The Aerogel RICH detector of the Belle II experiment

International audience; In the forward end-cap of the Belle II spectrometer, an innovative proximity focusing Ring Imaging Cherenkov counter with a multilayer focusing aerogel radiator has been installed. The detector is designed to be operated in a magentic field of 1.5 T, and consists of a double layer aerogel radiator, an expansion volume and a photon detector. In total 420 Hamamatsu hybrid avalanche photo sensors with 144 channels each are used to read out single Cherenkov photons with high efficiency. We expect the device to provide better than 4s separation of pions from kaons in the full kinmatic region of the experiment, from 0.5 GeV/c to 4 GeV/c. The detector components have been successfully produced and installed in the spectrometer. After the commissioning phase in 2018, the detector is now included in the Belle II data taking, and is expected to contribute substantially to the performance of the spectrometer in looking for rare decays of B and D mesons, and of tau leptons. In this contribution we review the detector design and present results of the first detector performance studies.