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Pentaquarks

Pentaquarks. Valery Kubarovsky Rensselaer Polytechnic Institute / Jefferson Lab. Introduction. More than 10 experiments have recently reported observation of the baryon with exotic quantum numbers Light: M=1525-1555 MeV Narrow: G < 9-25 MeV (possibly ~1 MeV)

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Pentaquarks

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  1. Pentaquarks Valery Kubarovsky Rensselaer Polytechnic Institute / Jefferson Lab

  2. Introduction • More than 10 experiments have recently reported observation of the baryon with exotic quantum numbers • Light:M=1525-1555 MeV • Narrow:G< 9-25 MeV (possibly ~1 MeV) • Strangeness S=+1 (opposite to the strangeness of the usual baryons) • This new state was identified as the Q+ pentaquark baryon with quark contents • There is also evidence for two related states with the strangeness S=-2, • And even charmed pentaquark Q0(3100) with quark contents Valery Kubarovsky PIC-2005

  3. Pentaquark in naïve quark model The spontaneous breakdown of the chiral symmetry would produce nonzero constituent mass and the massless pseudoscaler Goldstone bosons • Pentaquark mass = 4*350+470=1830 MeV • In addition there is some penalty for the p-wave (in case of the positive parity) • So the pentaquark mass must be more than 2 GeV in any constituent quark model • The predicted width is wide (>175 MeV) due to the allowed decay to the baryon and meson with mass well above the threshold • The ground state hasnegativeparity. Valery Kubarovsky PIC-2005

  4. “Correlated” Qurk Model Jaffe, Wilczek • The four quarks are bound into two • spin zero , color and flavor 3 diquarks [ud], ds, [su] (Bose statistics). • For identical diquarks , like [ud]2, the lightest state has negative space parity. So the q4q state has positive parity • The narrow width is described by relatively week coupling to the K+n continuum from which it differs in color, spin and spatial wavefunctions. _ L=0 (ud) s L=1 _ (ud) L=1, one unit of orbital angular momentum needed to obtain as in the cSM JP = ½+ Valery Kubarovsky PIC-2005

  5. Chiral Soliton Model _ • The extra QQ pair in the pentaquark is added in the form of a pseudo scalar Goldstone meson, which costs nearly zero energy. • In reality, to make the Q+ from the nucleon, one has to create a quasi-Goldstone K-meson and to confine it inside the baryon of the size >1/M. It costs roughly 600 MeV. • So the Q+ mass is near 1540 MeV. • G = 15 MeV • (D.Diakonov, V.Petrov and M.Polyakov, 1997) JP = ½+ Valery Kubarovsky PIC-2005

  6. Pentaquarks on the lattice • The only known method to derive hadronic properties from first principles is lattice QCD. • There have been a number of lattice studies to see if Q+ can be predicted from QCD. • It was consistently found that the lightest isospin I=0 state is below lightest isospin I=1 state but there was disagreement on the parity assignment of a possible Q+ baryon • There was also studies which did not find a pentaquark resonance, only scattering states of weakly-interacting Kaons and Nucleons. • So the consensus has not been reached but as was written in one of the articles “absence of evidence is not evidence of absence”. Valery Kubarovsky PIC-2005

  7. LEPS COSY-TOF pp  S+Q+. The initial evidence for Pentaquarks CLAS-D SAPHIR DIANA 4.8 4.4 5.2 4.6 CLAS-p Neutrino NA49 HERMES 6.7 7.8 4.2 6.7 ~5 1862MeV ZEUS SVD 5.6 5-6 ~5 4.6 Valery Kubarovsky PIC-2005

  8. Q+ Mass pK0 nK+ World Average: 1532.5±2.4 MeV • The pK0 peaks have the mass positions systematically lower than for the nK+ decay mode. • Shift could be due to different background shapes and interference effects • Or it may indicate the serious concern about the existence of the Q+ baryon Valery Kubarovsky PIC-2005

  9. JP = ½+ What do we know about the Q+ width? Widths seen in experimental analyses are dominated by resolution effects. More precise information is obtained in analyses with theoretical constraints. DIANA, Phys. Atom. Nucl. 66,1715,(2003) GQ <9 MeV HERMES, PLB585, 213 (2004) GQ = 17+/-9+/-3 MeV S. Nussinov et al., hep-ph/0307357 GQ < 6 MeV (non-observation) R. Arndt et al., PRC68, 42201 (2003) GQ < 1 MeV (non-observation) R. Cahn and G. Trilling, PRD69, 11401(2004) GQ= 0.9 +/- 0.3MeV (from DIANA results) A. Sibirtsev, et al., hep-ph/0405099 (2004) GQ < 1 MeV (K+d Kopp) W. Gibbs, nucl-th/0405024 (2004) GQ = 0.9 +/-0.2 MeV (K+d X) G(Q+) ~ 1 MeV Valery Kubarovsky PIC-2005

