25 May 2005SPring-8/LEPS GeV photon beam experiments at the SPring-8 laser-backscattering facility Introduction LEPS (Laser-Electron.

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Presentation transcript:

25 May 2005SPring-8/LEPS GeV photon beam experiments at the SPring-8 laser-backscattering facility Introduction LEPS (Laser-Electron Photon at SPring-8) facility Some experimental results using the forward spectrometer - photon beam asymmetry for the K + photoproduction -  meson photoproduction - search for pentaquark  + Present status and future prospect M. Yosoi, RCNP  Kyoto for the LEPS collaboration

25 May 2005SPring-8/LEPS Characteristics of BCS photons (BCS:Backward Compton Scattering) rather flat energy distribution with small spreading (Unlike the Bremsstrahlung, where low energy photons are dominated, ~1/E  ) high linear- or circular- polarization photon energy can be tagged by recoil electron E e =8 GeV =351 nm

25 May 2005SPring-8/LEPS Synchrotron Radiation Rings with laser-backscattering facilities 2.8 GeV 6 8GeV ~100mA

25 May 2005SPring-8/LEPS With LEPS, what can be aimed at ? Threshold region of  (ss) meson and hyperon resonances Key words : 1. Forward angle measurement including 0 deg. 2. Polarization observables 3. Strangeness

25 May 2005SPring-8/LEPS LEPS facility

25 May 2005SPring-8/LEPS Beam line map of Spring-8

25 May 2005SPring-8/LEPS Schematic view of the LEPS facility b) Laser hutch a) SPring-8 SR ring c) Experimental hutch Inverse Compton  -ray Laser light 8 GeV electron Recoil electron Tagging counter Collision Energy spectrum of BCS photons Bremsstrahlung Backward-Compton scattering 36m 70m SSD + Sc phodoscope ScFi + Sc phodoscope

25 May 2005SPring-8/LEPS LEPS forward spectrometer 2m Target LH 2, LD 2, etc. AC index = 1.03 to reject e + e - pairs SSD 120  m pitch DCs  ~ 200  m Magnet 135 x 55 cm 2, ( 35 o x 15 o ) B = 0.7T ( ~10 6 /sec )

25 May 2005SPring-8/LEPS Particle identification TOF : RF signal - TOF wall,  t = 120 ps Momentum : SSD, DCs, Tracking  p ~ 6 MeV/c for 1 GeV/c K Reconstructed mass spectra

25 May 2005SPring-8/LEPS Other detectors : 1. EM calorimeter for  0  2,   2 Main detector Lead scintillating fiber 252 modules Covered solid angle 2.08  (str)  : 30 o ~ 100 o  : 0 o ~ 360 o Length of each module 22cm ( 13.7 X 0 ) Angular interval (segment) 10 degree Energy resolution ( 6.8% for 1 GeV photon)

25 May 2005SPring-8/LEPS Other detectors : 2. TPC for Hyperon Resonances B 3/2 - 1/2 -  )  ) 30 MeV  (1405) : 3-quark state or KN bound state ? KN

25 May 2005SPring-8/LEPS Photon beam asymmetry for the p(,K + ) and p(,K + ) 0 reactions R.G.T. Zegers et al. PRL29,092001

25 May 2005SPring-8/LEPS Missing resonances N * and  * Knowledge of N* and D* is essential to understand the internal structure of baryons. Many nucleon resonannces predicted by quark models are still missing. So far,  N channel  K  or K  channel K+K+ Y(   ) p  N *,  * W~ 1.9 GeV without D 13 with D 13 SAPHIR data D 13 (1895) resonance ? Recent CLAS data show more than one resonance !

25 May 2005SPring-8/LEPS Description of K photoproduction Choice of included resonances Coupling constant Hadronic form factors Treatment of background terms Need more study to fix parameters. Additional observables are useful for further studies. There are the data of cross sections and recoil polarizations from SAPHIR and CLAS collaborations. Photon beam asymmetry is one of the good candidates. Ambiguities tree-level effective-Lagrangian approach - should be cautious to define conclusions with cross sections only

25 May 2005SPring-8/LEPS Missing mass spectrum and photon beam asymmetry  For all events N v(h) : normalized yield of K + photoproduction for vertical (horizontal) pol.  : K + azimuthal angle P  : Polarization of photon  and  0 events 2  cuts : contamination < 2%

25 May 2005SPring-8/LEPS Experimental Results and comparison with model calculations Mart & Bennhold PRC61, (with D 13 ) Janssen et al. PRC65, , PRC66, Currently no models reproduce our data consistently.  Strong conclusions for D 13 etc., are very premature. Data : positive sign become large as E 

25 May 2005SPring-8/LEPS  meson photoproduction on protons near threshold cf.)  photoproduction from Li, C, Al, and Cu has also been measured  study modification of  in the nuclear medium (T. Ishikawa et al. PLB608, 215)

25 May 2005SPring-8/LEPS Vector meson photoproduction  p    p  uud Pomeron   (~ss)  p uud ??? Pomeron exchange Meson exchange Glueball exchange ?? hep-ph/ ,

25 May 2005SPring-8/LEPS K  K  invariant mass distributions Missing mass ( , Kp)X (GeV) Selection of K  K  p final state: 3  cut in missing mass Missing mass resolution  =10 MeV Invariant mass resolution  =2-3 MeV K  K  invariant mass (GeV) Missing mass ( , K  K  )X K  K  invariant mass (GeV)  selection cut: |M KK |< 10 MeV KKKK K  p p K   BG from hyperons BG from KKp  BG subtraction by using weighted MC which fits to the real data.

