Likelihood analysis of beam data for time of propagation counter

We, Belle II PID group, present about analysis method and the obtained performance of particle identification (PID) for Time Of Propagation (TOP) counter using 120 GeV/c pion beam at CERN. We used the prototype of the TOP counter with focusing mirror. The TOP counter is a new detector of PID for barrel region at Belle II detector. The TOP counter is a kind of Ring Imaging Cherenkov counter (RICH), which detects internally reflected Cherenkov light, like a DIRC [1]. TOP counter reconstructs the velocity of particle "beta" from 3D Cherenkov ring image. The ring image consists of position and timing of detected photons. To evaluate the performance of TOP counter, we have to understand fine structures of ring image using experimental data. We can calculate the likelihood from Probability Density Function (PDF) provided by Monte Carlo (MC) simulation. We aligned the incident condition of particle and the time offset using maximum likelihood estimation successfully. Then, beta is evaluated by same method event by event. As a result, we obtained the beta resolutionσ_β = 2.2 × 10^-3.