Aerogel-Based Ring-Imaging Cherenkov counter in the Belle II experiment

In the charged particle identification system of the Belle II experiment, an aerogel-based proximity focusing ring-imaging Cherenkov (ARICH) counter is installed in the forward end-cap region of the Belle II detector. The goal is to enhance the power of separation between kaons and pions up to and above 4σ for momenta up to 4 GeV/c, which is critical to the study of CP violation involved in B decays. The counter is made of aerogel tiles with refractive indices n of 1.045 and 1.055, transmission lengths of 30-40 mm, and dimensions of 18 × 18 × 2 cm³. A 144-channel HAPD (Hybrid Avalanche Photo Detector) is adopted as a photo-detector, and the HAPD signal processing is performed using ASICs (Application Specific Integrated Circuit) to digitize the signal and FPGAs (Field Programmable Gate Array) for data processing.During Phase II operation of SuperKEKB, the commissioning of ARICH is done both with cosmic rays and beam collisions. The Cherenkov rings are clearly observed, and the performance of the hardware system and identification algorithm is being evaluated. In this paper, we will report on the construction of ARICH and on the initial performance of the detector obtained during the commissioning.