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Q + Search at LEPS/SPring-8

Q + Search at LEPS/SPring-8. Introduction Evidences for Q + Counter-evidences for Q + Preliminary results from new LEPS data Summary. T. Nakano （ RCNP, Osaka University ）. Exotic Hadron WS @ NWU, May 27, 2005. What are penta-quarks?. Baryon. Minimum quark content is 5 quarks.

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Q + Search at LEPS/SPring-8

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  1. Q+ Search at LEPS/SPring-8 • Introduction • Evidences for Q+ • Counter-evidences for Q+ • Preliminary resultsfrom new LEPS data • Summary T. Nakano （RCNP, Osaka University） Exotic Hadron WS @ NWU, May 27, 2005

  2. What are penta-quarks? • Baryon. • Minimum quark content is 5 quarks. • “Exotic” penta-quarks are those where the antiquark has a different flavor than the other 4 quarks • Quantum numbers cannot be defined by 3 quarks alone. Example: uudds Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1 Strangeness = 0 + 0 + 0 + 0 + 1 = 1 e.g. uuddc, uussd c.f. L(1405): uudsu or uds

  3. Prediction of the Q+ Baryon 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. Baryon masses in constituent quark model mu ~ md = 300 ~ 350 MeV, ms=mu(d)+130~180 MeV • Mainly 3 quark baryons: M ~ 3Mq + (strangeness)+(symmetry) • 5-quark baryons, naively: M ~ 5Mq + (strangeness) +(symmetry) 1700~1900 MeV for Q+

  5. 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

  6. JLab-d Spring8 DIANA ELSA ITEP SVD/IHEP JLab-p HERMES ZEUS COSY-TOF CERN/NA49 H1 pp  S+Q+. Evidence for Penta-Quark States This is a lot of evidence Nomad

  7. First evidence from LEPS g nK+K-n M = 1.540.01 MeV G < 25 MeV • Background level was estimated byusing events from a LH2 target. • 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 Q+ No L(1520) peak in events without a proton. Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020

  8. Mass Final state: K+ + n Ks + p (Ks + p ) A few % difference from zero, but ~20% difference from the KN threshold.

  9. Width • There is some inconsistency: • Most measurements are only upper limits. • DIANA has G < 9 MeV. • The cross-section implies G=0.9 MeV. • HERMES: G = 13 +- 9 stat. (+- 3 sys.) MeV • ZEUS: G = 8 +- 4 stat. (+- 5 sys.) MeV • Arndt et al. and Cahn et al. analysis of KN phase shifts suggests that G < 1 MeV !! • The small width is the hardest feature for theorists to understand…

  10. Negative Results • HERA-B (Germany): • reaction: p+A at 920 GeV • measured: K-p and K0p invariant mass • Clear peak for L(1520), no peak for Q+ • production rate: Q+/L(1520)<0.02 • BES (China): • reaction: e+e- J/y  Q+Q- • limit on B.R. of ~10-5 And many unpublished negative results (HyperCP, CDF, E690, BaBar, LEP,,,).

  11. Q+ Search at CDF:pK0S Mass Spectrum

  12. Slope for mesons Slope for baryons Slope for pentaquarks??

  13. CLAS: New high statistics exp.Search for Q+ in g pK+Ksn Limit on Q+ production:Total cross-section < 1~4 nb How could other low energy experiments with lower statistics could observe the signal? R. De Vita, APS April meeting, 2005

  14. LEPS New LD2 and LH2 runs • Data taken from Oct. 2002 to Jun. 2003. • ~2・1012photons on a 15cm-long LD2 target. • Less Fermi motion effect. • LH2 data were taken in the same period with ~ 1.4・1012photons on the target. #of photons: LH2:LD2 ≈ 2:3 we expect # of events from protons: LH2:LD2≈ 2:3 # of events: LH2:LD2≈ 1:3

  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) Can be consistent with the CLAS high stat result? preliminary preliminary  Next talk by S.I. Nam MMgK+(GeV) MMgK-(GeV)

  16. LEPS detector Good acceptance in the forward angle. g 1m

  17. New way to search for Q+ 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 p

  18. 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 LH2 LD2 γ L(1520) MMd(γ,K－p) GeV/c2 K+/K0 p/n Formation momentum Q+ n/p PK GeV/c

  19. Final State Interaction I. III. p K- γ γ K+ K- p K+ p p p K+ n n n n Small, especially at low PK+ Large at low Pp p II. p L(1520) γ γ K- K+ K- p K+ p K- K+ n n n n Contributions from II and III are suppressed.

  20. Event selection K mass is smeared by Fermi motion. (assumed proton at rest) Λ(1520) LD2 data: after selecting L(1520) LD2 select inelastic events γp→K－pKπ MMp(γ,K－p) GeV/c2 M(K－p) GeV/c2 Select Λ(1520) in 1.50–1.54 GeV/c2 ⇒calculate K- p missing mass of g d  K- p X reaction

  21. K－p missing mass in 1.50<M(K－p)<1.54 GeV/c2 g d L(1520) X 1.53 GeV K- p Good understanding of the background spectrum shape is crucial . preliminary “You see the peak because you want it. I see two dips.” by Prof. I at KEK MMd(γ,K－p) GeV/c2

  22. Major 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+

  23. f contribution f K+ momentum is estimated by missing momentum technique.  Smeared by Fermi motion. LD2 data M(K－K+) GeV/c2

  24. K－p invariant mass after f exclusion Eg > 2.2 GeV all energy L(1520) LH2 LH2 non-resonant KKp events non-resonant KKp events M(K－p) GeV/c2 M(K－p) GeV/c2 Non-resonant KKp (phase space) contribution reproduces the spectrum shape under L(1520)well.

