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The Aerogel Ring-Imaging Cherenkov (ARICH) detector provides particle identification information in the forward endcap part of Belle II. It efficiently separates charged kaons from pions for momenta between 0.5 GeV/c and 4.0 GeV/c. Optical photons are emitted in a double layer aerogel radiator and detected by Hybrid Avalanche Photo Detectors. ARICH was installed in Belle II at the end of 2017, the first phase of a complex detector calibration of electronic signals and preparation for physics signals calibration has been successfully concluded. Cherenkov rings, created by cosmic rays as well as by charged particles from beam collisions, have been observed. The kaon identification efficiency using D ± ∗ decays has been studied and its calibration is expected by the end of the commissioning phase when sufficient data is available. • The commissioning phase of Belle II is completed. • Cherenkov rings, created by cosmic rays and at collisions, were observed in ARICH. • The ARICH performance is studied using charged kaons and pions from D̂{+*} decays [ABSTRACT FROM AUTHOR]

For efficient separation of hadrons in the forward end-cap of the Belle II spectrometer, an aerogel proximity focusing Ring Imaging Detector is installed in the high magnetic field between the central drift chamber and electromagnetic calorimeter. Cherenkov photons, emitted in the double layer aerogel radiator are expanded through the empty space and detected on the photon detector consisting of Hybrid Avalanche Photo diodes. The readout electronics working in a threshold mode records hit patterns registered during beam collisions. A particle identification algorithm based on the two dimensional extended maximum likelihood technique is used to assign probabilities for different particle hypotheses of tracks traversing the aerogel RICH detector. For efficient discrimination, the Aerogel RICH detector has to be calibrated. We present the key calibration steps used to optimize the detector performance. [ABSTRACT FROM AUTHOR]