10-MeV Eg smearing • Real peak or fluctuation? • Within existing data, robustness of the peak has been checked by Eg smearing (smoothing of the spectra). • Fine structure (< experimental resolution) must be statistical fluctuations. All data & sideband BG data -BG Counts/5 MeV Counts/5 MeV preliminary preliminary MM(K－p) GeV/c2 MM(K－p) GeV/c2

17. γ Θ＋ L(1116) p p－ n forward angle detection p Search forg d  L(1116) Q+ • Normalization factor for LH2 data (green line) is 2.6.  No large p/n asymmetry. • Quasi-free process can be reproduced by free process.  small effect from Fermi motion. • Large cross-section compared with L(1520). • Missing Mass resolution is worse. • No excess at 1.53 GeV nor at 1.6 GeV. Counts/5 MeV preliminary MMd(γ,p－p) GeV/c2

18. Summary of Q+ search at LEPS Confirmation of + by using LD2 data • K+K- detection mode: • 1.53 GeV/c2 peak observed ~ expected # of events. • BG estimation is still underway. • K-p detection mode in MMd(g,pK-) spectrum: • 1.53 GeV/c2 peak (4-5σ,preliminary) + 1.6 GeV/c2 bump associated with L(1520) production • Signal-like behavior. [M(pK-) gate, Eg dependence, & robustness of the peak under smearing.]

19. Prospects Negative and positive results on Q+: • Negative results restrict the possibility of Q+ existence. • New CLAS@JLAB data denied their evidence. But their kinematical condition is different from LEPS. • There are still some positive evidences which cannot be excluded completely. • e.g. DIANA exp. (hep-ex/0603017) • If Q+ exist, production mechanism should be also exotic. Further measurements with wider kinematical coverage & high statistics are important. • LEPS: Experiment w/ Large solid angle TPC will start soon. • LEPS2: New beamline is proposed.

20. Beamline map of Spring-8 BL31ID

21. Schematic view of the LEPS2 facility High intensity：Multi laser LRNB Round beam ~10 7 photons/s（LEPS ~10 6 ） High energy：Re-injection of X-ray from undulator （Tohoku U., JASRI） Eg < 7.5GeV (LEPS < 3GeV) Backward Compton Scattering 8 GeV electron Recoil electron (Tagging) 30mlong line （LEPS 7.8m) Laser or re-injected X-ray modify： JASRI Acc. group a) SPring-8 SR ring GeV g-ray Inside building Outside building b) Laser hutch 5 m 4pg detector（Tohoku U.） Large decay spectrometer Forward spectrometer New DAQ system Plan to move the BNL-E949 detector to Spring-8 (helped by RIKEN) c) Experimental hutch

22. Schedule 2006 2007 2008 2009 Decomposition of E949 detector (BNL) Moving E949 det. Construction of the decay spectrometer Detector commisioning Construction of the forward spectrometer Submit LEPS2 proposal Apply BL modification High speed DAQ system Start experiment Modify SR ring Construction of the beamline Beam commisioning Building Exp hutch Infrastructure Radiation shield Laser injection system 4p photon detector X-ray re-injection system R&D of polarized HD target Experiment with HD target

23. LEPS2 Collaboration • Core institutes RCNP, Osaka U. LNS Tohoku U. JASRI (SPring-8) Accelerator group RIKEN Radiation Laboratory (new !) • Other LEPS members Kyoto U., Osaka U., Konan U., Chiba U., Yamagata U., Miyazaki U., Pusan U.(Korea), Seoul U.(Korea), Academia Sinica (Republic of China), Ohio U. (USA), Saskachewan U. (Canada) …etc. • New members National Defense Academy, … some members from the BNL-E949 experiment (domestic and foreign institutes (BNL, TRIUMF,…)) We are trying to expand the collaboration !!