New results on Θ + from LEPS Yuji Kato, for the LEPS collaboration Nagoya University ・ Introduction ・ LEPS experiment ・ Inclusive analysis – Blind analysis ・ Exclusive analysis ・ Summary 1/28

Introduction: Θ + 2.Narrow width Γ=0.39±0.10MeV/c 2 (DIANA) Γ<0.64MeV/c 2 (Belle) 1.Low mass Sum of the constituent quark mass ~1900MeV/c 2 Constituent quark model 1700 ～ 1800MeV/c 2 ⇔ ～ 1530MeV/c 2 F.Huang et al Baryon with S=+1,charge=+1 →minimal quark content=uudds Its existence is still controversial! 2/28

Previous result of the Θ + search by LEPS γ d →K + K - pn reaction ・ Data taken in ・ 2.0<E γ <2.4 GeV. ・ Significance of 5.1σ from shape analysis. (Δ(-2lnL) with/without signal) ・ Mass=1524±2 ＋ 3MeV/c 2. ・ CLAS reports null result with the same reaction but acceptance is almost exclusive. ・ New data was taken in with almost the same setup. To confirm the result.. arXiv: /28

Backward-Compton scattering(BCS) b) Laser hatch a) SPring-8 SR ring c) Experimental hatch 36m 70m Collision 8 GeV electron BCS photon Laser light ・ Two laser injection system to increase beam intensity. ・ 5W Ar laser → 8W solid state laser 。 ・ Beam intensity of 1→2Mcps was achieved at the maximum. ・ About 2.6 times statistics was collected in Upgrade since previous experiment 4/28

 1m1m TOF wall MWDC 2 MWDC 3 MWDC 1 Dipole Magnet (0.7 T) Start counter Silicon Vertex Detector Aerogel Cherenkov (n=1.03) Linearly polarized LEPS forward spectrometer Liquid deutelium Target (150 mm thick ) ・ The same setup for two experiments. ・ Symmetric acceptance for positive/negative charged particles. symmetry in Θ + /Λ(1520) production. 5/28

Search for Θ + in Fermi-motion corrected K - missing mass K-K- K+K+ n p d  detected P pn P min P rec Θ + from M(NK + ) Λ(1520) from M(NK - ) MM(K - )X 30 MeV/c 2 M(nK + ) by MMSA :11 MeV/c 2 (16 MeV/c 2 for Λ(1520) ) missing momentum Minimum Momentum Spectator Approximation (MMSA): Assume possible minimum momentum configuration for the spectator. γn→K - Θ + →K - K + n γp→K + Λ(1520)→K + K - p 6/28 undetected Resolution of MM(K - ) or MM(K + ) are not good due to Fermi-motion. Reactions of interest

Inclusive/Exclusive Analysis K-K- K+K+ n spectator p d γ K-K- K+K+ p spectator n d γ    Analysis which do not separate two channels → Inclusive analysis Analysis which separate two channels → Exclusive analysis ・ Previous analysis was inclusive analysis. ・ Blind analysis was performed to check previous result. (Selection cut is not changed from previous analysis. calibration fixed before opening the box) γp→K+K-pγp→K+K-p γn→K+K-nγn→K+K-n 7/28

Comparison of the Λ(1520) peak PreviousNew Peak position(MeV) ± ±1.0 Peak width (MeV)18.5± ±1.6 Peak height53.0± ±8.2 S/N ratio1.74± ±0.15 MMn(γ,K - )X>1.6GeV/c 2 to keep the condition of the blind analysis. Previous dataNew data γp →K + Λ(1520) →K + K - p K-⇔K+n ⇔ pK-⇔K+n ⇔ p γn →K - Θ + →K + K - n ✓ Λ(1520) peak was found to be consistent for two data sets. ✓ Other distributions were also checked and consistent. we decided to add two data sets after opening the box. ・ Is there any problems on new data? ・ Is it possible to add two data sets? 8/28

Box open for new data Statistical significance less than 2σ, 1512±5MeV/c 2 fitting w/o signal fitting w/signal Λ(1520) cut M(pK - )>1.55 GeV/c 2 9/28

Consistency check of final M(NK + ) spectrum The increment of the χ 2 from the best fit in the space of peak height and position of signal. →almost 3σ deviation from two data sets. New data previous data ・ Two data sets are normalized by the entry. ・ In total, spectrum are consistent. χ 2 /ndf=56.4/66 K.S test 58.8% Unlikely to happen → Overestimation of significance by shape analysis. 10/28

Exclusive Analysis K-K- K+K+ n spectator p d γ K-K- K+K+ p spectator n d γ    Analysis which do not separate two channels → Inclusive analysis Analysis which separate two channels → Exclusive analysis γp→K+K-pγp→K+K-p γn→K+K-nγn→K+K-n Separation of events from proton and neutron largely improves the sensitivity. 11/28

