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Search for pentaquarks: the experimental program at CLAS

Search for pentaquarks: the experimental program at CLAS. S. Niccolai, IPN Orsay for the CLAS collaboration. Epiphany Conference Krakov, 1-6-2005. Introduction Published CLAS results on Θ + Pentaquark program at CLAS Outlook. What are pentaquarks?. d. − 1/3. u. s. + 1/3. +2/3.

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Search for pentaquarks: the experimental program at CLAS

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  1. Search for pentaquarks: the experimental program at CLAS S. Niccolai, IPN Orsay for the CLAS collaboration Epiphany Conference Krakov, 1-6-2005 • Introduction • Published CLAS results on Θ+ • Pentaquark program at CLAS • Outlook

  2. What are pentaquarks? d −1/3 u s +1/3 +2/3 d u −1/3 +2/3 • “Exotic” pentaquarks (qqqqQ) • The antiquark has a different flavor from the other 4 quarks • Quantum numbersdifferent from any 3-quark baryon Example:uudds(exotic): • Baryon number = 1/3 + 1/3 + 1/3 + 1/3 - 1/3 = 1 • Strangeness = 0 + 0 + 0 + 0 + 1 = +1 • New form of quark matter: baryons whose • minimum quark content is 5 (qqqqq) • “Non-exotic” pentaquarks • The antiquark has the same flavor as one of the other quarks • Difficult to distinguish from 3-quark baryons Baryons with S = +1 cannot be made by qqq !!!!! S. Niccolai, IPN Orsay

  3. Existence of pentaquark is not forbidden by QCD • General idea of a five-quark states has been around since late 60’s (Jaffe, Lipkin, Strotteman…) • However… 1986, Particle Data Group: “The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided” Almost 15 years later... • 10 experiments reported evidences for pentaquark states since 2003 • Over 200 theory papers published • Started series of dedicated international workshops: JLab (2003), SPring-8 (2004), Genova (2005) • Q+ listed in PDG 2004 (***)! • Research on pentaquarks is one of the central programs in many labs

  4. uudds Q+(1539) N(1650-1690) : exotic states S(1760-1810) X+(1862) ddssu uussd The pentaquark anti-decuplet Experimental searches were motivated by predictions of chiral soliton modelDiakonov, Petrov, Polyakov, Z.Physics A359 (1997) S = +1 Baryon octets, decuplets and antidecuplets are seen as rotational excitations of the same soliton field S = 0 S = -1 S = -2 I=-1/2 I=-1 I=-3/2 I=1 I=3/2 I=0 I=1/2

  5. uudds S = +1 decay modes nK+,pK0 narrow width (few MeV) S = 0 Jp=1/2+ : exotic states S = -1 S = -2 ddssu uussd I=-1/2 I=-1 I=-3/2 I=1 I=3/2 I=0 I=1/2 The pentaquark anti-decuplet Experimental searches were motivated by predictions of chiral soliton modelDiakonov, Petrov, Polyakov, Z.Physics A359 (1997) Q+(1539) can be detected experimentally!! N(1650-1690) S(1760-1810) X+(1862)

  6. T. Nakano et al., PRL91, 012002 (2003) • n→K+K-(n) (the neutron is bound inside 12C) Q+ First observation: LEPS/SPring-8

  7. The Jefferson Lab electron accelerator Newport News, USA Emax ~ 6 GeV Imax ~ 200 mA Duty Factor ~ 100% sE/E ~ 2.5 10-5 Beam Pol ~ 80%

  8. The CLAS detector at JLAB • Toroidal magnetic field (6 supercond. coils) • Drift chambers (argon/CO2 gas, 35,000 cells) • Time-of-flight scintillators (684 PMTs) • Electromagnetic calorimeters • (lead/scintillator, 1296 PMTs) • Cherenkov Counters • (e/p separation, 216 PMTs) Bremsstrahlung photon tagger S. Niccolai, IPN Orsay

  9. The CLAS detector at JLAB • Performances: • large acceptance for charged particles • 8°<q<142°, 0.2< pp<4 GeV/c, 0.1<pp<4 GeV/c • good momentum and angular resolution • Dp/p ≤ 1.5%, Dq, Df ≤ 1 mrad Bremsstrahlung photon tagger S. Niccolai, IPN Orsay

  10. The CLAS detector at JLAB CLAS is designed to measure exclusive reactions with multi-particle final states Bremsstrahlung photon tagger S. Niccolai, IPN Orsay

  11. The bremsstrahlung photon tagger Tagging range: (20% - 95%)E0 Eg= E0 – Ee (E0 known, Ee measured)

  12. gd →K-pK+(n) CLAS-d: first Q+ exclusive measurement • Experimental data from 1999 run • Tagged photons with Eγmax =3 GeV • Target: 10 cm long liquid deuterium • Exclusivity: • No Fermi motion corrections • Final state identified with less background Possible reaction mechanism non-spectator proton: pK- rescattering required to detect p in CLAS (pmin(p) = 300 MeV/c)

