Yudai Ichikawa (Kyoto University/JAEA) 2013/07/26 RCNP 研究究会「核子・ハイペロン多体系におけるクラスター現 象」 1 J-PARC における d(π +, K + ) 反応を 用いた K 中間子原子核の探索

Contents  Introduction  Nuclear K bound states  K - pp  J-PARC E27 experiments  d （ π +,K + ） X reaction  Experimental set up K1.8 beam line + SKS + Range counter Array(RCA)  Coincidence with RCA  Preliminary result of first stage data  Summary 2

 Nuclear K bound state  The deeply bound state by strong interaction.  Strong attraction of the I =0 KN interaction （ KN I=0 ） plays an important role in nuclear K bound state.  K - pp nuclear bound state  The simplest nuclear K bound state.  Theoretical prediction of B.E. and Γ depends on the KN interaction models and the calculation method. PRC76, (2002) arXiv: v2[nucl-th] PRC76, (2007) PRC76, (2007) NPA804, 197 (2008) PRC80, (2009) Theoretical prediction B.E （ MeV ） Γ （ MeV ） T. Yamazaki and Y. Akaishi 4861 A. N. Ivanov, P. Kienle, J. Marton, E. Widman N. V. Shevchenko, A. Gal, J. Mares, J. Revai 50~70~100 Y. Ikeda and T. Sato 60~9545~80 A. Dote, T. Hyodo, W. Weise 20 ± 3 40~70 S. Wycech and A. M. Green 56.5~7839~60 Barnea et al ‘ PRL B712, (2012) 3 Introduction

T.Yamazaki et al., PRL 104, (2010) M.Agnello et al., PRL 94, (2005) Experimental approach of K - pp - FINUDADISTO reactionStopped K - absorption on 6,7 Li+ 12 Cp + Tp=2.85GeV methodInvariant mass of back to back Λ p pairs p+p → X+K + （ missing mass ） X → Λ +p （ invariant mass ） B.E105 ± 5 MeV Width118 ± 8 MeV M.Agnello et al., PRL 94, (2005)T.Yamazaki et al., PRL 104, (2010) K - +p+p~2.37GeV/c 2 4

d( π +, K + ) reaction for J-PARC K1.8 beam line π + + n → Λ (1405) + K + Λ (1405) + p → bound K - pp 1 % (→ quasi free Λ * 99% ） K - pp is produced via a Λ (1405) doorway π+π+ K+K+ Target 5 Y.Akaishi, T.Yamazaki, Phys. Rev. C (2007) P π = 1.7GeV/c 5

Expected Inclusive spectrum Background of d （ π +,K + ） Counts / 500keV - π + “n” -> Σ 0 K + - π + “n” -> Λ K + - π + “n” -> Λ (1405) K + - π + “n” -> Σπ K + - π + “n” -> Σ 0 (1385)K + - π + “n” -> Λπ K + - π + “n” -> pK - K + - π + “p” -> Σ + K + - π + “p” -> Σ + (1385)K + - π + “p” -> Λπ K + - π + “p” -> Σπ K μ b 76.7 μ b 124 μ b 13.7 μ b 40 μ b 28.9 μ b 40 μ b 470 μ b 106 μ b 246 μ b 38 μ b Missing Mass [GeV/c 2 ] K - + p+p~2.37GeV/c 2 6 K - pp ~1000events ( Λ (1405)1% sticking) Γ FINUDA =67MeV/c 2 K - pp Cannot observe K - pp signal from the Inclusive spectrum. ->Coincidence measurement. 6

Range Counter Array (RCA) for BG suppression RCA SKS Range counter Array(RCA)  6units  5 layers ( cm) of plastic scintillator.  39° – 122° (L+R)  50 cm TOF θ K - pp → Λ + p p+π-p+π- d （ π +,K + ） K - pp 7

K - pp → Λ + p 1, K - pp → Σ 0 + p 1, p 2 + π - Λ + γ, p 2 + π - π + + d → Λ * + K + + p 1s, Λ * → Σ + π, Σ + → p 2 + π 0 Background process P 1s < 250MeV/c Background suppression by protons tag In the quasi free background process, momentum of spectator proton(p 1s ) is smaller than 250MeV/c. K - pp decay 8