25 May 2005SPring-8/LEPS Differential cross sections Solid : fit with E  independent slope Dashed : fit with E  dependent slope

25 May 2005SPring-8/LEPS Differential cross section at t=-|t| min Curve : Pomeron + Pseudo scalar exchange model (A. Titov et. al, PRC 67, ) A peaking structure is seen in d  /dt near E  =2 GeV, which has not been explained by model calculation. Smaller t-slope near threshold.

25 May 2005SPring-8/LEPS Decay polarization observables with linearly polarized photon Decay Plane //  natural parity exchange (-1) J (Pomeron, 0 + glueball, Scalar mesons) Photon Polarization KK KK KK Decay Plane  unnatural parity exchange -(-1) J (Pseudoscalar mesons ,  )   Relative contributions from natural, unnatural parity exchanges (U/UN) Decay angular distribution of  meson  →K  K  KK

25 May 2005SPring-8/LEPS Decay angular distribution W ~ sin 2  : Helicity-conserving process is dominant. Natural parity exchange is dominant. (no energy dependence) Forward angles; -0.2 < t+|t| min < 0 GeV 2 around the peak region of the cross section above the peak region ~2 GeV peak : mainly natural parity exchange Need more data above 2.4 GeV

25 May 2005SPring-8/LEPS Search for pentaquark  +

25 May 2005SPring-8/LEPS What are pentaquarks ? M  *Y] MeV D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305 (Chiral Soliton Model) Exotic: S = +1 Low mass: 1530 MeV Narrow width: ~ 15 MeV J  =1/2 + Minimum quark content : 5 quarks Quantum numbers of “Exotic” pentaquarks : not 3-quark Theoretical Prediction of  

25 May 2005SPring-8/LEPS First evidence of   from LEPS  n    K   K  K  n M = 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 ++ T. Nakano et al., PRL91,

25 May 2005SPring-8/LEPS Positive results Final state: K s + p K + + n (K s + p ) A few % difference from zero, but ~20% difference from the KN threshold. Statistics is not enough.

25 May 2005SPring-8/LEPS Negative results Many negative results have been reported with good statistics from high energy facilities. (HERA-B, HyperCP, CDF, BaBar, LEP, Belle, …) New CLAS data for  p  K s  + has shown no peak ! (April APS meeting) Cross section  p    K s ) < 1-4 nb Inconsistent with SAPHIR data

25 May 2005SPring-8/LEPS LEPS new search for   1. n    K   K  K  n in deuteron MM   (GeV) MM   (GeV) preliminary  (1520)  p  K  K  p  n  K  K  n LEPS new LH 2 /LD 2 data (Oct – Jun, 2003) Fermi motion is corrected to get the missing mass spectra  exclusion cut is essential Background is estimated by mixed events

25 May 2005SPring-8/LEPS LEPS new search for   2. d  (1520)   K  p     is identified by K  p missing mass from deuteron.  No Fermi correction is needed.  p n  KK p  p n  K－K－ p  (1520)

25 May 2005SPring-8/LEPS Major background process Quasi free  (1520) production must be the major background. The effect can be estimated from the LH 2 data  p n  (1520) K+K+ n LH2 LH2 distribution: comparison with MC MMd(γ,K － p) GeV/c 2

25 May 2005SPring-8/LEPS Event selection MMp( ,K － p) GeV/c 2 M(K － p) GeV/c 2  (1520) Select  (1520) in 1.50–1.54 GeV/c 2 ⇒ calculate K  p missing mass K mass is smeared by Fermi motion. (assumed proton at rest) LD 2 data: after selecting  (1520) inelastic events select LD2

25 May 2005SPring-8/LEPS K  p missing mass for (1520) production from deuteron MMd( ,K － p) GeV/c 2 MMd( ,K － p) GeV/c 2 Quasi-free  and non-resonant KKp  preliminary 1.53 GeV, width <10 MeV

25 May 2005SPring-8/LEPS Extracting (1520) contribution M(K  p) GeV/c 2  (1520) contribution can extracted by sideband subtraction method. N W  p   p reaction and non-resonant KKp production can contribute in the signal region. But they do not make a peak in K  p invariant mass. LH 2 contrib utions from , KKp,,,.

