Performance test of a lead-glass counter for the J-PARC E36 experiment

Abstract The J-PARC E36 experiment will search for a violation of lepton universality by a precise measurement of the ratio of the kaon decay widths R K = Γ ( K + → e + ν ) / Γ ( K + → μ + ν ) . Charged particles will be identified by the combination of three independent systems: a lead-glass Cherenkov counter, an aerogel Cherenkov counter, and a time-of-flight measurement. The performance of the lead-glass Cherenkov counter was investigated with e + , μ + , and π + beams in the relevant momentum region from the K + decays. By using a polyethylene degrader to slow down the beam momentum in front of the lead-glass block, we succeeded in reducing the muon mis-identification probability down to 5% while maintaining a high e + detection efficiency of 98%.