Performance of the Belle II aerogel-based ring-imaging Cherenkov counter system in SuperKEKB 2019 phase 3 operation

In the Belle II experiment, an aerogel-based proximity focusing ring-imaging Cherenkov (ARICH) counter is used for charged particle identification (PID) in the forward end-cap region. The goal is to separate kaons from pions at above 4$\sigma$ significance level for momenta up to 4 GeV/$c$, which is critical for the measurements of rare $B$ decays and $CP$ violation in $B$ decays. Chrerenkov photons are emitted in aerogel tiles and 144-channel Hybrid Avalanche Photo Detector (HAPDs) are used as the photo-detectors. We utilize a two-layer aerogel design with different refractive indexes in a focusing configuration. In Phase 3 of the Belle II operation (from Apr. 2019), the ARICH system has been operating smoothly. The performance of particle identification with ARICH has been well validated and is in agreement with simulation.