  10. First Wave Experiments Historical Overview Valery Kubarovsky PIC-2005

  11. LEPS : Q+ published mass plot M = 1.540.01 MeV G < 25 MeV Gaussian significance 4.6s • 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 background Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020 Valery Kubarovsky PIC-2005

  12. DIANA/ITEP (Moscow) Charge exchange reaction K+N]Q+]pK0S K0S]p+p- • 750 MeV K+ beam incident on 700 liters Xe bubble chamber. • Interaction energy is determined by the range of the kaon. • Charged particles are identified by ionization, momentum is measured by the range. Valery Kubarovsky PIC-2005

  13. DIANA • Selecting forward going protons and kaons - Qk and Qp < 100o. • p and K0 are emitted back-to-back - cosFpK < 0. • Peak at 1.539 GeV in the invariant mass of K0p. • Statistical significance 4.4s. • Measured width G < 9 MeV The best limit for width Valery Kubarovsky PIC-2005

  14. Experimental Hall B CLAS detector Experiments at JLAB Valery Kubarovsky PIC-2005

  15. Bremsstrahlung tagged photon facility, photon energy resolution ~0.2% Review of experiments: CLAS/JLAB CEBAF/Hall B CLAS Detector 6 Superconducting toroidal coils Electromagnetic calorimeters Beam line and the target Drift chambers TOF counters Cherenkov counters Valery Kubarovsky PIC-2005

  16. CLAS-gd: Exclusive reaction • Experimental data from 1999 run; • Tagged photons with up to 3 GeV energy; • Target: 10 cm long liquid deuterium; Exclusive photoproduction on deuterium: Possible reaction mechanism • No correction for Fermi smearing is needed. • Aids significantly to reduce the background. Valery Kubarovsky PIC-2005

  17. CLAS M = 1.542 GeV G < 21 MeV (sM=9 MeV) Stat.sig. = 5.2s (4.6-5.8) Q+ Gaussian background Simulated background Distribution ofL(1520) events Valery Kubarovsky PIC-2005

  18. CLAS: Photoproduction onhydrogen After PID n Valery Kubarovsky PIC-2005

  19. Q+ • Cosq*(p+) > 0.8 CLAS-gp: with forward going p+ • Fitted mass 1.555 GeV • G< 28 MeV consistent with detector resolution g p+ K- p p N* Q+ M(nK+) Valery Kubarovsky PIC-2005

  20. L(1520) preliminary preliminary g nK+K-n g pK+K-p • Dedicated experiment • Aimed for 4x statistics • of 2003 result • Announced at N*2004 LEPS/SPring-8 gdK+K-n(p) • A proton is a spectator (undetected). • Fermi motion is corrected to get the missing mass spectra. • Background is estimated by mixed events. MMgK-(GeV) MMgK+(GeV) Valery Kubarovsky PIC-2005

  21. Conclusion of LEPS experimental group • LEPS high statistics experiment has reconfirmed the peak, very unlikely to be due to statistical fluctuations. • The preliminary study shows no indication that the peak is generated by kinematical reflections, detector acceptance, Fermi-motion correction, nor cuts. • “existence ranges from very likely to certain, but further confirmation is desirable” - “three-star” definition by PDG. Valery Kubarovsky PIC-2005

  22. Second Generation Dedicated Experiments Valery Kubarovsky PIC-2005

  23. Search for Pentaquarks at Jlab A comprehensive program to search for pentaquarks with high statistics and high resolutionphotoproduction experimentsis in progress atJefferson Lab g10deuteronEg~ 1.0-3.5 GeVcompleted in 2004 Hall-ASearch for Q++, Socompleted in 2004 Search for Q+ planned for 2006/7 g11 proton Eg~ 1.6-3.8 GeVcompleted in 2004 eg3deuteronEg~ 4.0-5.4 GeVcompleted in 2005 Super-gprotonEg ~ 3.8 – 5.7 GeV planned for 2006/7 Valery Kubarovsky PIC-2005

