Exclusive measurement of the Non-Mesonic Weak Decay of 5ΛHeE462 collaboration
It is known that the dominant weak decay mode of Λ particle bound in nuclei is Λ+N → N+N. This nonmesonic decay is the very unique strangeness-changing baryon-baryon weak interaction process, which can occur onlyin nuclei. Concerning the ratio of two non-mesonic decay modes, Γn/Γp ≡ Γ(Λ+n→n+n)/Γ(Λ+ｐ→n+ｐ),
recent experiments for $A=5,12$ suggests large ratios [1,2]. However these reported results have large errors of 30~
100% so the largeΓn/Γp ratio is not yet experimentally established. So far, several theoretical calculations arecarried out based on one-pion or one-boson exchange models or the hybrid quark model, but none of them could
simultaneously explain the non-mesonic weak decay(NMWD) rate and Γn/Γp ratio.
Up to now, most of the experiments concerning this ratio of NMWD measured only protons from Λp → n p
process and Γn was determined by the subtraction of all the other decay processes. Thus the obtained results mustbe much affected by the small change of the assumption on the rescattering process (final state interaction(FSI)
effect) and by the possible existence of the two-nucleon induced NMWD process, ΛNN→NNN.
In order to measure the ratio unambiguously, we choose light s-shell Λ hypernuclei, 5ΛHe, so as to minimize theFSI 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 ΛN→NN decay process for 5ΛHe is about 152MeV) for them, we can measure Γn/Γp ratio directly
only from the n+p- to n+n-double coincidence pair numbers. The result of this measurement is free from the strengthof FSI effect and also from the possible ΛNN→NNN contribution.
With Superconducting Kaon Spectrometer(SKS) at k6, we took 6Li(π+,K+) data during year 2000-2001. In theinclusive hypernuclear formation spectrum, we identified more than 50,000 5ΛHe formation events.
The left figure shows the preliminary neutron
spectra from the NMWD of 5ΛHe(about 30% of total).
Numbers of detected neutrons changes with detectionenergy threshold(top), but after the correction by thesimulated energy- dependent neutron detection
efficiency (bottom), all the data points agrees well.
The neutron spectrum (bottom) shows no peaking at
Q-value/2 ~ 76MeV, which strongly suggests large
contribution of FSI effect and/or ΛNN→NNN decay
process in the NMWD of hypernuclei even at A=5.
It should be noted that the statistics of neutron
detection from the NMWD of 5ΛHe is improved by
two-orders of magnitudes compare to the previous
measurement[1].
In the preliminary analysis of the 1st year data, wealso established the way to identify back –to-backn+p- and n+n-pairs.
Top figure of the next page shows the angularcorrelation of the neutron and proton which weredetected simultaneously from the NMWD of 5ΛHe(double coincidence acceptance is corrected). We
can clearly identify the back-to-back n+p-pair from Λp→
np NMWD process in this angular distribution plot. The
statistics of the n+p eventsin cosθ<-0.9 region is about 50 counts in year 2000 data.
We also analyzed n+n-double coincidence from the Λn→nn decay process and found that angular correlation plot of
two neutrons has very sharp peaking at cosθ<-0.9 (just 10counts in year 2000 data). This is the first experimental

evidence forthe existence of the Λn→nn NMWD decay process.
When we select the back-to-back component in n+p double coincidence events, the shape of the neutron spectrum
changes drastically as given in second figure in this page.
Although single neutron spectrum(top) shows no peaking at Q/2, the spectrum of neutron which have back-to-backproton coincidence pair detection (middle/bottom) gives broad peaking (due to the Fermi motion in the initial state) at
about Q/2~76MeV, which shows the clean identification of Λp→np NMWD process.
From the n+p to n+n back-to-back coincidence numbers, wecan estimate the Γ(Λ+n→n+n)/Γ(Λ+ｐ→n+ｐ)
ratios as 0.7 0.2 (very preliminary) from year 2000
data, which suggests that the ratio is slightly smaller thanone.
Recently, we finished the (π+,K+) data tracking for year
2001 data. From the yield ratio of the 5ΛHe peak formation
rate, we can expect the improvement of the statistics by 3.5-4
times when we add year 2001 data.
Thus we can achieve Δ(Γn/Γp) ~ 0.1 error level.
Also we finished data taking for 12ΛC in this year (E508).
We expect same error-level results for both of A=5 and A=12.
As by-products of the experiment, we are also analyzing thelifetime and asymmetry of proton emission from the weak
decay of 5ΛHe.
From year 2000 data, we already obtained the lifetime as 282
20 ps (preliminary), whose error level is slightly better thanthe previous measurement[1]. Since the present error ismainly determined by the statistics, we can expect ~10 pserror-level results in the E462 final result.
Concerning the proton decay asymmetry parameter, weobtained αNM =-0.07 0.18 (very preliminary) from year 2000 data, which is consistent with the previous SKS
experiment [3] (E278: 0.24 0.22).
We will obtain ΔαNM ~0.1 level results when we analyze all the E462 data.
From the theoretical point of view, it is very hard to simultaneously explain the “large” (close to unity) Γn/Γpratio and “very small” decay asymmetry parameterαNM. We may need to consider new decay mechanism (e.g. large
contribution of the NMWD from p-wave initial ΛN state) when both of them are established experimentally.
The off-line analysis of E462 is in progress now and we will get preliminary results of year 2001 data at around
September 2002.

References[1] J. J. Szymanski et al., Phys. Rev. C33 (1991) 849.
[2] O. Hashimoto et al., Phys. Rev. Lett. 88 (2002) 042503[3] S. Ajimura et al., Phys. Rev. Lett. 84 (2000) 4052; Phys. Rev. Lett. 80 (1998) 3471.