Development of positron timing counter with SiPM readout for MEG II experiment

The MEG experiment is now being upgraded to search for the charged lepton flavor violating decay μ+ → e+γ with an improved branching ratio sensitivity, 5 × 10-14. We are developing a new Timing Counter to precisely measure the timing of the decay positrons under the high rate environment. The Timing Counter consists of 512 counters and each counter is based on a fast plastic scintillator plate readout by multiple SiPMs. The positron timing can be measured with improved time resolution by averaging positron impact times over multiple counter hits. We performed a beam test with a prototype to demonstrate the time resolution under a high rate environment. As a result, a timing resolution of ∼30 ps was demonstrated at the expected positron rate. Furthermore, it is quite important to precisely time-align all the counters. We started to develop time calibration methods using a light pulse from a picosecond laser pulser distributed to all the counters. We tested the laser pulse injection to a single counter and measured the light amount, stability, and reproducibility, and the method is found to be promising for the precise time calibration in the MEG II experiment. The construction has been started and the part of timing counter will be installed in autumn 2015.