Performance evaluation of the aerogel RICH counter for the Belle II spectrometer using early beam collision data

TL;DR

This paper evaluates the early performance of the aerogel RICH counter in the Belle II spectrometer, demonstrating its effective particle identification capabilities using initial collision data.

Contribution

It provides the first performance assessment of the ARICH counter in Belle II using real collision data, including calibration and efficiency measurements.

Findings

Achieved 93.5% kaon identification efficiency

Kaon-pion misidentification rate of 10.9%

Validated the counter's performance with early collision data

Abstract

The Aerogel Ring Imaging Cherenkov (ARICH) counter serves as a particle identification device in the forward end-cap region of the Belle II spectrometer. It is capable of identifying pions and kaons with momenta up to $4 \, {\rm GeV}/c$ by detecting Cherenkov photons emitted in the silica aerogel radiator. After the detector alignment and calibration of the probability density function, we evaluate the performance of the ARICH counter using early beam collision data. Event samples of $D^{\ast +} \to D^0 \pi^+ (D^0 \to K^-\pi^+)$ were used to determine the $\pi(K)$ efficiency and the $K(\pi)$ misidentification probability. We found that the ARICH counter is capable of separating kaons from pions with an identification efficiency of $93.5 \pm 0.6 \, \%$ at a pion misidentification probability of $10.9 \pm 0.9 \, \%$. This paper describes the identification method of the counter and the evaluation of the performance during its early operation.