First ExclusiveMeasurement of the Non-Mesonic Weak Deacay of 12  C First Exclusive Measurement of the Non-Mesonic Weak Deacay of 12  C Seoul national University M.J.Kim KEK-PS E462/E508 Collaboration da  ne2004 Contents of talk Contents of talk n+n/n+p coincidence measurement  n /  p ~ 0.40±0.09 (statistical error)  n /  p ~ 0.40±0.09 (statistical error) Preliminary!!

E462/E508 Collaboration KEK, RIKEN, Seoul Univ., GSI, Tohoku Univ., Osaka Univ., Univ. Tokyo Osaka Elec. Comm. Univ., Tokyo Inst. Tech. S. Ajimura, K. Aoki, A. Banu, H. Bhang, S.Shin,T. Fukuda, O. Hashimoto, J. I. Hwang, S. Kameoka, B. H. Kang, E. H. Kim,J. H. Kim, M. J. Kim, T. Maruta, Y. Miura, T. Nagae, S. N. Nakamura, H. Noumi, S. Okada, Y. Okatasu, H. Outa, H. Park, P. K. Saha, Y. Sato, M. Sekimoto, T. Takahashi, H. Tamura, K. Tanida, A. Toyoda, K.Tsukada, T. Watanabe, H. J. Yim

Γ p Γ p (Λ+ ｐ → ｎ + ｐ ) Γ n Γ n (Λ+ ｎ → ｎ + ｎ ) Non Mesonic(NMWD) q ~ 400MeV/c 1/τ HY =Γ tot ΓmΓm Γ nm Γ π _ (Λ→ ｐ + π － ) Γ π 0 (Λ→ ｎ + π ０ ) Mesonic q ~ 100MeV/c Weak decay mode of  hypernucleus Γ N Γ 2N (ΛNN → NNN ) ? Direct determination of the ratio !!!! n/pn/pn/pn/p Purpose

Model  n /  p Ramos-Benhold et al. (1994) OPE OME Dubach et al. (1996) OPE OME Parreno et al.(1996) OPE Ramos et al. (1997) OPE  n /  p Szymanski et al. (BNL 1991) Noumi et al. (KEK 1995) Theory Experiment  n /  p exp >>  n /  p th  n /  p ratio puzzle! Status of  n /  p of 12  C(until recently ) ±1.21/ ±0.59 ±0.32/1.00 Problems in Experiment Problems in Experiment 1)high threshold 2) Large error bar Problems in determination method of  n /  p ratio from data Problems in determination method of  n /  p ratio from data 1)model dependent 2) Effects of FSI and 2N process * FSI: Final State Interaction

 12  C g.s. NMWD  N → NN N N Direct Measurement of the  n /  p ratio Top Detector Bottom Detector Angular correlation ( back-to-back, cos  <-0.8 ) Energy correlation ( energy sum ~ Q value~150MeV ) Angular correlation ( back-to-back, cos  <-0.8 ) Energy correlation ( energy sum ~ Q value~150MeV ) Select ΛN→NN events without FSI and 2N process effect Y np =N np x  np x  n x  p x (1-R FSI ) np Y nn =N nn x  nn x  n x  n x (1-R FSI ) nn By charge symmetry (1-R FSI ) np =(1-R FSI ) nn ~ = Y np =N np x  np x  n x  p x (1-R FSI ) np Y nn =N nn x  nn x  n x  n x (1-R FSI ) nn By charge symmetry (1-R FSI ) np =(1-R FSI ) nn ~ = Nucleon

p n ++ K+K+ NT: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm NT: 20cm×100cm×5cm T3: 10cm×100cm×2cm T2: 4cm×16cm×0.6cm Solid angle Neutral: 26.5% Charged : 10% Decay arm Active Target SETUP beam Charged particle ： ・ TOF (T2→T3) ・ tracking （ PDC ） Neutral particle ： ・ TOF (target→NT) ・ T3 VETO (KEK-PS K6 & SKS)NT

Excitation energy spectra for 12 C(  +,K + ) Excitation Energy(MeV) 12  C g.s. quasi free 11  B inclusive w/ proton w/ neutron w/ n+p w/ n+n 11 C +   B + p MeV 2.5 MeV 6.3 MeV 8.1 MeV C  11 B 12  Level scheme of C Proton decay  9.21MeV 10.76MeV

Particle identification Charged particleNeutral particle proton  d PID1 : total energy vs dE/dx PID2 : total energy vs TOF  (PID1+PID2) / PID1 : total energy vs dE/dx PID2 : total energy vs TOF  (PID1+PID2) / Neutron  1/  Constant back ground → very small Constant back ground → very small gated 12  C ground state 5MeV< energy < 150MeV Energy resolution  ~8MeV at ~75MeV

Coincidence Measurement Angular Correlation Energy sum n + p n + n cos θ MeV Estimated Contamination from  - absorption Number:109 cos θ ＜－ 0.8 E N >30MeV Number:33 cos θ ＜－ 0.8 E N >30MeV Q value ~150 E n + E p E n + E n

Estimation of N np,N nn (Nnn/Nnp = 0.40±0.09) Number:109 Number:33 Measured Y nn, Y np Simulated coincide efficiency  np (Ep,n=65MeV)  nn (En,n=65MeV) Angular Correlation(cos ) θ Estimated N nn, N np N np= Y np /  np ~ 5677±572 N nn= Y nn /  nn ~ 2267±437 Event by event correction cos θ ＜－ 0.8 E N >30MeV cos θ ＜－ 0.8

Coincidence Measurement(E462/E508) Estimated Contamination from  - absorption cos θ Counts Angular distribution Energy sum distribution Q value ~ 153MeV He C s-shell p-shell 5  12  n + p n + n

CHe Szymanski et al. Noumi et al. Kim et al.(1N) Sato et al.(1N) Sato et al.(1N+2N) n/pn/pn/pn/p Results Various Experiments Various Experiments E508 OME OME,DQ OPEOPE E462 E307E307E369 Szymanski et al. 5  12  5  He (E462):0.45 ± 0.11 ± 0.04 (Final !!) 12  C (E508):0.40 ± 0.09(statis.) (Preliminary !!) Theory(OME or DQ) Theory(OPE)

Summary The np- and nn-pairs from NMWD of 12  C were observed successfully. The energy sum of back-to-back np- and nn- pairs are approximately equal to the Q-values(~150MeV) of the NMWD.  N nN process were clearly identified !! We got  n /  p ratio of 12  C at cos 30MeV.  n /  p ~ N nn /N np ~ 0.40 ± 0.09 (statis. ) (Preliminary!! ) (expected systematic error ~ ± 0.04 ) Consistent with recent theoretical calculations !!!! → θ

Spare OHPS

1.87±0.59±0.32/ ±1.21/0. 81 Experimental results Syzmanski et al. (BNL) Noumi et al. (KEK) Sato et al.(KEKE307) Kim et al.(KEKE369) n/pn/pn/pn/p 0.87±0.09±0.21(1N) 0.60±0.11/0.09±0.23/0.21(1N +2N) 0.51±0.15(1N)

Neutron and proton spectra from 12  C N n,p /NMWD/10MeV (MeV) Neutron Proton Nucleon energy