A search for the K−pp bound state in the 3He(K−in-flight, n) reaction at J-PARC

We have collected the first physics data of an experimental search for the simplest kaonic nuclear bound state, “K− pp”, by the 3He(K− n) reaction at J-PARC. We confirmed that our spectrometer system works as designed and observed clear peak structure composed of the quasi-elastic K−“n” → K−n and the charge-exchange K−“p” → ¯̅K0n reactions in the forward neutron spectrum.