25 May 2005SPring-8/LEPS Side-band subtraction in K  p missing mass MMd( ,K － p) GeV/c 2 after side-band subtraction Confirmed that the observed excess is related with  (1520). Bump structure LD2 E  > 1.75 GeV preliminary

25 May 2005SPring-8/LEPS Summary has been in operation since 2000 for the study of the nuclear and particle physics. From the first LH 2 run with the forward spectrometer - photon beam asymmetry  of K + photoproduction  polarization observables are strong tools to pin down the missing baryon resonances. -  photoproduction near the threshold  found peaking structure around E=2 GeV, new mechanism ?  more study (LD 2, E upgrade) -  + was first discovered in n(,K - ) mode for n in the carbon of the trigger counter New LH 2 /LD 2 exp. Re-observed  + peak in two ways n(,K - ) mode (n in d), and d(,(1520)) mode

25 May 2005SPring-8/LEPS Present status and prospect  photoproduction from nuclear target (Li,C,Al,Cu) data taking completed in 2001, already published p(,K s ) experiment with gas cherenkov counter data taking completed in 2002 Experiment using the EM calorimeter with nuclear target (CH 2,C,etc) data taking completed in 2003 Hyperon resonance study with TPC and spectrometer for nuclear target (CH 2,C,Cu) data taking completed in 2004 Energy upgraded : 2.4 GeV  2.9 GeV now going on for the nuclear target LH 2 /LD 2 + TPC(new) GeV  will start after this summer

25 May 2005SPring-8/LEPS LEPS collaboration D.S. Ahn a, J.K. Ahn b, H. Akimune c, Y. Asano d, W.C. Chang e, S. Date f, H. Ejiri a,f, T. Emori j, H. Fujimura h,j, M. Fujiwara a,b, K. Hicks i, K. Horie a, T. Hotta a, K. Imai j, T. Ishikawa k, T. Iwata l, Y. Kato a,H. Kawai m, Z.Y. Kim h, K. Kino a, H. Kohri a, N. Kumagai f, S. Makino n, T. Matsumura a, N. Matsuoka a, T. Mibe a, K. Miwa j, M. Miyabe j, Y. Miyachi o, M. Morita a, N. Muramatsu d, T. Nakano a, Y. Nakatsugawa j, M. Niiyama j, M. Nomachi p, Y. Ohashi f, T. Ooba m, H. Ookuma f, D. S. Oshuev e, C. Rangacharyulu q, A. Sakaguchi p, T. Sasaki j, T. Sawada a, P. M. Shagin a, Y. Shiino m, H. Shimizu k, Y. Sugaya p, M. Sumihama a, T. Tsunemi a, H. Toyokawa f, A. Wakai o, C.W. Wang e, S.C. Wang e, K. Yonehara c, T. Yorita f, M. Yosoi j and R.G.T. Zegers a, a Research Center for Nuclear Physics (RCNP), Ibaraki, Osaka , Japan b Department of Physics, Pusan National University, Pusan , Korea c Department of Physics, Konan University, Kobe, Hyogo , Japan d Japan Atomic Energy Research Institute, Mikazuki, Hyogo , Japan e Institute of Physics, Academia Sinica, Taipei 11529, Taiwan f Japan Synchrotron Radiation Research Institute, Mikazuki, Hyogo , Japan h School of physics, Seoul National University, Seoul, Korea i Department of Physics, Ohio University, Athens, Ohio 45701, USA j Department of Physics, Kyoto University, Kyoto, Kyoto , Japan k Laboratory of Nuclear Science, Tohoku University, Sendai , Japan l Department of Physics, Yamagata University, Yamagata, Yamagata , Japan m Department of Physics, Chiba University, Chiba, Chiba , Japan n Wakayama Medical College, Wakayama, Wakayama , Japan o Department of Physics, Nagoya University, Nagoya, Aichi , Japan p Department of Physics, Osaka University, Toyonaka, Osaka , Japan q Department of Physics, University of Saskatchewan, Saskatoon, S7N 5E2, Canada r Department of Applied Physics, Miyazaki University, Miyazaki , Japan

25 May 2005SPring-8/LEPS Backup slides

25 May 2005SPring-8/LEPS

25 May 2005SPring-8/LEPS Recent experimental setup with TPC A TPC(Time Projection Chamber) is newly installed with a superconducting solenoidal Magnet (~ 2 T)

25 May 2005SPring-8/LEPS A possible reaction mechanism  + can be produced by re-scattering of K +. K momentum spectrum is soft for forward going  (1520).  p/n n/p  (1520) K + /K 0  Formation momentum LD2 P K GeV/c MMd( ,K  p) GeV/c 2 LH2

25 May 2005SPring-8/LEPS K  p missing mass in sideband regions MMd( ,K － p) GeV/c 2   (1520) K + /K 0   p  KK No peak!  + formation cross-section by simple kaon re-scattering should be small. LD2 A. Titov estimated it is small.