Present and Future of LEPS and LEPS2 Takashi Nakano (RCNP, Osaka Univ.) for the LEPS&LEPS2 collaboration HYP2015, September 8th,

Outline LEPS Overview Some recent results HD target LEPS2 Overview First experiment Summary 2

Laser Electron Photon beamline at SPring-8 3 Operated since 2000.

Backward-Compton Scattered Photon 4 PWO measurement tagged Linear Polarization of  beam photon energy [GeV]photon energy [MeV] 8 GeV electrons in SPring nm(260nm) laser  maximum 2.4 GeV(2.9 GeV) photon Laser Power ~6 W  Photon Flux ~1 Mcps E  measured by tagging a recoil electron  E  >1.4 GeV,  E  ~10 MeV Laser linear polarization % ⇒ Highly polarized  beam

LEPS spectrometer K-K- K+K+ DC3 DC2 DC1 Beam Start counter (STC) SSD AC ・ Dipole magnet : 0.7 Tesla ・ Acceptance : Hori : ± 20° Vert : ± 10° ・ AC index : 1.03 (reject 0.6 GeV/c π ) x z y ・ E γ = 1.5 ~ 2.4 GeV ・ tagger rate : ~10 6 cps ・ trigger rate : ~ 100 cps

γ n p K+K+ K-K- p n Θ+Θ+ spectator signal events γ n p K+K+ K-K- p n Λ(1520) spectator reference events Θ + study at LEPS Θ + production via γd → K - Θ + → K - K + pn Spectator can not escape from the target. signal events : γn → K - K + n reference events : γp → K - K + p n/p separation is possible by improving the proton detection efficiency.

Effect of proton rejection proton rejection cut z-vertex cut 90% of proton events (with a neutron spectator) are identified by selecting events stemming from the downstream part of the target. Enhancement is seen in the Θ + signal region. Repeat the experiment with improved proton detection efficiency & data

Large STC (LSTC) x : 780 mm y : 340 mm z : 10 mm Large STC (LSTC) x : 780 mm y : 340 mm z : 10 mm Small STC x : 150 mm y : 94 mm z : 5 mm Small STC x : 150 mm y : 94 mm z : 5 mm

proton detection with STC Proton detection efficiency is improved by using large-area start counter (STC) in run. energy loss at STC p K+K+ K-K- n K+K+ K-K- or proton untagged with STC p K+K+ K-K- proton tagged with STC signal K +,K - detect K +,K - detect K +,K -,p detect K +,K -,p detect LH LH 2 K +,K - detect K +,K - detect K +,K -,p detect K +,K -,p detect

% point % point efficiency becomes ~90 % for events with z-vertex > -960 mm for data with z-vertex > mm for data z-vertex [mm] Counts : LH 2 : LH 2 z-vertex point dependence Overall proton detection efficiency is: ~60% for data ~85% for data Events from LH2 target. Measured proton detection efficiency

γp → K + X γd → K + X MM(K + ) [GeV/c 2 ] Counts assuming proton mass for missing mass calculation Quality of K + missing mass spectra are the almost same. K + missing mass spectra

S.Y. Ryu, phD thesis, 2015

Kaonic nuclei search 17  K/K *  Virtual K, K * p p n n N N N N K K π direct K-exchange is forbidden in (π +,Κ + ) reaction. π+π+ Κ+Κ+ K0K0 K-N interaction is strongly attractive (I=0). weakly attractive (I=1).  KNN bound state (K - pp, K - pn, K - nn)

K - pp search via  + d  K + +   + X From likelihood ratio method;  = 20 MeV :  b  = 60 MeV :  b  = 100 MeV :  b n       Search region 2.22 – 2.36 GeV/c 2 Significant peak cannot be observed.  ~ 10% of Q.F. processes MM(K +  - ) Upper limit of cross section (95% C.L.) ~10% of Q.F processes So far, no peak was observed in inclusive modes. We will try to detect decay products. → Larger acceptance, LEPS2.

Development of HD polarized target RCNP ~100 km

Calibration data taken in Dec. T=0.52 K T=4.2 K P H = % Measurements Time (day)  = days about 8 months T = 300 mK B = 1 Tesla P P P H Polarization & Relaxation time in HD

Recoil electron (tagging) LEP (GeV  -ray) Laser room Inside SR bldg 30m long line 8 GeV electron Laser Outside SR bldg Experimental bldg Beam dump BGOegg LEPS2 spectrometer Storage ring 10 times high intensity: Multi-laser injection & Laser beam shaping Best e-beam divergence (12  rad)  Photon beam does not spread out  Construct experimental apparatus outside SR bldg Backward Compton scattering BGO EM calorimeter Large LEPS2 spectrometer using BNL/E949 magnet  expect better resolutions ~135 m

BL31 =14  rad. BL33 =58  rad. Reaction region (30m) Reaction region (7.8m) Tagging point e- e-  e- e-  Divergence of LEP beam Better divergence  Better tagging resolution Smaller beam size at long distance LEPSLEPS2

LEPS2 Detector 24  TPC DC  counter RPC TOP B=1 T :  p/p  1% for  TPC Prototype Residual RMS=117  m RPC ToF time distribution 2.22 m 5 m >3  K/  GeV/c 2

 + Search at LEPS2 25 pK s invariant mass K * missing mass (t-channel K-exchange is possible) No Fermi motion correction. No φ background. To measure angular dependence of production rate in large angle region, up to CLAS acceptance. A large acceptance and better resolution detector is necessary.