Summary of first step data  Goal  Inclusive d( π +,K + )X < Mx <2.5GeV/c 2. The first measurement of this reaction and this missing mass region.  p( π +,K + )X data Check the contribution of “p” in “d”  Check the feasibility of coincidence measurements. One proton tag/ two proton tag (with RCA )  Beam time summary  p( π +,K days; pion number ~ 7.6*10 9  d( π +,K days; pion number ~ 3.3*

Spectrometer performance  p( π +,K + ) Σ GeV/c [LH 2 Target] ~ Energy Calibration & Cross section normalization ~ ○ Resolution ; Δ M ~ 2.41MeV(FWHM) ○ Mean ; ± 0.03 MeV (PDG = MeV) MissingMass[GeV/c 2 ] Counts / 0.05MeV preliminary 10 Σ + Cross section [μb/sr] Scattering angle(Lab)[degree] Red: Present data Blue: D.J. Candlin preliminary

p( π +,K + )1.7GeV/c  Σ + production  Δ M = 3.2MeV(FWHM)  Mass = MeV  Σ + (1385) production  Y π production Missing Mass[GeV/c 2 ] Counts / 1MeV Simulation Data Σ+Σ+ Σ*+Σ*+ preliminary - π + “p” -> Σ + (1385)K + - π + “p” -> Λπ K + - π + “p” -> Σπ K + Zoom Missing Mass[GeV/c 2 ] 11 Scattering angle(Lab)[degree] Red: Present data Blue: D.J. Candlin Σ + Cross section [μb/sr]

 Fitting result for Σ (1385) region p( π +,K + )1.7GeV/c Black:Data Green: Σ(1385) + Red: Λπ Pink: Σπ Missing Mass[GeV] χ 2 =2.270 Counts/ 1MeV Σ (1385) + Mass & Width =PDG value 12 preliminary

Counts / 500keV Missing Mass [GeV/c 2 ] Black: Simulation Red: Data Λ Σ Y* Inclusive spectrum Proton + Σ 0 = GeV Neutron + Σ + = GeV Zoom 13 preliminary 13 d( π +,K + )1.7GeV/c

Differential cross section for d( π +,K + ) reaction θ Lab = 2 - 4° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] 14 θ Lab = 4 - 6° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] θ Lab = 6 - 8° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] θ Lab = ° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] θ Lab = ° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] θ Lab = ° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] θ Lab = ° dσ/dΩ/dm[ub/sr/2MeV] Missing Mass[GeV] 14 preliminary

Cross section for Λ and Σ 0 production dσ/dΩ/dm[ub/sr/2MeV] dσ/dΩ/dm for θ=2~4° Residual Missing Mass[GeV] Cross section for Λ production Cross section for Σ 0 production dσ/dΩ[ub/sr] Black: data Pink: ΣN cusp Red: Λ Green: Σ + +Σ 0 Blue: Simulation all 15 preliminary

Summary  E27 experiment  Search for K - pp bound state via the d( π +,K + ) reaction.  Coincidence measurement with RCA.  Inclusive analysis  We have obtained d( π +, K + ) at 1.7 GeV/c for the first time.  Together with the p( π +, K + ) data, the inclusive spectra are understandable.  Cusp structure in Σ N threshold is observed.  There are some differences between data and simulation in Y* region.  Coincidence analysis with RCA  Analysis is on going. –> Need detail study. 16

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K - pp → Λ + p 1, → p 2 + π - proton2 theta_Lab proton1 theta_Lab angular distribution of 2protons （ >300MeV/c) from K - pp 18 π + + d → Λ * + K + + p 1s, Λ * → Σ + π, Σ + → p 2 + π 0 Background process P 2 > 250MeV/c θ P2 < 30° Background suppression by protons tag We can suppress the quasi-free BG by requiring high momentum proton with RCA. RCA K - pp → Λ + p 1 p2+π-p2+π- 18

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chisqr 2_4deg= _6deg= _8deg= _10deg= _12deg= _14deg= _16deg=

Result from CLASS for Λ (1405)