1 Introduction and status of the LEPS Status of the  + Study New LEPS LD2 data Summary Recent results from LEPS T. Nakano （ RCNP, Osaka University ） Bejing, August 30, 2004

2 The LEPS collaboration Research Center for Nuclear Physics, Osaka University D.S. Ahn, M. Fujiwara, T. Hotta, K. Kino, H. Kohri, T. Matsumura, T. Mibe, T. Nakano 、 A. Shimizu, M. Sumihama Pusan National University J.K. Ahn, J.Y. Park Konan University H. Akimune Japan Atomic Energy Research Institute / SPring-8 Y. Asano, N. Muramatsu Institute of Physics, Academia Sinica, Taiwan W.C. Chang, T.H. Chang, D.S. Oshuev, C.W. Wang, S.C. Wang Japan Synchrotron Radiation Research Institute (JASRI) / SPring-8 S. Date, H. Ejiri, N. Kumagai, Y. Ohashi, H. Ookuma, H.Toyokawa, T. Yorita Ohio University K. Hicks Kyoto University K. Imai, M. Miyabe, M. Niiyama, T. Sasaki, M. Yosoi Chiba University H. Kawai, T. Ooba, Y. Shiino Yamagata University T. Iwata Wakayama Medical University S. Makino Nagoya University T. Fukui Osaka University H. Nakamura, M. Nomachi, A. Sakaguchi, Y. Sugaya, University of Saskatchewan C. Rangacharyulu Institute for High Energy Physics (IHEP), Moscow P. Shagin Laboratory of Nuclear Science, Tohoku University H. Shimizu, T. Ishikawa University of Michigan K. Yonehara Michigan State University R.G.T. Zegers Seoul National University H. Fujimura Miyazaki University T. Matsuda, Y. Toi

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

4  LEPS detector 1m1m

5 Current Setup of TPC experiment Main groups: Kyoto University Academia Sinica (Taiwan) Pusan National University (Korea)

6 Current experimental Setup Solenoid Magnet Dipole Magnet TPC Buffer Collimator 24Φ Collimator 2 １ Φ Beam Physics Run started from Jan. 2004

7 Near future plan Run with the current TPC setup until December, Energy upgrade to E MAX =2.9 GeV in September, –Above the theshold of  p   + K -  +. New TPC with LH 2 or LD 2 targets in January, –Use the same readout system of the current TPC.

8  + Baryon M  *Y] MeV D. Diakonov, V. Petrov, and M. Polyakov, Z. Phys. A 359 (1997) 305. Exotic: S=+1 Low mass: 1530 MeV Narrow width: ~ 15 MeV J p =1/2 +

9 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 First evidence from LEPS  n  K + K - n  peak in events without a proton.  1.54  0.01 MeV  < 25 MeV Gaussian significance 4.6  Phys.Rev.Lett. 91 (2003) hep-ex/ ++

10 Summary of Experiments WhereReactionMassWidth  ’s* LEPS  C  K + K - X < DIANAK + Xe  K 0 p X < 94.4 CLAS  d  K + K - p(n) < SAPHIR  p  K + K 0 (n) < ITEP A  K 0 p X < CLAS  p   + K - K + (n) < HERMESe + d  K 0 p X ~5 ZEUSe + p  e’K 0 p X ~5 COSYpp  K 0 p  < *Gaussian statistical significance: estimated background fluctuation

11 Mass M = 1.54 GeV: SPring-8, ITEP, CLAS-d –These were the first 3 publications M = GeV: CLAS-p –This one has over 7  significance! M = 1.53 GeV: HERMES, ZEUS, COSY –These are the most recent ones … A few % difference from zero, but ~20% difference from the KN threshold. –We need to find better ways to estimate the experimental uncertainties.

12 COSY result Before the acceptance correction.  MeV  MeV  b pp  K 0 p  

13 Width Again, there is inconsistency: –Most measurements are only upper limits. –DIANA has  < 9 MeV. –The cross-section implies  =0.9 MeV. –HERMES:  = stat. (+- 3 sys.) MeV –ZEUS:  = stat. (+- 5 sys.) MeV –Arndt et al. and Cahn et al. analysis of KN phase shifts suggests that  < 1 MeV !! The small width is the hardest feature for theorists to understand …

14 DIANA/ITEP Result hep-ex/  MeV  MeV K +  Xe  →    p X    n →    p) P K+ < 530 MeV/c Require  K <100deg. &  p <100 deg. Remove cos  pK <0  back-to-back    n →   Cahn and Trilling hep-ph/    →    n  MeV consistent with KN phase shift analysis.