  13. pp+p- ppp- pK+K- CLAS-d: pK+K- event selection Main background: pp+p- S. Niccolai, IPN Orsay

  14. K+ p K- gd → p K+K─ (n) in CLAS K- p K+

  15. gd →K-pK+(n) CLAS-d: neutron ID The neutron is detected by missing mass • 15% of non pK+K- events within ±3s of the peak • Almost no background under the neutron peak with tight timing cut, DtK S. Niccolai, IPN Orsay

  16. CLAS-d : cut on known resonances • Several other known processes can contribute to the pK+K-(n) final state: • gd→fp(n), f→K+K- • gd→L(1520)K+(n), • L(1520)→pK- • Both reactions proceed predominantly on the proton (neutron is a spectator) • Kinematics of both reactions are not a good match for Q+ production. S. Niccolai, IPN Orsay

  17. CLAS-d: background reduction • Cut on the missing neutronmomentum, pn>0.08 GeV/c • Cut on the K+ momentum, pK+<1 GeV/c, based on the 3-body phase space Monte Carlo (gd→Q+K-p, Q+→nK+) Before pK+ cut S. Niccolai, IPN Orsay

  18. S. Stepanyan et al., PRL91, 252001 (2003) gd →K-pK+(n) Gaussian background Simulated background Distribution of L(1520) events CLAS-d: result • M = (1.542 ± 0.005) GeV/c2 • = 21 MeV/c2 • Only ~40 events in the Q+ peak • Statistical significance: 4.6s - 5.8s, depending on the background shape. • No significant Q+ signal was found in the spectator analysis (like SPring-8) due to not optimal torus field settings (limited forward acceptance for K-). • No Q++ peak observed in M(pK+) S. Niccolai, IPN Orsay

  19. Is “Θ+”a kinematical reflection? High mass mesons (a2(1320), f2(1270)) decaying to K+K-can produce structures inM(NK) at low mass (A. Dzierba et al., hep-ph/0311125)

  20. Is “Θ+”a kinematical reflection? K. Hicks et al., hep-ph/0411265, found inconsistencies in Djerba’s approach

  21. Is “Θ+”a kinematical reflection? Open issue: needed measurements of Θ+ in different final states (no K+K-)

  22. First observation on the proton: CLAS-p gp→K-p+K+(n) After PID: neutron identified by missing mass Eg = 3.0 – 5.47 GeV Cut on f: M(K+K-)<1.06 GeV/c2 No clear Θ+signal!! S. Niccolai, IPN Orsay

  23. First observation on the proton: CLAS-p gp→K-p+K+(n) V. Kubarovski et al., PRL92, 032001 (2004) t-channel process a) selected and background processes eliminated with the cuts (c.m.): cosθ*p+ > 0.8 andcosθ*K+ < 0.6 M = (1.555 ± 0.010) GeV/c2 Statistical significance: (7.8 ± 1.0) σ G = 26 MeV/c2

  24. cut First observation on the proton: CLAS-p gp→K-p+K+(n) V. Kubarovski et al., PRL92, 032001 (2004) • Q+ production through • N* resonance decays? to understand production mechanism look at M(nK+K-)

  25. First observation on the proton: CLAS-p gp→K-p+K+(n) outside Q+ region Cut on M(nK+) in Q+ region • intermediate baryon state? • p-p cross section data in PDG have a gap in the mass range 2.3–2.43 GeV S. Niccolai, IPN Orsay

  26. HERMES Spring-8 SAPHIR CLAS-d CLAS-p DIANA COSY ZEUS IHEP ITEP Summary on Θ+: positive results • 10 observations • different reaction mechanisms BUT: • ~ 10 MeV gap between • measured masses • statistical significance • between 4 and 7.8 σ • often heavy cuts • backgrounds not well understood

  27. _ _ _ _ _ HERA B: pA→pK0X CDF: pp→pK0X Phenix: Au+Au→nK-XBaBar: e+e-→pK0X Belle: e+e-→B0B0→ppK0 BES:e+e-→J/Ψ→ΘΘ • in e+e-: • there are no quarks in initial state • no theoretical predictions on QQ production • in the other cases: • fragmentation processes, Q+ could be suppressed (Titov et al., Phys. Rev. C 70, 042202 (2004)) • high multiplicity in the final state, combinatorial backgrounds _ Summary on Θ+:negative results • high statistics • clearly see some of the known resonances BUT: NEEDED DEDICATED, HIGH-STATISTICS, EXCLUSIVE MEASUREMENTS!!!!!

