1 Overview of Pentaquark Searches T. Nakano RCNP, Osaka University UCLA, March 27, 2006

2 Outline Introduction Status of  + study New results from LEPS Other recent results Summary

3 Pentaquark 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 + The antiquark has a different flavor than the other 4 quarks.

4 DPP predicted the   with M=1530MeV,  <15MeV, and J p =1/2 +. Naïve QM (and many Lattice calc.) gives M=1700~1900MeV with J p =1/2 -. But the negative parity state must have very wide width (~1 GeV) due to “fall apart” decay. Theory For pentaquark Fall apart Ordinary baryons qq creation Positive parity requires P- state excitation. Expect state to get heavier. Need counter mechanism. diquark-diquark, diquark- triquark, or strong interaction with “pion” cloud? Positive Parity?

5 Time dependent experimental status of  + γ + d (n ) reactions γ + p → p K s 0 γ + p → n K + K - p + K + (N) → p K s 0 lepton + D, A → p K s 0 p + A → pK s 0 + X p + p → pK s 0 +  + Other  + Upper Limits BaBar CLAS-d2 BELLE ALEPH, Z SVD2 LEPS-d2 LEPS-C CLAS-d1 DIANA SAPHIR SVD2 COSY-TOF Hermes JINR CLAS-p LEPS-d BC ZEUS BES J,  CLAS g11 SPHINX HyperCP HERA-B FOCUS WA89 CDF : Positive result : Negative result

Non-evidence for Pentaquarks FOCUSBABAR BES CDF FOCUS SPHINX CDF DELPHI HyperCP HERA-B CDF 0c0c  -- + more

7 Slope for mesons Slope for baryons Slope for pentaquarks??

8 M(nK + )(GeV) Counts/4 MeV M(nK + )(GeV) Counts/4 MeV -0.8 < cos  CM < -0.6 preliminary 0.6 < cos  CM < 0.8 CLAS: New high statistics exp. Search for  + in  p  K + K s n R. De Vita, APS April meeting, % )N(Λ ) Θ *  

9 Impact of the CLAS proton result A 5  evidence turned out to be wrong! If there is no large isospin asymmetry in the elementary process, the   d and   A experiments with lower statistics should not be able to see the signal, and they are also wrong.  t-channel K* exchange amplitude does not have isospin asymmetry. However, many quark model predicts. Moreover, suppression of K* exchange leads to small cross-section in general. Theoretical attempts to explain large isospin asymmetry: Nam, Hosaka and Kim, hep-ph/ , hep-ph/ [PRD], hep-ph/ Lipkin and Karliner, hep-ph/ K0K0 p  K*K* ++ K+K+ n

10 New and old CLAS data Two distributions statistically consistent with each other: 26% c.l. for null hypothesis from the Kolmogorov test (two histograms are compatible). Reduced  2 =1.15 for the fit in the mass range from 1.47 to 1.8 GeV/c 2 G10 mass distribution can be used as a background for refitting the published spectrum. Preliminary

11 Final state interactions Minimum momentum of protons in CLAS >0.35 GeV/c. Detecting the high energy spectator, proton in the reaction  d ’ pK - K + n requires re-scattering (FSI). The upper limit on the measured cross section in the reaction gd   + pK -, with P p >0.35 GeV/c, is about 450 pb (95.4% CL). The upper limit on the cross section of the elementary process gn   + K - is 4-20 nb, model dependent.

12 From Carl Carlson ’ s talk at Hawaii pentaquark workshop Don’t give up so easily... pentaquark negative evidence

13 First evidence from LEPS  n  K + K - n Phys.Rev.Lett. 91 (2003) hep-ex/ ++ Low statistics: but Tight cut: 85% of events are rejected by the  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.

14 LEPS LD 2 runs Collected Data (LH 2 and LD 2 runs) Dec.2000 – June 2001 LH 2 50 mm ~5×10 12 photons published data May 2002 – Apr 2003 LH mm ~1.4×10 12 photons Oct – June 2003 LD mm ~2×10 12 photons #neutrons × #photons in K ＋ K － detection mode LD 2 runs = 5mm-thick STC in short LH 2 runs × ~5

15 Search for  + in  n  K + K - n MM  (GeV) MM  (GeV) A proton is a spectator (undetected). Fermi motion is corrected to get the missing mass spectra. Tight  exclusion cut is essential. Background is estimated by mixed events. preliminary L(1520)  p  K + K - p  n  K + K - n Counts/12.5 MeV

16  + search in  d   (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. If ss(I=0) component of a  is dominant in the reaction, the final state KN has I=0. (Lipkin) γ p n Θ＋Θ＋ K－K－ p  (1520) detected

17 A possible reaction mechanism  + can be produced by re-scattering of K +. K momentum spectrum is soft for forward going  (1520). γ p/n n/p  (1520) K + /K 0  missing momentum Formation momentum LD2 P miss GeV/c LEPS acceptance has little overlap with CLAS acceptance. Exchanged kaon can be on- shell.

