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Experimental Review of Pentaquarks: Positive and Null Results

Experimental Review of Pentaquarks: Positive and Null Results. Forum on Pentaquarks (DESY) February 1, 2005 Ken Hicks (Ohio University). Outline. Preliminary Comments and Opinions Evidence for the Q + The Null Experiments Some common “myths” New Data (SPring-8) Conclusions.

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Experimental Review of Pentaquarks: Positive and Null Results

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  1. Experimental Review of Pentaquarks:Positive and Null Results Forum on Pentaquarks (DESY) February 1, 2005 Ken Hicks (Ohio University)

  2. Outline • Preliminary Comments and Opinions • Evidence for the Q+ • The Null Experiments • Some common “myths” • New Data (SPring-8) • Conclusions K. Hicks, Ohio U.

  3. Preliminary Remark Congratulations to the ESA on a MAJOR success: K. Hicks, Ohio U.

  4. Opinions on Pentaquarks: • There are valid criticisms for both positive and null experimental results. • A “scorecard” approach will not work. We need better, higher-statistics, data. • Science versus emotion • There have been strong statements on both sides of the existence question. • Let’s make scientifically sound statements. K. Hicks, Ohio U.

  5. More Opinions • If the Q+ exists, data suggests it likely favors certain production mechanisms. • This is an exotic baryon. • It may have an exotic production mechanism. • To make solid scientific statements: • Calculate the expected rate of production. • Understand the rate of the background. • Compare with acceptance-corrected data. K. Hicks, Ohio U.

  6. If it exists, what is it? • The first Q+ search was motivated by the chiral soliton model of DPP. • Is it is possible that there is another interpretation of the Q+? We should not be biased toward a particular theory. • Lattice QCD suggests that the Q+ has negative parity (opposite to DPP). • But these are not “gold-plated” calculations Diakonov, Petrov and Polyakov, Z. Phys. A359, 305 (1997). K. Hicks, Ohio U.

  7. Positive results K. Hicks, Ohio U.

  8. Comparison of Q+ Experiments *Gaussian statistical significance: estimated background fluctuation K. Hicks, Ohio U.

  9. JLab-d Spring8 DIANA SAPHIR ITEP SVD/IHEP JLab-p HERMES ZEUS COSY-TOF CERN/NA49 H1 pp  S+Q+. Evidence for Pentaquark States This is a lot of evidence Nomad K. Hicks, Ohio U.

  10. Critical Comments • For many experiments, the background shape is not clearly known. • Some experiments have harsh angle cuts that could affect the mass spectra. • In all cases, the signal is weak compared with standard resonances. • Cuts are necessary to lower background. K. Hicks, Ohio U.

  11. CLAS: deuterium result • Mass = 1.542 GeV • < 21 MeV Significance 5.2±0.6 s NQ= 43 events Q+ Significance = ? ? Two different background shapes Events in the L(1520) peak. K. Hicks, Ohio U.

  12. Official CLAS statement • “Further analysis of the deuterium data find that the significance of the observed peak may not be as large as indicated.” • We really need a calculation of the background before the statistical significance of the peak can be known. • Eventually the new experiment, with much higher statistics, will settle the question. • The g10 experiment (x10 statistics) is now complete, and final results are expected at end of Feb. 2005. • “Why is it taking so long?” --> It’s only 8 months!! K. Hicks, Ohio U.

  13. Results from ZEUS NOTES: 1. Q+ peak is evident only for Q2 > 20 GeV2. --> ZEUS suggests that this condition gives the Q+ enough transverse momentum to get into their detector acceptance. 2. There is an assumption of background shape. --> A different background changes the stat. signifig. K. Hicks, Ohio U.

  14. HERMES Q+ spectra add additional p • signal / background:1:3 • signal / background • 2:1 • standard cuts applied • + K* and L veto K. Hicks, Ohio U.

  15. no enhancement in D* Monte Carlo no enhancement in wrong charge D Results from H1 (From Karin Daum) Apply mass difference technique M(D*p)=m(K p)-m(K)+MPDG(D*) Background well described by D* MC and “wrong charge D” from data narrow resonance at M=3099 3(stat.)  5 (syst.) MeV • Signal is visible in different data taking periods • But no signal seen in ZEUS data (question: different D* accep.?) K. Hicks, Ohio U.

  16. Null Results K. Hicks, Ohio U.

  17. Published Null Experiments K. Hicks, Ohio U.

  18. Critical Comments • Inclusive versus Exclusive measurement • inclusive has better resolution, but more background (especially at higher energy) • Backgrounds: combinatorial and from other resonances. Can we estimate? • Production mechanism: projectile or target fragmentation? • Is it calculable in some model? K. Hicks, Ohio U.

  19. Titov: inclusive production(fragmentation region) fast slow Ratio: pentaquark to baryon production Regge exchange dominates (2 = diquarks as quasi-partons) K. Hicks, Ohio U.

  20. Slope for mesons Slope for baryons Slope for pentaquarks?? K. Hicks, Ohio U.

  21. Slope for p.s. mesons Slope for baryons Slope for Pentaquark?? Hadron production in e+e- Slope: Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair) Baryons: ~ 10-4 /GeV/c2 (need two more pairs) Pentaquarks: ~ 10-6 /GeV/c2(?) (need 4 more pairs) we don’t know the production mechanism!! K. Hicks, Ohio U.

