Summary of the $K^- pp$ bound-state observation in E15 and future prospects

The possible existence of deeply-bound $\bar K$-nuclear bound states, kaonic nuclei, has been widely discussed as products of the strongly attractive $\bar K$Ninteraction in I = 0 channels. Investigations of those exotic states will provide us unique information of the $\bar K$Ninteraction below the threshold, which is still not fully understood so far. Recently, we observed the simplest kaonic nuclei, $\bar K$NN, having a much deeper binding energy than normal nuclei via inflight (K−,N) reactions at the J-PARC E15 experiment. For further studies, we have proposed a series of experimental programs for the systematic investigation of light kaonic nuclei, from $\bar K$N(Λ(1405)) to $\bar K$NNNN. We will measure the $\bar K$NNN(A= 3) system at the new experiment approved as J-PARC E80, as a first step toward a comprehensive study.