Performance Estimation of the Belle II Aerogel RICH Counter in the First Beam Collision

Abstract

International audienceThe Belle II experiment at the SuperKEKB facility started the beam collision in 2018, aiming search for the New Physics beyond the Standard Model. The Aerogel Ring Imaging Cherenkov (ARICH) counter is a particle identification (PID) device in the forward endcap of the Belle II detector to provide 4$\sigma$ separation of charged kaons and pions of momenta up to 3.5 GeV. The ARICH counter is a proximity-focusing RICH counter to identify particle species based on the angular distribution of Cherenkov photons located in 30 cm depth of narrow space and 1.5 T of high magnetic field. Two layers of the silica aerogel with different refractive indices emits photons as Cherenkov ring image. A total of 420 of Hybrid Avalanche Photo Detectors (HAPDs) are used to measure the 2-dimensional distribution of photons. We have collected ring images in the first beam collision data during the Phase II commissioning run and studies for the PID performance estimation are being carried out toward the Phase III physics data taking starting in 2019