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Aerogel RICH Counter at the Belle II Detector

I. Adachi, N. Akopov, D. Augueste, J. Bonis, L. Burmistrov, S. Dey, R. Dolenec, G. Ghevondyan, R. Giordano, A. Hvala, T. Iijima, S. Iwata, H. Kakuno, G. Karyan, H. Kawai, T. Kohriki, T. Konno, S. Korpar, P. Krizan, S. Kurokawa, Y. Lai, A. Lozar, M. Mrvar, G. Nazaryan, S. Nishida, S. Ogawa, R. Pestotnik, I. Prudiiev, L. Santelj, A. Seljak, L. Senekovic, M. Shoji, K. Spenko, T. Sumiyoshi, M. Tabata, K. Uno, E. Waheed, M. Yonenaga, Y. Yusa

TL;DR

The Aerogel RICH (ARICH) detector for Belle II's forward end-cap employs a novel multilayer focusing aerogel radiator and Hybrid Avalanche Photodetectors (HAPDs) to achieve efficient π/K, π/p, and K/p separation up to 4 GeV/$c$. The paper details the design, construction, commissioning, and operation, including Geant4-based simulation within Basf2, digitization, and a likelihood-based particle identification framework. It reports on performance metrics from data-taking (e.g., ~11 signal photons per saturated muon track and a Cherenkov angle resolution around 12.7 mrad), and demonstrates good agreement with simulation as well as robust, stable operation over extended periods. The ARICH provides critical forward-region PID, enabling precise heavy-flavor measurements and rare decay studies in Belle II, and is supported by a comprehensive slow control, data-quality monitoring, and calibration infrastructure.

Abstract

We report on the design, operation, and performance of a novel proximity-focusing Ring Imaging Cherenkov (RICH) detector equipped with a multilayer focusing aerogel radiator, developed for the forward region of the Belle II spectrometer at the SuperKEKB $e^+e^-$ collider. The system achieves effective separation of charged pions, kaons, and protons across the full kinematic range of the experiment, from 0.5 GeV/c to 4 GeV/c. To date, the detector has successfully operated in data-taking, contributing to the collection and analysis of nearly 600/fb of Belle II $e^+e^-$ collision data.

Aerogel RICH Counter at the Belle II Detector

TL;DR

The Aerogel RICH (ARICH) detector for Belle II's forward end-cap employs a novel multilayer focusing aerogel radiator and Hybrid Avalanche Photodetectors (HAPDs) to achieve efficient π/K, π/p, and K/p separation up to 4 GeV/. The paper details the design, construction, commissioning, and operation, including Geant4-based simulation within Basf2, digitization, and a likelihood-based particle identification framework. It reports on performance metrics from data-taking (e.g., ~11 signal photons per saturated muon track and a Cherenkov angle resolution around 12.7 mrad), and demonstrates good agreement with simulation as well as robust, stable operation over extended periods. The ARICH provides critical forward-region PID, enabling precise heavy-flavor measurements and rare decay studies in Belle II, and is supported by a comprehensive slow control, data-quality monitoring, and calibration infrastructure.

Abstract

We report on the design, operation, and performance of a novel proximity-focusing Ring Imaging Cherenkov (RICH) detector equipped with a multilayer focusing aerogel radiator, developed for the forward region of the Belle II spectrometer at the SuperKEKB collider. The system achieves effective separation of charged pions, kaons, and protons across the full kinematic range of the experiment, from 0.5 GeV/c to 4 GeV/c. To date, the detector has successfully operated in data-taking, contributing to the collection and analysis of nearly 600/fb of Belle II collision data.
Paper Structure (28 sections, 1 equation, 30 figures)

This paper contains 28 sections, 1 equation, 30 figures.

Table of Contents

  1. Introduction
  2. Design choices
  3. Aerogel
  4. Photon detector - HAPD
  5. Readout Electronics
  6. Firmware Design
  7. Mechanical structure and services
  8. Construction
  9. Quality assurance of the components
  10. Construction
  11. Slow control system
  12. High Voltage and Low Voltage Systems
  13. Data quality monitors
  14. Environmental monitors
  15. Commissioning and Operation
  16. ...and 13 more sections

Figures (30)

  • Figure 1: The Belle II spectrometer (left) and the geometry of the ARICH detector (right) with its main components, aerogel radiator (edges depicted in light gray) and photo-sensors (in dark red).
  • Figure 2: Proximity focusing RICH with a dual radiator in the focusing configuration.
  • Figure 3: Cherenkov angle for pions and kaons for two refractive indices of the Cherenkov radiator, 1.045 and 1.055.
  • Figure 4: Proximity focusing RICH, proof of principle in a beam test: the distribution of Cherenkov photon hits of the Cherenkov angle for a 4 cm homogeneous radiator (right) and for a focusing configuration with two 2 cm thick layers with $n_{1}$ = 1.046, $n_{2}$ = 1.056 (left) Korpar:2007vxs.
  • Figure 5: Recovery of photons emitted at the outer edge of the ARICH detector acceptance: planar mirrors on the side wall of the ARICH vessel.
  • ...and 25 more figures