1 / 17

Q + Search at LEPS

Q + Search at LEPS. First evidence from LEPS. g n K + K - n. Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020. Low statistics : but Tight cut : 85% of events are rejected by the f exclusion cut.

nibal
Télécharger la présentation

Q + Search at LEPS

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Q+ Search at LEPS

  2. First evidence from LEPS g nK+K-n Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020 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+

  3. 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) preliminary preliminary MMgK+(GeV) MMgK-(GeV)

  4. 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 p

  5. 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 verylittle overlap with CLAS acceptance. Pmiss GeV/c

  6. 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

  7. Background processes • 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.

  8. Sideband subtraction to remove non-resonant background Eg > 1.75 GeV LH2 LD2 M(K－p) GeV/c2 M(K－p) GeV/c2 1.50 < M(K－p) < 1.54 1.45 < M(K－p) < 1.50 or 1.54 < M(K－p) < 1.59 - 0.4 S =

  9. Remove fluctuation by smearing Eg Counts/5 MeV Counts/5 MeV correction for f contribution MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2 • Fluctuations in the sideband spectra are removed by smearing Eg with 10 MeV smearing (nearly equal to the resolution). • Eg smeared spectrum gives c2/n.d.f ~ 1 when compared with the original spectrum. • f contribution in the signal region is slightly larger than that in the the sideband region. The underestimtion is corrected by using the MC simulation.

  10. Comparison of Eg smeared spectra 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

  11. Estimate L* contribution from LH2 data • Estimate quasi-free L* contribution using LH2 data. • Missing mass is calculated by assuming deuteron mass in the initial state. • MC study shows the Fermi motion effect is small. • Non-resonant and f contributions are subtracted by sideband subtraction method. • Small fluctuations in the large missing mass region (MM>1.55 GeV) could not be completely removed. Counts/5 MeV MMd(γ,K－p) GeV/c2 MC MMd(γ,K－p) GeV/c2

  12. K－p missing mass spectrum Excesses are seen at 1.53 GeV and at 1.6 GeV above the background level. Counts/5 MeV 1.53-GeV peak: preliminary (in the 5 bin = 25 MeV) 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.

  13. Variation of width for the signal region M(K－p) GeV/c2 M(K－p) GeV/c2 L* : -17% EX: -12% (BG: -50%) L* : +13% EX: +22% Counts/5 MeV Counts/5 MeV preliminary preliminary MMd(γ,K－p) GeV/c2 MMd(γ,K－p) GeV/c2

  14. Background estimation • If we fit the full missing mass region (instead of MMd<1.52 GeV), the L* contribution increases by 33%. • The background level in the Q+ region becomes 6.5% more, and the significance drops to 3.8. • The c2 of the fit is bad: c2/ndf=2.8 in the full region and c2/ndf=2.4 in the region MMd<1.52 GeV. preliminary Counts/5 MeV MMd(γ,K－p) GeV/c2 • Sideband method overestimate BG level because of L* leakage into the sideband region. • However, a slight change of the BG level does not change the fitting result much. • 10% reduction of BG level requires 15% increase of L* contribution. It results in a 5% smaller BG level in the Q+ region. preliminary Counts/5 MeV x 0.9 MMd(γ,K－p) GeV/c2

  15. Photon energy dependence • 1.53 GeV peak: • No change in the peak position. not likely due to kinematical reflections. Counts/5 MeV • 1.60 GeV bump: • Only seen in the low energy region.  threshold effects? • Not seen in LH2 data • Associated with L(1520). • Different reaction mechanism from that of the 1.53 peak. MMd(γ,K－p) GeV/c2 Eg < 2.1 GeV Eg > 2.1 GeV

  16. K－p missing mass in sideband regions 1.45<MpK<1.50 or 1.55<MpK<1.59 Counts/5 MeV γ p K- K+/K0 Q+ 10 MeV away from the L* region 1.44<MpK<1.49 or 1.56<MpK<1.60 γ L(1520) Counts/5 MeV K+/K0 Q+ Q+ formation cross-section by simple kaon re-scattering issmall. MMd(γ,K－p) GeV/c2 A theoretical estimation by Titov is small (nucl-th/0506072) .

  17. Summary of LEPS new result • We searched for Q+ in the in the g d L(1520) KN reaction • A ~5 sPeak is seen at ~1.53 GeV/c2 in the missing mass of the (g,L*). • The peak can be explained by the narrow S=+1 resonance Q+ , and we think it is likely. • 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*) reaction. • Width of the 1.53 peak is consistent with the detector resolution.

More Related