γ ‘n’ → K - Θ + → K - K 0 s p → K - π + π - p K- π p LEPS1 CLAS LEPS2 Wide acceptance for K -. Covers CLAS acceptance. K - ID in p < 1.4 GeV/c. Large acceptance for multi-particle productions. Search for Θ + in the invm(K 0 s p). No need for Fermi-motion correction. No  background 。 E γ = 2.4 GeV Expected invariant mass resolution 26

Two pole structure of  (1405) 27 D. Jido, et al. NPA725(2003) V.K. Magas, E. Oset and A. Ramos, PRL 95

E  K* K  p  K-K- K*(890) Λ(1405) photoproduction with linearly polarized photon 28

E  K* K  p  K-K- K*(890) Λ(1405) photoproduction with linearly polarized photon T.Hyodo et. al, PLB593 29

Exp. hall was constructed. (2010.Oct-2012Jan) Installation of the E949 magnet (2011.Nev-Dec)  counters were installed. (2012.June) Beam pipe (2012.May) 30

Comparison of LEPS and LEPS2 LEPSLEPS2 Beam Intensity (~2.4 GeV)2~3x10 6 (2 lasers)<10 7 (4 high-power lasers) Beam Intensity (~2.9 GeV)2~3x10 5 (2 lasers)<10 6 (4 high-power lasers) PolarizationLinear/Circular Detector Area42m 2 x 3m(h)198m 2 x 10m(h) Charged Particle Acceptance 0~30 degrees7~120 degrees Momentum Resolution0.5% (for 1-GeV kaon)1~1.5% (for 1-GeV kaon) Photon Coveragenone30~110 degrees 31

Experimental Setup 32  BGOegg Inner Plastic Scintillator (IPS) Drift Chamber (DC) Resistive Plate Chamber (RPC) Cylindrical Drift Chamber (CDC) Target 6.8  z=0 m 1.28 m E949  -counter 2 m (Vertical) x 3.2 m (Horizontal) 6.8  z=12.5 m 21  E949 Magnet z=1.6 m Upstream Charge Veto Counter Tagged Photon Energy : 1.3 – 2.4 GeV (355 nm UV laser) Tagged Photon Intensity : 1.4 – 1.8 Mcps (3 or 4 laser injection) From 2015A UpVeto counter has been enlarged since 2015 Jan.

BGO-Egg : ELPH, Tohoku U. Large acceptance photon detector (BGO-Egg) 1320 BGO crystals Covering 24 o ~144 o polar angle 1.3% energy resolution for 1 GeV

Summary of Data Collection 34 PeriodTarget Integrated # of  ’s (tagged E  region) = tagger counts  dead time corr.  DAQ eff. 2014A (Apr.  July) Carbon/CH 2 [20 mm] C: 1.31  10 12, CH 2 : 1.58  (In total, C: 4.29  10 12, CH 2 : 2.56  ) Additionally, empty target 2014B (Nov.  Feb.) LH 2 [40 mm] Hori: 2.24  10 12, Vert: 2.01  Additionally, empty target & nuclear targets 2015A (Apr.  July) Carbon [20 mm] 9.77  (Vert: 8.97  ) Additionally, empty target & CH 2 target All the detector systems have been stably operated in 2015A.

Experimental setup 35 γ + 12 C -> η’ x 11 B + p 12.5m 7°7° 24 ° 144 ° BGOegg calorimeter  Forward TOF target Tagging counter Storage Ring LEPS2 building Drift Chamber Tracking e-e- 150 MeV H.Nagahiro et al. PRC74(2006)45203

γ + 12 C -> η’ x 11 B + p Experimental method 36 Identify η’ production by η tag BGOegg calorimeter 1m 2 γ 3π 0 ->6 γ → → η (39%) 2m 3.2m Search for a bound state Forward TOF 12.5m from the target Vert : ± 7 ° Hori: ± 4 ° (33%)

Expected energy spectrum GeV γ 0°0° NJL model calculation No shift bound 150MeV shift ΔE=28MeV bound multi π Secondary η E ex -E 0 (MeV) ΔE=28MeV η tag (6 γ ) E ex -E 0 (MeV) H. Nagahiro Small background See signals in bound region counts

η tag by BGOegg (C target run) 3π 0 invariant mass 3 neutral cluster combinations minimizing η -> 3 π 0 -> 6 γ decay mode BGOegg cluster = 6 or 7 3π 0 InvM (MeV) 2 γ InvM (MeV) Energy re-calculate sigma 8.0±0.1MeV (PDG:547.9MeV) η tag (3σ)side band (3-6σ) Choose best π 0 combination small combinatorial (multi π) BG mean 548.0±0.1MeV MeV 2 γ invariant mass

Proton missing mass spectra γ + 12 C -> X + p (γ + 12 C -> η’ x 11 B + p) Mx – M 11B - M η’ η tag (signal region masked) missM ’ (MeV ) large η related BG ηX (ηπ, ηππ) secondary η (πN→ηN) MC simulation missM(MeV) side band 20MeV/bin Signal region (-200~200MeV) masked cos (ηp Opening angle) -200 < MisM <200MeV masked no events in < -0.9 cosOpen

Mass spectra from CH 2 target run 6-gamma missing mass 2-gamma missing mass MeV Select proton 6-gamma invariant mass2-gamma invariant mass MeV require   and    ’’

Combination of BGOegg and RPC(TOF) Proton missing mass for 2 gammas Momentum conservation Invariant mass and proton missing mass Invariant mass of 2 gammas Invariant mass of 6 gammas Proton missing mass for 6 gammas By using both BGOegg and RPC, background events are cleaned up. MeV    ’’ ’’ ’’   ’’  require   and  presented by T.

Summary 42 LEPS Updates on Θ +,  interference, kaonic nuclei search. Development of the HD target LEPS2 x10 luminosity. ~10Mcps. Two different experimental setups. Solenoid spectrometer Θ +, Λ(1405) BGO EGG + TOF(RPC) Backward meson production from proton and nuclei BGO EGG experiment was started last year.