Measurement of the proton spectra from Non-Mesonic Weak Decay of $^5_{\Lambda}He$, $^7_{\Lambda}Li$ and $^{12}_{\Lambda}C$

Abstract

The results of a measurement of the proton spectra following the Non--Mesonic Weak Decay of $\mathrm{^5_{\Lambda}He}$, $\mathrm{^7_{\Lambda}Li}$ and $\mathrm{^{12}_{\Lambda}C}$ are presented and discussed. The experiment was performed at the ($e^+$ $e^-$) collider DA$\Phi$NE at Laboratori Nazionale di Frascati of INFN. It is the first measurement for $\mathrm{^7_{\Lambda}Li}$, and for all the spectra the lower limit on the energy of the protons is 15 MeV, never reached before. All the spectra show a similar shape, namely a peak at around 80 MeV as expected for the free $\Lambda p \to np$ weak reaction, with a low energy rise that should be due to Final State Interactions and/or two--nucleon induced weak processes. The decay spectrum of $\mathrm{^5_{\Lambda}He}$ is somehow similar to the ones reported by previous measurements and theoretical calculations, but the same doesn't happen for the $\mathrm{^{12}_{\Lambda}C}$ one.