Exclusive analysis by using Start Counter (SC) Energy loss of SC – expectation of KK – Half of expectation of proton KKp only KKn and part of KKp M(NK - ) ・ S/N ratio of Λ(1520) improved by tagging proton because KKn events are rejected. ・ Conversely, we can enhance the neutron events. Proton not tagged (Proton rejected) Proton tagged K+K+ K-K- p n K+K+ K-K- p K+K+ K-K- or 12/28 Λ(1520)

previous new Proton rejected M(NK + ) for exclusive samples ・ Peak is seen in tagged events for the previous data while not seen in the new data. ・ An enhancement is seen in proton rejected events in the both data. ・ After the proton rejection, spectrum for two data sets are very similar Proton tagged 13/28

Exclusive samples for summed data Proton tagged Proton rejected ・ Structure seen in proton tagging becomes much smaller. ・ Enhancement is seen in proton rejected events. → Further rejection of the proton events. 14/28

Neutron enhanced sample Efficiency Proton rejection efficiency becomes 60%→90% by selecting downstream of target M(nK + ) 15/28

To check the enhancement in the exclusive sample K+K+ K-K- p Exclusive analysis is.. model independent rejection of proton events using SC. However, acceptance of the vertex cut is only ~40%. proton “tagged” sample K+K+ K-K- p proton “leaked” sample Extrapolate with a help of MC Subtract from full data sample. →MC based exclusive analysis. 16/28

1. Distributions for proton-tagged events were fitted with realistic MC distributions. 2. The whole proton contributions including events which leaked from SC are estimated based on the fitting results. 3. The estimated proton contributions are subtracted from full data sample (without z-vertex and proton tagging cut). Procedure of MC based exclusive analysis 17/28

Fitting proton-tagged events φ and non-resonant KK Λ(1520) Λ(1405) Summed χ 2 /ndf = 33.3/37 χ 2 /ndf = 34.4/37 χ 2 /ndf = 33.9/37 ・ Extended maximum-likelihood un-binned fit. ・ M(pK + ), M(pK - ), cos(Θ) of K + are simultaneously fitted. ・ Ratio of φ to non-resonant KK is determined from M(KK). ・ Λ(1405) to explain threshold enhancement of M(pK - ) ・ χ 2 /ndf is close to one. M(pK + ) M(pK - ) 18/28

MC-based exclusive analysis M(NK - ) Real data Estimated proton contribution M(NK + ) ・ No Λ(1520) peak after the proton subtraction in M(nK - ) distribution. ・ Enhancement is seen in M(nK + ) Subtract proton contribution M(nK - )M(nK + ) 19/28

overlay with normalization by entry dE/dX-based exclusive anslysis Subtract leaked proton contribution M(nK + ) with two methods MC-based exclusive analysis 20/28

Proton/neutron ratio from two methods 1.MC based Ratio of estimated proton contribution to the neutron contribution for the full data sample. → 4616/2831 = dE/dx based. Ratio of tagged proton contribution to the neutron contribution for the sample with vtz cut (proton tagging efficiency of 0.9 was taken into account). → 1770/1119 = 1.58 We understand/control proton contribution very well. Consistent in the both exclusive analyses. 21/28

Problem and Improvement of Exclusive analysis 1.Events from a proton are rejected. 2.Leaked proton contributions estimated by MC are subtracted. We know there is a fluctuation at 1.53 GeV/c 2 in M(pK + ) in the previous data. 22/28

n+p and p(leaked) subtracted M(nK - ) distribution The peak did not appear in M(nK - ) 23/28

subtracted n+p and p(leaked) M(nK + ) distribution The peak appeared in M(nK + ) 24/28

subtracted New data ( ) n and p(leaked) The peak appeared in the new data. 25/28

Horizontal Vertical Pol. dependence The large polarization dependence of the S/N ratio was seen. 26/28

The spectrometer acceptance has approximately rectangular shape. K+K+ K-K- K+K+ K-K- If K + and K - prefer to fly parallel to the polarization, the acceptance is larger for horizontal case. → Suggesting non-resonant KK has p-wave component Vertical Horizontal Origin of polarization dependence 27/28

The Θ + is studied via γd→K + K - pn reaction with high statistics data at SPring-8/LEPS. 2.6 times higher statistics compared with previous data are collected. The inclusive M(NK + ) spectrum for new data does not show a strong narrow peak, which is inconsistent with the previous shape analysis. -The significance of the peak in new data is less than 2 σ by the shape analysis. Exclusive analysis is performed by identifying the proton contribution using energy loss in SC. -A part of the inconsistency was due to fluctuation in proton tagged events. -Enhancement of events are seen in the region of 1.5<M(nK + )<1.55 GeV/c 2 for proton rejected events. -The enhancement is seen in the both new and previous data. These results are checked and confirmed by MC-based exclusive analysis. S/N ratio strongly depends on the polarization. Mass and significance estimation of the enhancement is underway. LEPS collaboration just started new experiment with large SC. Summary 28/28