15   signal in the KN data? Input mass background (non-reson.) Gibbs, nucl-th/ Widths range: MeV 0.9 MeV = solid Conclude: width  must be ~1 MeV

16   width from the KN database Nussinov (hep-ph/ ):   < 6 MeV Arndt et al. (nucl-th/ ):   < 1 MeV Haidenbauer (hep-ph/ ):   <5 MeV Sibertsev (hep-ph/ ):   < 1 MeV Gibbs (nucl-th/ ):   ~ 0.9

17 HERMES: e + d  K 0 p X Detect K 0      Nice clean peak. Complicated background due to  * resonances  shaded: Pythia MCmixed events Session 8B Dueren

18 ZEUS : e + p  e’K 0 p X The peak only shows up clearly for Q 2 > 20 GeV 2. Note (inset) that the peak appears in both K 0 p and K 0 p albeit with different widths.

19 HERA-B (Germany): –reaction: p+A at 920 GeV –measured: K - p and K 0 p invariant mass –Clear peak for  (1520), no peak for  + –production rate:  + /  (1520)<0.027 BES (China): –reaction: e + e -  J/    +  - –limit on B.R. of ~10 -5 Null Results And many unpublished negative results (HyperCP, CDF, E690, BaBar, LEP,,,). Session 13B Zivko

20 Slope for mesons Slope for baryons Slope for pentaquarks??

21 Questions to be answered To be or not to be? What is the true mass? How narrow is the width? What is the Spin and Parity? How to measure? Is there J + =3/2 + partner? Still narrow? Other members of the anti-decuplet? Can the peak seen in the previous LEPS data be seen in the new data again?

22 LEPS New LD2 and LH2 runs Data taken from Oct to Jun ~ 2 ・  photons on a 15cm-long LD2 target. Less Fermi motion effect. LH2 data were taken in the same period with ~ 1.4 ・  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

23 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

24  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. Realistic MC  event generator is available.  see Mibe’s poster.

25 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)

26 Energy dependent  exclusion cut E  (GeV) R=0.01 R=0.05 R=0.20

27 Cut dependence of K + missing mass for the LH2 data MM  (GeV) R=

28 Cut dependence of K + missing mass for LD2 data MM  (GeV)

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

30 Comparison of MM d ( ,K + K - ) MM  (GeV) Remove  d  KKd contributions by requiring MM d ( ,KK)>1.89 GeV

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

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

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

34 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 -.

35  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.

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

37  + 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.

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

39 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.)

40 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!

41 Can we remove fluctuations?

42 Can we remove fluctuations? MM  (GeV)

43 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.

44 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.

45 Summary Evidence for an S=+1 baryon around 1.54 GeV with a narrow width has been observed by many experimental groups. There are some inconsistencies in the measured masses and widths. No signal has been observed in high energy experiments with high statistics and good mass resolution. The  + does not exist or its production in high energy reactions must be highly suppressed.

46 LEPS new exp. re-observed the peak. –Unlikely to be due to statistical fluctuations. –Further checks and BG study are in progress. The issue will be settled in near future. –New dedicated experiments with high statistics are on-going, scheduled, or planned at several labs (Jlab, KEK, BNL, JINR, e.t.c.).

47 Comparison of Fermi motion corrections MM  (GeV) No large difference among the three Fermi motion corrections.

48 Comparison of Fermi motion corrections MM  (GeV) No large difference among the three Fermi motion corrections. The resolution of the 2 nd order correction is slightly worth than the others.

49 MM( ,K + ) vs. MM( ,K - ) MM  (GeV) MM  (GeV) LH2LD2

50 After removing  (1520) contribution No large difference among the three Fermi motion corrections.

51 E  vs. MM( ,K) MM  (GeV) MM  (GeV) Acceptance correction is necessary to get true E  dependence.

52 MM( ,K - ) –no MM d cut

53 Summary LEPS took new data with LD2 target for the search of the  +. Energy dependent  exclusion cut was designed without looking at K missing mass spectra and its validity was checked with  (1520) study. The “  + ” peak (dip) was reproduced in the K - missing mass of LD2 data. – unlikely to be due to statistical fluctuations. Background level was estimated by a mixed event technique using LH2 data. Further study is in progress to have better understanding of the background.