Study of some two-body non-mesonic decays of $^4_\Lambda$He and $^5_\Lambda$He

The Non-Mesonic (NM) decay of $^4_\Lambda{\mathrm{He}}$ and $^5_\Lambda{\mathrm{He}}$ in two-body channels has been studied by the FINUDA Collaboration. Two-body NM decays of hypernuclei are rare and the existing observations and theoretical calculations are scarce. The $^4_\Lambda{\mathrm{He}}\rightarrow d+d,\; p+t$ decay channels simultaneously observed by FINUDA on several nuclei are compared: the $pt$ channel is the dominant one. The decay yields for the two decay channels are assessed for the first time: they are $(1.37\pm 0.37)\times 10^{-5}/K^-_{stop}$ and $(8.3\pm 1.0)\times 10^{-5}/K^-_{stop}$, respectively. Due to the capability of FINUDA of identifying $^5_\Lambda{\mathrm{He}}$ hypernuclei, a few $^5_\Lambda{\mathrm{He}}\rightarrow d+t$ decay events have also been observed. The branching ratio for this decay channel has been measured for the first time: $(3.0\pm 2.3)\times 10^{-3}$.