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 $K^-$.

$Lambda d$ correlations from the $^4$He(stopped-$K^-$,$d$) reaction

We have observed an intense high-energy component in an inclusive $^4$He(stopped $K^-$, $d$) spectrum. For back-to-back $Lambda d$ pairs a prominent event cluster has been found just below the $m_{^4textrm{He}}+m_{K^-}-m_{n}$ mass threshold in the $Lambda d$ invariant mass spectrum of $Lambda dn$ events, which is evidence for a three-nucleon absorption process of $K^-$ in $^4$He. In addition, an appreciable strength is revealed below $sim$3220 MeV/$c^2$. Well separated $Sigma^0 dn$ events show a peak similar to the case of $Lambda dn$.

$Lambda N$ correlations from the stopped $K^-$ reaction on ${}^4$He

We have investigated correlations of coincident $Lambda N$ pairs from the stopped $K^-$ reaction on ${}^4$He, and clearly observed $Lambda p$ and $Lambda n$ branches of the two-nucleon absorption process in the $Lambda N$ invariant mass spectra. In addition, non-mesonic reaction channels, which indicate possible exotic signals for the formation of strange multibaryon states, have been identified.