  22. Some common “myths” K. Hicks, Ohio U.

  23. Myth #1 • “Kinematic reflection of the a2 and f2 tensor mesons explain the CLAS data” Near theshold (Eg<3 GeV) pion exchange dominates Regge exchange. --> For T=(a20 and f2), the g-p-T vertex violates C-parity! --> calculations using diagrams that do not violate C-parity (Y. Oh et al., hep-ph/0412363) give sT far too small to explain CLAS data as a2/f2 “reflections”. Some people use a Regge trajectory (p, p1, p2, etc.) K. Hicks, Ohio U.

  24. Myth #2 • “Ghost tracks could be responsible for the peaks seen in the pK0 mass spectra” This only can happen if there is an error in the tacking software. --> The same track must be used twice! --> All pentaquark (pK0) data analysis has been checked, and no such tracking error is found. K. Hicks, Ohio U.

  25. New Data K. Hicks, Ohio U.

  26. New data: LEPS deuterium* Minimal cuts: vertex, MMgKK=MN, no f, Eg < 2.35 GeV L(1520) Q+ Preliminary Preliminary MMgK-(GeV) MMgK+(GeV) *in collaboration with T. Nakano K. Hicks, Ohio U.

  27. MMgK+(GeV) MMgK+(GeV) • No large difference among the three Fermi motion correction methods MMgK+(GeV) LEPS: Fermi motion corrections L(1520) resonance K. Hicks, Ohio U.

  28. Fermi motion corrections: Q+ MMgK-(GeV) MMgK-(GeV) • No large differences among the three Fermi motion corrections. MMgK-(GeV) K. Hicks, Ohio U.

  29. LEPS: K－p detection mode(New and Preliminary results) • Inclusiveproduction: • Θ＋ is identified by K－p missing mass from deuteron. ⇒ No Fermi correction is needed. γ Θ＋ γ Θ＋ L(1520) p K－ p K－ n n p K. Hicks, Ohio U. p

  30. Event selections in K－p mode K＋ mass Λ(1520) Non-resonant KKp γp→K－pKπ π－ mis-ID as K－ MMp(γ,K－p) GeV/c2 M(K－p) GeV/c2 Λ(1520) is tightly selected in 1.50–1.54 GeV/c2 K. Hicks, Ohio U.

  31. K－p missing mass for events in the L(1520) peak Small enhancement at 1.53 GeV. But the statistics is not large enough. Hydrogen target data MMd(γ,K－p) GeV/c2 K. Hicks, Ohio U.

  32. A possible reaction mechanism • Q+ can be produced by re-scattering of K+. • K momentum spectrum is soft for forward going L(1520). PK obtained by missing momentum γ L(1520) K+/K0 p/n Formation momentum Q+ n/p PK GeV/c K. Hicks, Ohio U.

  33. K－p missing mass for events with missing momentum > 0.35 GeV/c sideband regions VERY PRELIMINARY! MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 select K. Hicks, Ohio U.

  34. Summary • There is reason for caution about the existence of the Q+. • Need better experiments (pos. and null). • Experiments need to have better control over the background shape. • Can backgrounds be calculated? • The new LEPS data for the Q+ is interesting, but not conclusive. • CLAS data: internal review in ~1 month. K. Hicks, Ohio U.

  35. Outlook • There are several new experiments that will help settle the existence question: • SPring-8: LEPS (deuterium: higher statistics) • JLAB: CLAS (g10, g11, eg3) • COSY: TOF • DESY? • We still need to understand the null experiments: • background? production mechanism? K. Hicks, Ohio U.

  36. p p Q+ S+ Model-independent Parity At threshold S-wave dominant T = 1 K, or K* If S = 0, then Li = even, P = even ==> P(Q) = + If S = 1, then Li = odd, P = odd ==> P(Q) = - Thomas, Hosaka, KH, Prog. Theor. Phys. 111, 291 (2004). See full calculation: C. Hanhart et al., hep-ph/0410293. K. Hicks, Ohio U.

  37. Width: Indirect Limits • Nussinov (hep-ph/0307357): GQ< 6 MeV • Arndt et al. (nucl-th/0308012): GQ< 1 MeV • Haidenbauer (hep-ph/0309243): GQ< 5 MeV • Cahn, Trilling (hep-ph/0311245): GQ~ 0.9 MeV • Sibertsev et al. (hep-ph/0405099): GQ< 1 MeV • Gibbs (nucl-th/0405024): GQ~ 0.9 MeV K. Hicks, Ohio U.

  38. Width: Possible Q+ Signal? Input mass Conclude: width G must be ~1 MeV Gibbs, nucl-th/0405024 Widths range: 0.6-1.2 MeV 0.9 MeV = solid background (non-reson.) K. Hicks, Ohio U.

  39. Comments: Width and Parity • If the KN database is correct, it is likely that the Q+ width is G~1 MeV. • If the width is 1 MeV, the parity is almost surely positive. • negative parity width goes up by ~50. • If the lattice results are correct, the width is almost surely negative. This problem of width/parity is the most worrisome aspect to the existence of the Q+. K. Hicks, Ohio U.

  40. (ud) L=1 s (ud) 200 MeV Decay Width: G » » 8 MeV ( ) 2 2 6 A di-quark model for pentaquarks JW hep-ph/0307341 JM hep-ph/0308286 L=1, one unit of orbital angular momentum needed to get J=1/2+ as in cSM Uncorrelated quarks: JP = 1/2− Additional width suppression may come from w.f. overlap. K. Hicks, Ohio U.

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