J-PARC E27 Experiment to Search for a K−pp Bound State

An antikaon and a nucleus may form a bound state (a kaonic nucleus), due to the strong attraction of K bar -N in I=0. A K − pp bound state, which is the bound state of an antikaon and two nucleons, with the total charge +1 and the isospin 1/2, is expected to be the simplest kaonic nucleus. Various theoretical calculations for a K − pp bound state have been carried out and the existence of the bound state is supported in these calculations. However, the binding energy and the decay width depend on the K bar -N interactions and the calculation methods [1]. Several experiments have been performed to look for a signal of kaonic nuclei [2,3]. However, any firm experimental evidence has not been found. An experimental search for the K − pp bound state is performed by using the d(π + , K + ) reaction at J-PARC K1.8 beam line (J-PARC E27experiment). Fig.1 shows the experimental setup of this experiment. The first data taking as a pilot run was carried out in June, 2012. The missing-mass spectrum of this reaction studied at the beam momentum of 1.7 GeV/c, which allows the production of Λ(1405), was obtained for the first time and a significant peak shift by ~30 MeV was observed in the Y* region. In a preliminary proton-coincidence analysis, a sharp spike due to the ΣN-ΛN coupling and a broad enhancement around 2.3 GeV/c 2 , which might be attributed to the K − pp bound state, was clearly observed. At this symposium, the preliminary results of the inclusive and coincidence analysis will be discussed.