Purpose

Role of the HKS water Cherenkov counter is to suppress protons in a kaon($P_c$ = 1.2 GeV/$c$ $\pm$ 12.5 %) trigger on the online level. In the case of 100 mg/cm${}^2$ ${}^{12}$C target, stuggered two layors of this counter were designed to separate 0.34 kHz of kaons from 280 kHz of protons, and $\sim$100 Hz protons are expected to remain in the trigger. Additional precise particle identification should be done by offline analysis using beta information derived from TOF counter.