Two nucleon induced Lambda decay and the neutron to proton induced ratio

Three-body nonmesonic weak decay of the (Lambda)12C hypernucleus

We have measured the branching ratio of the three-body process in the nonmesonic weak decay of Lambda12C to be 0.29+/-0.13. This result was obtained by reproducing the nucleon and the nucleon pair yields introducing a measured final state interaction. At the same time, we have determined the absolute decay widths, Gamma(n) and Gamma(p), along with Gamma2N, whose relative ratio has been a long-standing puzzle. Including the three-body process, we have successfully reproduced the nucleon energy distribution, the coincidence two-nucleon angular correlation, and the momentum sum distribution simultaneously.

Nucleon-nucleon coincidence spectra in the nonmesonic weak decay of lambda hypernuclei and the gamma(n)/gamma(p) puzzle

By combining a one-meson-exchange model for the LambdaN-->nN transition in finite nuclei with an intranuclear cascade code, we have obtained nucleon-nucleon (angular and energy) coincidence distributions from the nonmesonic weak decay of (5)(Lambda)He and (12)(Lambda)C hypernuclei. Although, due to the elimination of interferences, two-nucleon coincidences are expected to give a cleaner extraction (with respect to single nucleon observables) of the ratio Gamma(n)/Gamma(p) identical with Gamma(Lambdan-->nn)/Gamma(Lambdap-->np), we show that the effect of the final state interactions is still important even when applying favorable energy and angular cuts. The agreement of our results for the ratio N(nn)/N(np) between the number of nn and np emitted pairs with preliminary KEK coincidence data allows us to conclude that Gamma(n)/Gamma(p) for (5)(Lambda)He should be small and close to the value of 0.46 predicted by our meson-exchange model.

Proton energy spectra in the nonmesonic weak decay of 12lambdaC and 28lambdaSi hypernuclei

Numbers of protons per Gamma hypernuclear weak decay were measured as a function of proton energy above 40 MeV, explicitly identifying production of Gamma hypernuclei by the (pi+,K+) reaction. The ratios between the neutron-stimulated to proton-stimulated nonmesonic decay widths, Gamma((Lambda)n-->nn)/Gamma((Lambda)p-->np) ( = Gamma(n)/Gamma(p)) were extracted by fitting the proton energy spectra. The present result claims that the proton yields are suppressed and the Gamma(n)/Gamma(p) ratios are close to 1 both for 12LambdaC and 28LambdaSi in contradiction to theoretical expectations based on meson exchange models.