Exclusive Measurement of the Non-Mesonic Weak Decay of 5LHe (6)

1
2004.06.08 da?ne2004
First Exclusive Measurement of the Non-Mesonic
Weak Deacay of 12?C
Contents of talk nn/np
coincidence measurement ?n/?p
0.400.09(statistical error)
Preliminary!!
Seoul national University
M.J.Kim
KEK-PS E462/E508 Collaboration
2
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
3
Purpose
Weak decay mode of ? hypernucleus
Mesonic q 100MeV/c
Gp_ (?? p p-) Gp0 (?? n p0 )
Gm
1/tHY Gtot
Gp (? p? n p) Gn (? n ? n n)
Non Mesonic(NMWD) q 400MeV/c
Gnm
G2N (?NN ? NNN ) ?
Direct determination of the ratio !!!!
?n/?p
4
Status of ?n/?p of 12?C(until recently)
Theory
Model G n / G 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/?pexp gtgt ?n/?pth ?n/?p ratio puzzle!
0.19
0.27
0.20
0.83
0.12
0.10
Problems in Experiment 1)high threshold 2)
Large error bar Problems in determination
method of ?n/?p ratio from data 1)model
dependent 2) Effects of FSI and 2N process

Experiment
?n / ?p
Szymanski et al. (BNL 1991)
Noumi et al. (KEK 1995)
1.331.21/0.81
1.870.59 0.32/1.00
FSI Final State Interaction
5
Direct Measurement of the ?n /?p ratio
Select ?N?NN events without FSI and 2N process
effect
Top Detector
Angular correlation ( back-to-back, cosqlt-0.8
) Energy correlation ( energy sum Q
value150MeV )
Nucleon
N
?
NMWD
?N ? NN
N
YnpNnpx ?npx ?nx ?px (1-RFSI)np YnnNnnx ?nnx
?nx ?nx (1-RFSI)nn By charge symmetry
(1-RFSI)np(1-RFSI)nn

12LC g.s.
Bottom Detector
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(KEK-PS K6 SKS)
SETUP
Decay arm
?
Solid angle Neutral 26.5 Charged 10
NT
p
K
n
NT 20cm100cm5cm T3 10cm100cm2cm T2
4cm16cm0.6cm
Charged particle TOF (T2?T3)
tracking(PDC) Neutral particle TOF
(target?NT) T3 VETO
Active Target
beam
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Excitation energy spectra for 12C(p,K)
12LC g.s.
quasi free
inclusive
11C ?
10.76MeV
Proton decay
11
9.21MeV
w/ proton
?B p
11
B
8.1 MeV
?
6.3 MeV
w/ neutron
w/ np
2.5 MeV
0.0 MeV
12
C
w/ nn
?
12
Level scheme of C
?
Excitation Energy(MeV)
11LB
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Particle identification
Charged particle
Neutral particle
proton
5MeVlt energy lt 150MeV
?
Neutron
?
d
1/?
Constant back ground ? very small
PID1 total energy vs dE/dx PID2 total energy
vs TOF ? (PID1PID2) /
Energy resolution ?8MeV at 75MeV
gated 12LC ground state
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Coincidence Measurement
Angular Correlation
Energy sum
En Ep
cos ?lt-0.8
ENgt30MeV
n p
Number109
cos ?lt-0.8
En En
ENgt30MeV
n n
Q value 150
Number33
MeV
cos ?
Estimated Contamination from ?- absorption
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Estimation of Nnp,Nnn (Nnn/Nnp 0.400.09)
Measured Ynn,Ynp
Simulated coincide efficiency
Estimated Nnn,Nnp
?np(Ep,n65MeV)
NnpYnp/?np 5677572
cos ?lt-0.8
ENgt30MeV
Number109
Event by event correction
NnnYnn/?nn2267437
cos ?lt-0.8
?nn(En,n65MeV)
ENgt30MeV
Number33
Angular Correlation(cos )
?
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Coincidence Measurement(E462/E508)
Angular distribution
Energy sum distribution
Counts
n p
s-shell
5
He
?
n n
Q value 153MeV
n p
p-shell
12
C
?
n n
cos ?
Estimated Contamination from ?- absorption
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Results
5?He (E462)0.45 0.11 0.04 (Final !!)
12?C (E508)0.40 0.09(statis.) (Preliminary !!)
?n/?p
5
12
C
He
Noumi et al.
?
?
Szymanski et al.
Sato et al.(1N)
Sato et al.(1N2N)
Szymanski et al.
Kim et al.(1N)
OME
OME,DQ
OPE
OPE
E307
E307
E369
E508
E462
Theory(OPE)
Various Experiments
Theory(OME or DQ)
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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 lt-0.8 and
  ENgt30MeV .
• ?n/?p Nnn/Nnp 0.40 0.09 (statis. )
  (Preliminary!! )
• (expected systematic error 0.04)
• Consistent with recent theoretical
  calculations !!!!

?
?
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Spare OHPS
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Experimental results

Syzmanski et al. (BNL)
Noumi et al. (KEK)
Sato et al.(KEKE307)
Sato et al.(KEKE307)
Kim et al.(KEKE369)
?n/?p
1.331.21/0.81
1.870.590.32/1.00
0.870.090.21(1N)
0.600.11/0.090.23/0.21(1N 2N)
0.510.15(1N)
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Neutron and proton spectra from 12?C
Neutron
N n,p/NMWD/10MeV
Proton
Nucleon energy
(MeV)