The ARICH detector is used to identify charged particles in the front-end endcap region of the Belle II spectrometer. Its main objective is to separate kaons from pions in the full kinematical region of the experiment (about 0.5-4.0 GeV).

ARICH relies on the emission of light by very fast charged particles (Cherenkov effect), more specifically on the relation between the velocity of the particle v and the emission angle of Cherenkov light in a transparent medium with the refractive index n,  cos Q = c/vn (here c is the speed of light in vacuum). At a given momentum, lighter particles are faster than heavier ones, and thus emit Cherenkov light at larger angles. In ARICH, a 4cm thick layer of aerogel with n=1.045 is used as the medium (‘radiator’) where Cherenkov photons are emitted. Cherenkov photons then propagate through an expansion volume and hit the photon detector. Because all photons are emitted at the same angle, the detected photons form a ring at the photon detector. For a pion with a given momentum, the ring radius is larger than the one corresponding to a kaons of the same momentum as illustrated below. At 3.5 GeV/c, this difference amounts to about 6mm.

Aerogel is an amorphous, highly porous solid form of silicon dioxide (SiO₂). In the ARICH the 3.5 m² Cherenkov radiator plane is covered by two layers of wedge-shaped aerogel tiles of dimensions 17 x 17 x 2 cm³.

As a photo-sensor a recently developed Hybrid Avalanche Photo Detector (HAPD) is used.   The principle of its operation is shown below. Incident photon is converted into photo-electron by a bi-alkali photo-cathode with peak quantum efficiency of ~30% at 400 nm. The electron is then accelerated in a vacuum tube with high electric field towards the segmented avalanche photo-diode (APD) with 5.1 × 5.1 mm pads. After the photo-electron hits the APD, an electric signal is produced which is then registered by a dedicated read-out electronics board with a preamplifier, a shaper and a comparator for each of the 144 channels, followed by an FPGA (Xilinx Spartan-6 XC6SLX45), where the hit information is recorded and communicated to further stages of the data acquisition system.
In total, ARICH consists of 420 HAPD modules arranged in seven concentric rings (between  56cm and 114 cm away from the beam axis) and of 2 × 124 aerogel tiles of wedge shape, in a donut-like detector set-up as shown below.

— Luka Santelj, Peter Križan, 2016-07-23