Exclusive measurement of the Non-Mesonic Weak Decay of 5ΛHeE462 collaboration
We are studying non-mesonic weak decay(NMWD) ofΛ hypernuclei(Λ+N → N+N). This NMWD is thevery unique strangeness-changing baryon-baryon weak interaction process, which can occur only in nuclei.
Concerning the ratio of two possible NMWD modes, Γn/Γp ≡ Γ(Λ+n→n+n)/Γ(Λ+ｐ→n+ｐ), theoretical
calculation based on one-pion exchange model predicts Γn<<Γp, whereas recent experiments for A=5,12
suggests large ratios close to unity [1,2]. However these reported results have large errors of 30~100%. Up to now,
most of the experiments concerning this ratio measured only protons from Λp → n p process and Γn wasdetermined by the subtraction of all the other decay processes. Thus the obtained results must be much affectedby the small change of the assumption on final state interaction(FSI) effect and by the possible existence of thetwo-nucleon induced NMWD process, ΛNN→NNN.
In order to measure this ratio unambiguously, we choose light s-shell Λ hypernuclei, 5ΛHe, so as to minimize
the FSI effect. In addition, we measured both of n+p- or n+n-pairs emitted from Λ+p→n+p or Λ+n→n+n
NMWD process. When we select two-nucleon pairs which has back-to-back angular correlation and applied energy
sum cut (Q-value for NMWD for 5ΛHe is 152MeV) for them, we can measure Γn/Γp ratio directly only from theratio of n+p- to n+n-double coincidence pair numbers. The result of this measurement is free from the strength of
FSI effect and also from the possible ΛNN→NNN contribution. We took 6Li(π+,K+) data during year 20002001 with SKS.
Fig. 1 shows the neutron and proton spectra from the NMWD of 5ΛHe. Energy loss of proton inside the target iscorrected. Whereas the theoretical calculation predicts the observation of the broad peak at Q-value/2 [3]especially in the neutron spectrum, the observed neutron spectrum shows no peaking. This suggests large
contribution of FSI effect and/or ΛNN→NNN decay process in the NMWD even at A=5. The number of neutron
above 50-60MeV is only 2.1 times larger than that of proton. This ratio is significantly lower than the naïveexpectation of Γn/Γp=1 (initial number ratio=3), indicating the dominance of Λ+ｐ→n+ｐ decay process.
We also established the way to identify back–to-back n+p- and n+n-pairs. Fig. 2 shows the angular correlation of
the n+p- and n+n-pairs from the NMWD of 5ΛHe (double coincidence acceptance is corrected). In both of n+p/n+n,
we observed clear back-to-back correlation. Also the sum energy distributions of two nucleons show peak at the Qvalue of NMWD.
From the back-to-back coincidence ratio, we obtained the Γn/Γp ratio as
Γn/Γp = 0.42+-0.10 (preliminary).
Recent theoretical calculation considering heavier meson predicts the Γ n/ Γ p ratio close to this value[4].
Although the result is still preliminary, both of the dominance of Λ +ｐ →n+ ｐ decay process and thesignificant contribution of Λ+n→n+n process are established for the first time.
As by-products of the experiment, we are also analyzing the asymmetry of proton emission from the NMWD ofpolarized 5 Λ He. Concerning the proton decay asymmetry parameter, we obtained smallα NM =0.09 0.08(preliminary) and confirmed the result of previous SKS experiment [5] (E278: 0.24 0.22) with improved error.
From the theoretical point of view, it is very hard to simultaneously explain the observed
Γn/Γp ratio and small decay asymmetry parameter αNM . This requires reconsideration of the decay mechanismof NMWD.
References[1] J. J. Szymanski et al., Phys. Rev. C33 (1991) 849.
[2] O. Hashimoto et al., Phys. Rev. Lett. 88 (2002) 042503[3] G. Garbarino et al., nucl-th/0212084 (2002)[4] A. Parreno and A. Ramos, Phys. Rev. C. 65 (2001) 015204[5] S. Ajimura et al., Phys. Rev. Lett. 84 (2000) 4052; Phys. Rev. Lett. 80 (1998) 3471.

Fig. 1

Fig. 2

Neutron and proton spectra from the NMWD of5HeΛ

Two-nucleon angular correlation(top) and energy sum(bottom) from the NWMD offor n+p(left) and n+n(right), showing the dominance of Λ+ｐ→n+ｐ process.

5HeΛ