  28. Decuplet partners of Θ+: search for X3/2−−,X3/20 uudds ddssu uussd Exotic, S = -2, Q = -2 Non exotic, S = -2, Q = 0

  29. Decuplet partners of Θ+: search for X3/2−−,X3/20 HERA-B p+A 920 GeV/c X-p- + X-p+ pp NA49 Ecm = 17.2 GeV C. Alt, et al., Phys.Rev.Lett.92, 042003 (2004) hep-ex/0403020 Combined spectra M=1.862± 0.002 GeV Statistical significance = 5.6σ G = 18 MeV/c2 No peaks observed! Ξ0 also observed No Ξ0

  30. Pentaquark today: open issues • confirmation of existence of Θ+ with high statistics • precise determination of the mass of Θ+ • properties of Θ+: spin, isospin, parity • production mechanisms • possible excited states of Θ+ • coupling N* to Θ+ • confirmation existence of other exotic members of decuplet An experimental program is currently underway at CLAS to address these issues S. Niccolai, IPN Orsay

  31. Pentaquark Searches at CLAS Experimental program approved and underway at CLAS • Search for Q+ on deuterium - G10 run gd→K-K+p(n)and other final states(data taking completed) • Search for Q+, Q*+, Q*++ on the proton - G11 run gp → various final states (data taking completed) • Search for X pentaquarks - EG3 run gvd→X5--, X5- X (data taking just finished) • Exotic hadron spectroscopy -G12 run gp at 6 GeV, high luminosity (to run in 2005/6) S. Niccolai, IPN Orsay

  32. tagged photons in the energy range 0.8 GeV to 3.59 GeV target: 24 cm long liquid deuterium data were taken at 2 settings of CLAS toroidal magnet (2250 A and 3375 A), lower field to increase acceptance for K- (Spring-8) at each setting integrated luminosity (2.5pb-1) is about 10 times higher than in published deuterium data Reactions channels to study: gd→pK-K+n Θ+→nK+ gd→ΛK+(0)n(p); Θ+→nK+,pK0 gd→pK-K0 (p) Θ+→pK0 g”n”→K-K+n Θ+→nK+ (with Fermi momentum corrections, to compare with SPring-8) CLAS G10: search for Θ+ on deuterium • data taking finished end of May • 10 billion of events collected • data processing almost done • results in the spring S. Niccolai, IPN Orsay

  33. CLAS G10: how to avoid a fake signal • Consider only signals with statistical uncertainty <10%, and statistical significance greater than 7s • Study of the NK system: • for pK0 and nK+ final states • in both the missing mass and invariant mass distributions • Production of NK system together with a hyperon (L, S) • Divide data set into two independent sets. Analyze one set, then apply the same analysis (cuts, etc) to the second set • Inspect Q+ candidates in CLAS event display • Full simulation of possible background final states S. Niccolai, IPN Orsay

  34. n gd →K-pK+(n) non-spectator neutron events f CLAS G10: data quality 50% of the high-field data L(1520)

  35. L p K+ Q+ n d CLAS G10: γd→ΛΘ+ Reaction already studied on 3He CLAS data, but statistics were too low • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  36. Decay modes under study: • Λ→pπ- Θ+→K+n d CLAS G10: γd→ΛΘ+ p p- L p K+ n Q+ n K+ • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  37. Decay modes under study: • Λ→pπ- Θ+→K+n • Λ→pπ-Θ+→K0p K0 → π+π- d CLAS G10: γd→ΛΘ+ p p- L p K+ p Q+ n p+ K0 p- • No possibility of kinematical reflections • (only one K, from Θ+ decay, in the final state) • S=+1 both for nK+ and pK0, thanks to Λ • No background channels to remove S. Niccolai, IPN Orsay

  38. Decay mode: • Λ→pπ- Θ+→K+n d CLAS G10: γd→ΛΘ+ p p- L p K+ n Q+ n K+ L n

  39. Decay mode: • Λ→pπ-Θ+→K0p K0 →π+π- d CLAS G10: γd→ΛΘ+ p p- L p K+ p Q+ n p+ K0 p- L K0

  40. Reaction channels to study: gp→K0K+(n) Θ+→nK+ gp→K0K0pΘ+→pK0 gp→K-K+p+(n) Θ+→nK+ gp→pK-K+Θ++→pK+ CLAS G11: search for Θ+ on proton • data taking finished end of July • 10 times more statistics than old gp • Eγ = 0.8 – 3.59GeV • search for ground and first excited states of Q+ • search for Q++ • data are being processed S. Niccolai, IPN Orsay

  41. CLAS G11: data quality 6% of statistics p- p L f S. Niccolai, IPN Orsay

  42. CLAS G11: data quality 6% of statistics S- n K0 L(1520) L S0 S+

  43. the new CLAS experimental program should solve these issues, with: • two experiments completed (G10, G11), results coming soon • one experiment finishing in these days (EG3) • one experiment to run later on in 2005 Summary and outlook • possible existence of pentaquarks gave new boost to hadronic physics and QCD spectroscopy • 10 Θ+ signals published so far, but: • several reports of non-observations • need of high-statistics dedicated experiments to: • low statistics • discrepancies in measured masses STAY TUNED… • establish existence of Θ+ • study its properties • find possible other pentaquarks (X5’s ?)

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