Development of a Cerenkov trigger detector for the COMET experiment

Theoretical models beyond the Standard Models predict that a branching ratio of the muon-to-electron (µ–e) conversion increases to about 10^-15, although the Standard Model contribution restricts it to be less than 10^-54. The COMET experiment searches for the µ–e conversion with signal event sensitivity of 10^-15 in the first phase (Phase-I). A cylindrical detector consisting of a cylindrical drift chamber and a trigger detector will be used for detecting signal electrons. We designed the CDC trigger detector for the COMET Phase-I, and constructed a 1st prototype. Using cosmic rays, we studied the performance of the prototype. A signal to noise ratio (S/N) of 16.9 ± 0.5 was measured. To improve S/N, we started construction of a 2nd prototype. (author)