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

19 Counts/5 MeV Sideband subtraction to remove non- resonant background S = < M(K － p) < < M(K － p) < 1.50 or 1.54 < M(K － p) < 1.59 LD2LH2 M(K － p) GeV/c 2  (1520) MMd(γ,K － p) GeV/c 2 LD2 Fluctuations in the sideband spectra are removed by smearing Eg by 10 MeV (nearly equal to the resolution).

20 BG estimation with two independent sideband regions M(K － p) GeV/c 2 MMd(γ,K － p) GeV/c 2 correction for  contribution Validity of the sideband method with E  smearing was checked by using two independent regions of the sideband. Channel-to-channel comparison gives mean=-0.04 and RMS=2.0. Counts/5 MeV

21 K － p missing mass spectrum MMd(γ,K － p) GeV/c 2 sideband  * sum Counts/5 MeVC Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level GeV peak: preliminary MMd(γ,K － p) GeV/c 2 Counts/5 MeV Normalization of  * is obtained by fit in the region of MMd < 1.52 GeV. (in the 5 bin = 25 MeV) preliminary No visible signal in sidebands. ++ ~

22 Remove high frequency fluctuations by 10-MeV E  smearing MM(K － p) GeV/c 2 Counts/5 MeV preliminary

23 M(K + K - ) GeV/c 2 Remove f background by rejecting events with P p <0.55 GeV/c MM(K － p) GeV/c 2 Counts/5 MeV preliminary The  + peak nor the bump at 1.6 GeV is not associated with  events.

24 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  (1520). Missing Mass resolution is worse. No excess at 1.53 GeV nor at 1.6 GeV. MMd(γ,  － p) GeV/c 2 Counts/5 MeV preliminary Search for  d   (1116)  + γ p n Θ＋Θ＋ －－ p  (1116) forward angle detection

25 K + + Xe  + +Xe’ (K 0+ +p) +Xe’ DIANA/ITEP Kaon Scattering Secondary kaons produced in the detector materials and then interacting within bubble chamber. More pictures were under analysis. p beam <445 MeV/cp beam >525 MeV/c 445<p beam < 525 MeV/c hep-ex/

26 momentum, GeV/c 1 / 50MeV momentum spectra of K + and K - K+K+ n ++ Momentum range possibly contributing to  + formation. BELLE – Low energy K + N scattering e + e -  K +/- X, K +/- A  pK 0, pK - R. Mizuk Detector Tomography  Determine resonance width

27 Belle – Limit on  + Width 397 fb -1 K + A  pK 0 s Belle limit 90%CL    MeV (90% M = 1.525–1.545 GeV Not inconsistent with previous results.    from K + A  pK 0 s X & K + D  inclusive analysis Belle:  < 0.64 MeV (90% M = GeV Γ Θ+ = Δm N Θ+ N ch σ ch 107mb B i B f Cahn,Trilling,PRD69,11501 (2004 ). m pKs (GeV/c 2 ) N / 2 MeV/c 2 DIANA

28 Existence of the  3/2 (1862) and the  c 0 is questionable. For the both cases, only one experiment has observed the positive evidence, while other experiments claim that their null results are incompatible with them. Other pentaquarks

29 pK + and pK - from 18.6 M d+Au at 200 GeV Background – Combinatorial and Correlated Pairs dAu results M (GeV/c 2 )  ++  p + K + STAR Pentaquark Search

30 dAu results The invariant mass distribution is fitted to a Gaussian plus a linear function. A sigma signal is seen Measured mass is about 1.53 GeV/c 2. Full width is about 15 MeV  ++

31 Summary Recent null results restrict the possibility that a pentaquark exists severely. If it exists, its production mechanism should be also exotic. There are still some positive evidences which cannot be excluded completely. Experiments with positive results should be repeated with higher statistics. Formation experiment with low energy K + beam will conclude the case for  +.