The Aerogel Ring Imaging CHerenkov (ARICH) counter provides particle identification of charged kaons and pions at the endcap of the Belle II detector at the SuperKEKB the asymmetric electron-positron collider in KEK. ARICH consists of silica aerogel radiators, an expansion volume and Hybrid Avalanche Photo-Detectors (HAPD) as photodetectors. Cherenkov photons are radiated when a charged particle passes through a silica aerogel. The detected photons are observed as a ring image on HAPDs. The particle identification is performed based on the difference in radius of Cherenkov light ring image that is originated from the difference in mass of charged particles. Since we perform particle identification based on the position information of Cherenkov light, the mis-alignment needs to be measured and taken into account to ensure the efficient performance of the ARICH detector.
ARICH was installed into Belle II detector in September 2017. The commissioning of Belle II with positron and electron beams started in April 2018. We report the study of the alignment of the ARICH detector. The position of the ARICH is aligned using e + e - → μ + μ – events in the beam collision.
In this study, we develop the alignment method using Monte Carlo simulation of e + e - → μ + μ - events. In order to quantify how the installed position of ARICH differs from the ideal position, translational or rotational misalignment of the ARICH relative to the entire Belle II detector is added to the Monte Carlo simulation. We compare the distributions of the Cherenkov angle with respect to the azimuthal angle (phi) for different mis-alignment. In the ideal position, Cherenkov angle is constant with regards to phi, but when the ARICH position is shifted from the ideal place, it has cosine-dependence on phi. From the shape of Cherenkov angle dependency on phi, we can calculate the combination of translations and rotations.
In this presentation, we report on development of alignment method. We also report the result of alignment using the real data of beam collision.