Backup

Start counter Light collection is not good near the edge of the counter. → Efficiency was estimated by using both LH 2 and LD 2 data

Other checks: φ events P min for φ events χ 2 /ndf=96.6/90 K.S test 52.4% New data Previous data (normalized by entry) Other distributions are also checked and consistent. We decided to add two data sets after opening the box. 31/28

Photon energy spectrum previous data new data GeV GeV GeV 1mm pitch SF 0.1mm pitch SSD Multi line laser Single line laser Tagger GeV

Summing two data sets Statistical significance of σ, peak position ~1520 MeV although it may contain large systematic error. Λ(1520) cut is implemented fitting w/o signalfitting w/signal Y. Kato FB /8/23 10

subtracted Downstream(vtz>-980 mm) n and p(leaked) The peak appear in low proton-leakage region.

subtracted Upstream (vtz<-980 mm) n and p(leaked) The peak appear in high proton-leakage region. The number of neutron events is consistent with the acceptance.

Comparison of data and MC with loose φ cut Data MC ・ φ events are excluded by M(KK)>1.03 GeV/c 2 ・ z-vertex, proton tagging cut is applied ・ Good consistency between data and MC M(pK - )M(nK + )

LH2 p(miss) vs. p(dEdx) LD2 p(miss) vs. p(dEdx) LH2 p(MMSA) vs. p(dEdx)

Expected mass resolution for Θ + Signal MC with realistic DC and E γ resolution Previous data 10.9±0.1 MeV/c 2 New data 10.8±0.1 MeV/c 2

M(KK)>1.03 GeV/c 2 M(KK)>1.04 GeV/c 2 M(KK)>1.05 GeV/c 2 dE/dX based and MC based M(nK+) with constant φ cut

Check the bias of proton tagging by LH2 data ・ No significant structure for un-tagged events ・ Number of events are actually very small MMγ(p,K - )XMMγ(p,K + )X Proton detected Proton not detected

Subtract proton contribution for previous data M(pK - ) subtract Real data Proton events from MC M(nK + ) subtract

Subtract proton contribution for data M(pK - ) subtract Real data Proton events from MC M(nK + ) subtract

New Previous MMp(γ,K + )X MMp(γ,π + π - )X MMp(γ,K + K - )X MMp(γ,K - p)X Comparison of the missing mass spectrum for LH2 data

Fitting new data only

Subtract proton contribution from full data sample (B.G from new data) M(NK - ) Real data Estimated proton contribution from new data M(NK + ) Subtract proton contribution M(nK - )M(nK + )

Simple K - missing mass for proton rejected sample

Photon energy spectrum previous data new data GeV GeV GeV 1mm pitch SF 0.1mm pitch SSD Multi line laser Single line laser Tagger GeV

φ exclusion cut M(K + K - ) for real data Offset=1.02, slope=0.09 non-resonant KKn MC Before cut After cut Reject medium mass region. Do not create a peak by the cut. E γ eff VS M(nK + )

Randomized minimum momentum method for BG estimation P min VS MMγ(n,K - )X signal MC non-resonant KKN MC Signal events has strong correlation between Pmin and MMγ(n,K - )X P min VS magnitude of Fermi correction approximated M(nK + ) M(nK+) appro M(nK+) MMγ(n,K - )X ΔM=12MeV/c 2

Real data MMn(γ,K - )X P min Mean±1σ MMγ(n,K-)X randomized Pmin approximated M(nK + ) times all the events RMM spectrum MC non-resonant KKN RMM spectrum

w/o Λ(1520) cut w Λ(1520) cut ・ RMM spectrum is divided into three regions. ・ Significance is obtained from Δ-2ln(L) with and without the Gaussian ・ The peak height is 27.5±5.0, Δχ2/ndf=30.4/2 = 5.1σ. peak position 1524.±2 ＋ 3MeV Application of the RMM to previous data

CutExaminedPassedRejection KK ID Vertex DIF Tagger P min E γ eff φ Ntrk= Unphy CutExaminedPassedRejection KK ID Vertex DIF Tagger P min E γ eff φ Ntrk= Unphy Summary of the cut statistics previous dataNew data

χ 2 /ndf=53.9/62 K.S test 57.6% K - missing mass with all cuts

Search for Θ + in Fermi-motion corrected K - missing mass K-K- K+K+ n p d  detected P pn P min P rec Θ + from M(NK + ) Λ(1520) from M(NK - ) MM(K - )X:30 MeV/c 2 M(nK + ) by MMSA:11 MeV/c 2 (16 MeV/c 2 for Λ(1520) ) MM(K + K - ) Minimum Momentum Spectator Approximation (MMSA): assumes possible minimum momentum configuration for the spectator. Resolution of MM(K - ) or MM(K + ) are not good due to Fermi-motion 54/28 undetected γn→K - Θ + →K - K + n γp→K + Λ(1520)→K + K - p Rection of interest