Development of an aerogel Cherenkov counter for the LEPS2/SPring-8 experiment to search for the $K^- pp$ bound state

The kaon bound nuclear system has been studied extensively to understand dense nuclear matter, particularly neutron stars. The $K^- pp$ bound state is the simplest kaonic nucleus and many experiments to search for this bound state have been performed by using various reactions. However, the existence of this bound state has not been established yet. To obtain more detailed information on the the $K^- pp$ bound state, we plan a new experiment via the $\gamma d \rightarrow K^+ \pi ^- X$ reaction at LEPS2/SPring-8. Produced particles are detected with the LEPS2 large acceptance spectrometer in a solenoid magnet. For $¥pi /K$ separation in the high momentum region more than 1.0 GeV/c, we plan to use an aerogel Cherenkov counter (AC). The geometry of a prototype has been determined by using an optical simulation code, guide-7. Based on a result of the simulation, the first prototype detector was constructed. We present the design of the detector, and performance test of the first prototype.