  24. Q+(1540) ? • no structure is observed at a mass of ~1540 MeV nK+ Mass Spectrum Counts/4 MeV preliminary M(nK+)(GeV) • the nK+ mass spectrum is smooth Valery Kubarovsky PIC-2005

  25. Upper Limit on theQ+Cross Sectionmass dependence preliminary Upper limit (95% CL) s g p Q+ K0< 2 nb Valery Kubarovsky PIC-2005

  26. Kinematics • Selection of forward angles of the K0 in the g-p center of mass • Energy limited to 2.6 GeV • no hyperon rejection K0 g qCM p Q+ Comparison with SAPHIR results SAPHIR Observed Yields SAPHIR N(Q+)/N(L*) ~ 63/630 ~ 10% CLAS N(Q+)/N(L*) <110/53000 <0.2% (95%CL) Cross Sections SAPHIR sg p Q+ K0 ~ 300 nb reanalysis 50 nb CLAS sg p Q+ K0 < 2 nb (95%CL) Counts Counts cosqCM(K0) > 0.5 cosqCM(K0) > 0.5 M(nK0) (GeV) M(nK+) (GeV) g11@CLAS Counts Counts L(1520) preliminary cosqCM(K0) > 0.5 cosqCM(K0) > 0.5 Q+(1540) ? M(nK0) (GeV) M(nK+) (GeV) Valery Kubarovsky PIC-2005

  27. The new data show no signal • Set upper limit on cross section nosignal Fermi momentum only sQ+ < 5 nb (95% CL) model dependent. corrected for r.s. with L(1520) as a guide. CLAS – 2nd Generation Experiment II • In previous result the background is underestimated. New estimate of the original data gives a significance of ~3s, possibly due to a fluctuation. Valery Kubarovsky PIC-2005

  28. Effective Lagrangian method (hep-ph/0505134)

  29. Total cross section from proton and neutron Total cross sections from proton and neutron do not show differences The new CLAS results do not exclude a state of < 1 MeV width. Valery Kubarovsky PIC-2005

  30. Published Null Experiments - - Valery Kubarovsky PIC-2005

  31. Belle – Limit onQ+Width LP2005 153 GQ+ from K+A -> pK0sX & K+D -> inclusive analysis K+A -> pK0s Belle limit 90%CL 397 fb-1 No no signal Belle: G < 0.64 MeV (90% CL) @ M = 1.539 GeV Not inconsistent with previous results. G< 1 MeV (90% CL) @ M = 1.525–1.545 GeV Valery Kubarovsky PIC-2005

  32. DELPHI CDF K-p Min.bias events Jet events L(1520) K0sp Examples of collider searches Valery Kubarovsky PIC-2005

  33. q e+ e- q No Q+ Quark fragmentation as a source ofQ+? • BaBar searches for Q+ in quark fragmentation with high statistics LP2005 299 e+e--> pK0s+X Valery Kubarovsky PIC-2005

  34. Baryon fragmentation s s u u Q+ d d d q Needs fewer quark pairs from the vacuum e Pentaquark less suppressed ? Pentaquark in fragmentation? Quark fragmentation u u d q Q+ e Pentaquark strongly suppressed ? Valery Kubarovsky PIC-2005

  35. ZEUS M(GeV) High energy production mechanism ep -> eK0spX Q+produced mostly at forward rapidity hLab > 0, and medium Q2 > 20 GeV2. Consistent with Q+ production in baryon fragmentation. Valery Kubarovsky PIC-2005

  36. pp-> pXX gA FOCUS < 0.0025/B range Cascade PentaquarkX5--(1862) ? HERA-B CDF • State not produced in quark fragmentation or is severely suppressed. gA FOCUS Valery Kubarovsky PIC2005

  37. Charmed PentaquarkQ0c(3100) ? FOCUS H1 expected FOCUS events H1 expected FOCUS events • Upper limit factor 4 lower than H1 results. Claim is that results are incompatible with H1. LP2005 #16 LP2005 #391 • Signal also in photo- production • Claim kinematical uniqueness. • FOCUS experiment claims incompatibility with H1. Valery Kubarovsky PIC2005

  38. Comments on X- -(1862), Q 0c(3100) • Each one observed by single experiment. • Strong evidence against both states from several other experiments with comparable kinematics and claimed higher sensitivity Valery Kubarovsky PIC2005

