Three- and four-nucleon absorption processes observed in the $K^-$-$^4$He reaction at rest

Correlations of back-to-back coincident $Lambda d$ and $Lambda t$ pairs from the stopped $K^-$ reaction on $^4$He had been investigated, thereby $Lambda d$ and $Lambda t$ branches of non-mesonic three- and four-nucleon absorption processes of antikaon at rest were identified as well-separable processes, respectively. The branching ratio of the three-nucleon process, ($^4$He-$K^-$)$_{atomic} rightarrow Lambda d n$, is estimated to be $(0.9 pm 0.1 (stat) pm 0.2 (syst)) times 10^{-3}$ from the normalized $Lambda d$ spectrum in a $Lambda d n$ final state, while the fraction of the four-nucleon process, ($^4$He-$K^-$)$_{atomic} rightarrow Lambda t$, is obtained to be $(3.1 pm 0.4 (stat) pm 0.5 (syst)) times 10^{-4}$ per stopped $K^-$.