Coincidence Measurement of the Weak Decay of 12
Λ C (E508)
H. Bhang for E508 collaboration
Department of Physics, Seoul National University, Seoul 151-747, Korea

The experiment is to measure the exclusive decay observables in the weak decay of 12
Λ C. The aimof the experiment is to obtain the mass dependent information of the observables by adding p-shell
55hypernuclei, 12
Λ C, in the data set, in addition to that of s-shell Λ He of E462. The data set of Λ He and12
Λ C will give the mass dependence of 1N-, 2N-NMWD and FSI whose information will help to sortout the 2N-NMWD and FSI. There has been intriguing inconsistency of the asymmetry parameterof NMWD among those of the s-, p-shell hypernuclei and theoretical predictions of baryonic weakinteraction. To shed light on the situation of the asymmetry parameter of NMWD is anotherimportant purpose of the experiment.
We have measured the decay product particle spectra from the Λ hypernucleus 12
Λ C by detecting
both neutral (n and γ) and charged particles (p and π − ) in coincidence for the total ∼2000 Gπ + beam.
The experiment was carried out at K6 beam line of KEK 12 GeV-PS with 1.05 GeV/c π + beams via
12C(π + , K + ) reaction using the large acceptance SKS spectrometer for the detection of the outgoingkaons. The decay product particles, neutrons, protons, pions and gammas were detected by three
decay coincidence counter systems placed around the target. Top and bottom counter systems
are symmetrically placed and the third side arm counter was installed for the angular correlation
measurement of two nucleons from NMWD process. Top and bottom counters covered ∼ 9 percent
of solid angles and the side one ∼7 percent. Each counter system consisted of a decay product starttiming counter, a drift chamber, stop timing counter and neutron counters of 6 layers of 5 cm thickplastic sintillation counters.
The analysis has been going on smoothly and now actively progressing. The neutral particle
identification was made from the time of flight spectrum as shown in Fig.1. The separation of n and
γ is good. A preliminary result on the neutron spectrum shown in Fig.2, though the normalizationis still changing, but not drastically, confirmed the strong neutron deficiency observed in E369 whichshowed the proton channel dominance in NMWD of Λ hypernuclei finally resolving the long standing
Γn /Γp puzzle [1, 2]. The charged particle identification was made combining the ∆E-E method andTOF-E methods and a typical PID for charged particles is shown in Fig.1. In order to show acomparison with that of neutron, a typical proton number spectrum, though very preliminary, also
is shown in Fig.2.
An important discovery has been made in the analysis of decay asymmetry parameter, αN M , inthe NMWD of 12Λ C with much improved statistics over that of E160. We have obtained αN M closeto zero, with one sigma level of about 10 percent, which is contradicting the previous result of large
negative value and establishing a new result consistent with that of E278.
Another important result has been obtained, though preliminary again, for Γπ0 with a much
reduced error bar which has been the largest error source in the study of NMWD due to the large
uncertainty of the previous Γπ0 measurement.

[1] O. Hashimoto et al., Phys. Rev. Lett. 88, 042503 (2002).
[2] J. Kim et al. Submitted to Phys. Rev. Lett.

2CountCharged particle ID for 12
Λ C400

proton350

accept

300
250
200

π

150
100

d50
0-2-1.5-1-0.5

00.5

11.5

2

PID function

12Count

Neutral particle ID for Λ C350

accept300
250

γ200150

neutron100
500-4-20246810

12141/βFIG. 1: The above one shows the particle identification of the detected charged particles and the below thatof the neutral particles.
12Count / DECAY ( / 10MeV )Energy spectrum from Λ C
0.18

Neutron ( 2 MeVee )0.16

Proton

0.14
0.12
0.1
0.08
0.06
0.04
0.02
0020

40

60

80

100120140160MeVFIG. 2: Preliminary spectra of the neutron and proton number per hypernuclear decay.