  39. New Positive Results • SVD2 • Spring8/LEPS • STAR Valery Kubarovsky PIC2005

  40. M = 1522 MeV s = 12 MeV Nevnt = 205 M = 1523 MeV s = 12 MeV Nevnt = 165 SVD2 New Improved Analysis, Ep=70 GeV Two independent data set: KS decays inside or outside the Vertex Detector preliminary preliminary Valery Kubarovsky PIC-2005

  41. 1.53 preliminary Counts/5 MeV 1.60 (?) MMd(γ,K－p) GeV/c2 Breaking News from LEPS gD -> K-p X ; M(K-p)=L*(1520) Q+ Valery Kubarovsky PIC-2005

  42. d-Au d-Au STAR STAR Preliminary Au-Au Q++ ? Q++? M(pK+) Q++? STAR: a Q++ pentaquark? “The STAR observed yield is so small such that many experiments would not have the sensitivity to see it.” (Huan Z. Huang). • Strong signal at M~1.53 GeV in both d-Au and Au-Au collisions. Valery Kubarovsky PIC-2005

  43. s/ b+s New CLAS-p New CLAS-d ? HERA-B, CDF ? ZEUS Pentaquark Status Group Signal Backgr. Significance publ.Comments ---------------------------------------------------------------------- SPring8 19 17 4.6s 3.2s SPring8 56 162 ? 3.8s SAPHIR 55 56 4.8s 5.2s DIANA 29 44 4.4s 3.4s CLAS(d)** 43 54 5.2s 4.4s CLAS(p) 41 35 7.8s 4.7s 18 9 6.7 3.5s HERMES 51 150 4.3-6.2 3.6s COSY 57 95 4-6 4.7s ZEUS 230 1080 4.6 6.4s SVD 41 87 5.6 3.6s SVD-2 370 2000 7.5sImproved analysis NA49 38 43 4.2 4.2s H1 50.6 51.7 5-6 5.0s SVD-2 370 2000 ? 7.5s Improved analysis SPring8 200 285 5.0s L*(K+n) STAR 2,250 150,000 5.5sQ++candidate ? New Clas-d ? New Clas-d Valery Kubarovsky PIC-2005

  44. Conclusion • Two high statistics experiments on protons and deuterium (CLAS) contradict results that observed ~5s signals with same kinematics. • The new CLAS results do not exclude a state of < 1 MeV width. • Sensitivity to Q+ at high energy appears in baryon fragmentation not in quark fragmentation. This could explain some of the null results at high energies. • New data from LEPS, SVD2, and STAR, support the Q observations. Valery Kubarovsky PIC2005

  45. Outlook • Analysis is continuing at Spring8, Jlab, COSY, HERMES, H1, ZEUS, SVD2, STAR, PHENIX • New measurements planned at SPring8, JLab • H1, ZEUS, HERMES high luminosity run until 2007 • Higher statistics data from STAR, PHENIX • More statistics from B-factories • Focus moved from bump hunting to more quantitative estimations of cross sections or upper limits Valery Kubarovsky PIC2005

  46. Concluding Remarks • Many labs are involved now in the search for pentaquarks. We have positive and negative results. • There is also, in parallel much theoretical activity. • Do pentaquarks exist? The final conclusion is not straightforward. • Theoretical interest to pentaquarks will not disappear in any case. More over, “if high precision experiments will not find Q+ and it’s partners, this may be even more difficult to understand than the Q+ small mass and small width” (Praszalowicz, May 2005) Valery Kubarovsky PIC2005

  47. QCD instantons DiQuarks are building blocks of multiquark states: pentaquarks and dibaryons Mesons Baryons (8) 4Q Mesons A nucleon is made of a quark and deeply bound scalar-isoscalar diquarks, absent in the decuplet. In the instanton liquid model there are two kind of diquarks, the scalar and the tensor (with spin=1) Baryons (10) 5Q Baryons DiBaryons Valery Kubarovsky PIC2005

  48. 1% of statistics + - + M ( π π K X) X + M (K X) X g11@CLAS Valery Kubarovsky PIC2005

  49. Hermes: Q+ Mass spectrum with additionalp 27.6 GeV positron beam is incident on a deuterium target in a search for Q+ in quasi-real inclusive photoproduction. • standard cuts applied • + K* and L veto • signal/background: 2:1 • signal/background: 1:3 Valery Kubarovsky PIC2005

  50. Q+(1540)/L*(1520) Ratio preliminary N(Q+) (95%CL) < 210 95% CL N(L*) ~ 91000 Valery Kubarovsky PIC2005

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