  25. K－p missing mass for L(1520) and non-resonant events in the non-resonant KKp L(1520) MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 Energy dependences are set to be the same. No angular dependence has been introduced in the both reactions.

  26. K－p missing mass for L(1520) Quasi-free L(1520) and non-resonant KKp f preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

  27. K－p missing mass in the sideband regions 1.46< M(K－p) < 1.50 GeV/c2 1.54< M(K－p) < 1.58 GeV/c2 LD2 LD2 MMd(γ,K－p) GeV/c2 MMd(γ,K－p ) GeV/c2 f: Energy and angular dependences were obtained by fitting to the data. KKp: Enegy dependence  fit to the data. Angulardependence  flat.

  28. Test by side-band subtraction Narrow gate (1.52<M(K－p)<1.54 GeV/c2) = L + N Λ(1520) is enhanced. Wide gate (1.46<M(K－p)<1.58 GeV/c2) = L + 3N Non-resonant KKp is enhanced. ⇒L = 1.5 (L+N)－ 0.5 (L+3N) narrow gate wide gate L N N N M(K－p) GeV/c2

  29. K－p missing mass for non-resonant KKp events (phase space distribution) 1.46< M(K－p) < 1.50 GeV/c2 sidebands averaged 1.50< M(K－p) < 1.54 GeV/c2 1.54< M(K－p) < 1.58 GeV/c2 MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

  30. Side-band subtraction in KK invariant mass Since φ is not related with Λ(1520), φ peak disappears. LD2 LD2 M(K+,K-) GeV/c2 M(K+,K-) GeV/c2 after side-band subtraction

  31. K－p missing mass in L(1520) and sideband region. LD2 LH2 preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 1.50 < M(K－p) < 1.54 0.5(1.46 < M(K－p) < 1.50) +0.5(1.54 < M(K－p) < 1.58)

  32. Side-band subtracted K－p missing mass Excess is related with Λ(1520). Most events are below 1.52 GeV/c2 Bump structure LD2 LH2 preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 Events in MMd(γ,K－p) < 1.52 GeV/c2: LH2:LD2 = 1:1.3 # of photons: LH2:LD2 ≈ 2:3  s(p) >> s(n) for quasi free L(1520) production.

  33. Energy dependence 2.1 GeV < Eg < 2.4 GeV(Emax) 1.75 GeV < Eg < 2.1 GeV LD2 LD2 preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

  34. K－p missing mass inhighand low energy region • 1.53 GeV Peak: • No change in the peak position. not likely due to kinematical reflections. • Narrower width in the low energy region. consistent with the detector resolution. Below 2.1 GeV Above 2.1GeV • 1.60 GeV Bump: • Only seen in the low energy region.  threshold effects? • Not seen in LH2 data • Associated with L(1520). preliminary MMd(γ,K－p) GeV/c2

  35. f contribution removed by angle cut Require cos qKp > 0 LD2 LH2 Excess remains. preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

  36. K－p missing mass in sideband regions γ p LD2 K- No peak! K+/K0 Q+ γ L(1520) K+/K0 Q+ Q+ formation cross-section by simple kaon re-scattering should be small. MMd(γ,K－p) GeV/c2 A theoretical estimation by A. Titov is small.

  37. Width: Comparison with a MC spectrum MC gives ~10 MeV resolution while Real data resolution is about ~ 8 MeV. The statistical uncertainty of ~2 MeV is mainly due to fluctuations of the background at the tails of the peak. preliminary MMd(γ,K－p) GeV/c2

  38. Summary • Evidence for an S=+1 baryon around 1.54 GeV with a narrow width has been observed by several experimental groups. • There are some inconsistencies in the measured masses and widths. • No signal has been observed in high energy experiments with high statistics and good mass resolution. • If the Q+ does exist, its production in high energy reactions must be highly suppressed. • The Q+ is not seen in the new CLAS high statistics data from a proton. • If the Q+ does exist, its production must have a large isospin asymmtery.

  39. Summary of LEPS new result • Peak is seen at ~1.53 GeV/c2 in the missing mass of the (g,L(1520)) reaction from a deuteron. • If is is real, the width seems to be very narrow. • The peak cannot be seen in the K-p invariant mass region outside of the L(1520). • If the peak is due to the Q+, its production by re-scattering seems to be small in our kinematic region. • Enhancement (bump structure) around 1.6 GeV is also observed in the (g,L(1520)) reaction but only in the low Eg region. • Non-resonant KKp and f contributions can be removed by sideband subtraction. • For quasi-free L(1520) production, s(p)>>s(n).

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