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

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 $4\>$GeV$\>$c$^{-1}$ 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.

Cherenkov detectors

Particles passing through a medium with a velocity larger than that of light in that medium emit electromagnetic radiation, called Cherenkov radiation. In this chapter the physical phenomenon and characteristic parameters of Cherenkov radiation, such as Cherenkov angle, spectrum and emission intensity, are introduced and the applications for particle detection and identification are discussed. It follows a presentation of the relevant detector types, such as threshold and differential Cherenkov detectors, ring imaging detectors (RICH and DIRC) as well as Cherenkov detectors in astroparticle experiments. The obtainable resolutions for particle identification via Cherenkov ring imaging and their limitations are discussed as well.