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Overview of Pentaquark Searches

Overview of Pentaquark Searches. T. Nakano RCNP, Osaka University. SQM2006 @ UCLA, March 27, 2006. Outline. Introduction Status of Q + study New results from LEPS Other recent results Summary. Pentaquark. The antiquark has a different flavor than the other 4 quarks.

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Overview of Pentaquark Searches

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  1. Overview of Pentaquark Searches T. Nakano RCNP, Osaka University SQM2006 @ UCLA, March 27, 2006

  2. Outline • Introduction • Status of Q+ study • New results from LEPS • Other recent results • Summary

  3. Pentaquark 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

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

  5. : Positive result : Negative result Time dependent experimental status of Q+ 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 2002200320042005

  6. FOCUS FOCUS HERA-B HyperCP SPHINX + more CDF CDF CDF Non-evidence for Pentaquarks BABAR BES X-- DELPHI X-- Q0c

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

  8. + N( Θ ) < 0.2% * N( Λ ) CLAS: New high statistics exp.Search for Q+ in g pK+Ksn Counts/4 MeV Counts/4 MeV -0.8 < cosqCM < -0.6 preliminary M(nK+)(GeV) Counts/4 MeV 0.6 < cosqCM < 0.8 M(nK+)(GeV) M(nK+)(GeV) R. De Vita, APS April meeting, 2005

  9. g K0 K* n p Q+ K+ Impact of the CLAS proton result • A 5s evidence turned out to be wrong! • If there is no large isospin asymmetry in the elementary process, the g(*)d and g(*)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/0502143, hep-ph/0503149[PRD], hep-ph/0505134 Lipkin and Karliner, hep-ph/0506084

  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). • Reducedc2=1.15 for the fit in the mass range from 1.47 to 1.8 GeV/c2 • 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 gd’pK-K+n requires re-scattering (FSI). • The upper limit on the measured cross section in the reaction gdQ+pK-, with Pp>0.35 GeV/c, is about 450 pb (95.4% CL). • The upper limit on the cross section of the elementary process gnQ+K- is 4-20 nb, model dependent.

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

  13. 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+

  14. LEPS LD2 runs • Collected Data (LH2 and LD2 runs) Dec.2000 – June 2001LH2 50 mm~5×1012 photons published data 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 = 5mm-thick STC in short LH2 runs × ~5

  15. Search for Q+ in g 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 L(1520) Counts/12.5 MeV Counts/12.5 MeV preliminary preliminary MMgK+(GeV) MMgK-(GeV)

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

  17. 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 • LEPS acceptance has little overlap with CLAS acceptance. • Exchanged kaon can be on-shell. Pmiss GeV/c

  18. 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.

  19. Sideband subtraction to remove non-resonant background L(1520) LD2 L(1520) LH2 LD2 Counts/5 MeV M(K－p) GeV/c2 M(K－p) GeV/c2 MMd(γ,K－p) GeV/c2 1.50 < M(K－p) < 1.54 Fluctuations in the sideband spectra are removed by smearing Eg by 10 MeV (nearly equal to the resolution). 1.45 < M(K－p) < 1.50 or 1.54 < M(K－p) < 1.59 - 0.4 S =

  20. BG estimation with two independent sideband regions Counts/5 MeV correction for f contribution M(K－p) GeV/c2 MMd(γ,K－p) GeV/c2 • Validity of the sideband method with Eg smearing was checked by using two independent regions of the sideband. • Channel-to-channel comparison gives • mean=-0.04 and RMS=2.0. MMd(γ,K－p) GeV/c2

  21. K－p missing mass spectrum Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level. 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 L* MMd(γ,K－p) GeV/c2 sum Normalization of L* is obtained by fit in the region of MMd < 1.52 GeV.

  22. Remove high frequency fluctuations by 10-MeV Eg smearing Counts/5 MeV Counts/5 MeV preliminary preliminary MM(K－p) GeV/c2 MM(K－p) GeV/c2

  23. Counts/5 MeV Counts/5 MeV preliminary preliminary MM(K－p) GeV/c2 M(K+K-) GeV/c2 MM(K－p) GeV/c2 Remove f background by rejecting events with Pp<0.55 GeV/c The Q+ peak nor the bump at 1.6 GeV is not associated with f events.

  24. γ Θ＋ L(1116) p p－ n forward angle detection p Search for g 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

  25. 445<pbeam< 525 MeV/c pbeam<445 MeV/c pbeam>525 MeV/c Kaon Scattering DIANA/ITEP K+ + XeQ++Xe’(K0++p) +Xe’ Secondary kaons produced in the detector materials and then interacting within bubble chamber. More pictures were under analysis. hep-ex/0603017

  26. K+ Q+ n BELLE – Low energy K+N scattering R. Mizuk e+e--> K+/- X, K+/-A -> pK0, pK- Detector Tomography momentum spectra of K+ and K- 1 / 50MeV momentum, GeV/c Momentum range possibly contributing to Q+ formation. =>Determine resonance width

  27. DIANA NΘ+ Nch σch 107mb Bi Bf ΓΘ+= Δm N / 2 MeV/c2 mpKs (GeV/c2) Belle – Limit on Q+ Width GQ+ from K+A -> pK0sX & K+D -> inclusive analysis K+A -> pK0s Belle limit 90%CL 397 fb-1 Belle: G < 0.64 MeV (90% CL) @ M = 1.539 GeV G< 1 MeV (90% CL) @ M = 1.525–1.545 GeV Cahn,Trilling,PRD69,11501 (2004). Not inconsistent with previous results.

  28. Other pentaquarks Existence of the X3/2(1862) andthe Qc0 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.

  29. pK+ and pK- from 18.6 M d+Au at 200 GeV Background – Combinatorial and Correlated Pairs STAR Pentaquark Search dAu results Q++  p + K+ M (GeV/c2)

  30. dAu results D++ The invariant mass distribution is fitted to a Gaussian plus a linear function. A 3.5-5.0 sigma signal is seen Measured mass is about 1.53 GeV/c2. Full width is about 15 MeV

  31. Summary • Recent null results restrict the possibility that a pentaquarkexists 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 Q+.

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