Search for $K^-pp$ deeply bound state via in-flight $^3$He($K^-,n$) reaction at J-PARC K1.8BR

We acquired the first step data of the experimental search for deeply bound kaonic state at J-PARC beamline K1.8BR (J-PARC E15[1]) in May, 2013. We measure $K^-pp$ bound state by detecting forward scattered neutron from $^3$He($K^-,n$) reaction with the incident kaon momentum of 1 GeV/c. The neutron momentum is measured by TOF counter placed 15 m ahead in the forward direction. The decay charged particles from $K^- pp$ are tracked in the cylindrical detector system (CDS) surrounding the liquid $^3$He target. The invariant masses ($p\pi$) and ( $\Lambda p$) are reconstructed by the tracking for a search of $K^-pp\to\Lambda p \to p\pi^- p$. The in-flight reaction and CDS can distinguish all the background process such as kaon-nucleon absorption. $5.1\times10^9$ kaons hit on the target in the first E15 physics run data. Some structure below $\bar{K}NN$ or threshold was observed from the missing mass spectrum of $^3$He($K^-,n$). In this presentation, we will present an overview and the preliminary results of the first E15 physics run.