The TOP counter of Belle II: status and first results

Abstract: High-efficiency and high-purity particle identification are fundamental requirements for the success of the Belle II experiment, whose main goal is to explore the new-physics scenarios in the CP-violating decays of the B mesons. To achieve the required particle identification performances, the Time-of-propagation counter has been installed in the central barrel region. This unique device consists in 16 bars of fused silica that act simultaneously as radiator and as light guide for the Cerenkov light. Unlike in the DIRC detectors, the PID information is mostly extracted measuring the time of propagation of the Cherenkov light in the radiator rather than its purely geometrical patterns. We will present here a general overview of the status of the TOP counter, including the estimation of the time resolution, the calibration strategies and performances, and the first result obtained in the commissioning phase, both using cosmic rays and $e^+e^-$ collision events collected during the {\it phase II} pilot run of the Belle II experiment. These are the first measurements of the particle identification performances of a time-of-propagation detector in a full HEP experimental setup.