Design of a Ring Imaging Cherenkov detector for the next generation heavy-ion experiment at the LHC

The ALICE Collaboration has proposed a new experiment, ALICE 3, to investigate the properties of the Quark–Gluon Plasma (QGP) beyond current limits in LHC Run 5. A key subsystem for charged-particle identification (PID) is a proximity-focusing Ring–Imaging Cherenkov detector, composed of a central barrel (bRICH) and forward end-caps (fRICH). With its target PID performance, this detector will enable unique contributions to dielectron physics, to the heavy-flavour sector, and to precision studies of event-by-event fluctuations of conserved charges. The main goal of my PhD research was to design the bRICH system through dedicated feasibility and optimization studies, proposing an efficient and cost-effective solution and demonstrating, via simulations and measurements, that it meets the ALICE 3 physics requirements in the most channelling environment of central (0-5%) Pb-Pb collisions. I proposed a modular, projective bRICH concept based on aerogel as Cherenkov radiator and SiPMs as photon sensors pointing to the interaction point to enhance the PID performance while minimising the total active area and costs. I further explored an eco–friendly heavy–gas mixture to enhance electron identification, and I demonstrated Cherenkov–based charged–particle timing with the same SiPM modules, achieving time resolutions down to tens of picoseconds. The simulation activity included the development of fast machine–learning algorithms based on Boosted Decision Trees to learn Cherenkov ring topologies and to improve PID in the high-multiplicity Pb-Pb environment under realistic dark count rate (DCR) conditions. I complemented these studies by characterizing irradiated SiPMs and developing a realistic operation and ageing model that combines cooling below -40°C with annealing cycles above 70°C. To further mitigate the DCR, I investigated the use of smaller SiPMs relative to the baseline 2 × 2 mm2 size coupled to light concentrators. Characterization measurements of aerogel tiles revealed a refractive index gradient along their thickness, producing a natural focusing effect that improves the angular resolution in the proposed geometry. The detector concept was validated with multiple prototype beam tests at the CERN–PS T10 facility between 2022 and 2025, validating the detector parameters and confirming the expected angular resolution, photon yield, separation power, and the stability of the reconstruction at DCR levels foreseen for ALICE 3. Overall, my work demonstrates the feasibility and performance of the projective proximity-focusing bRICH design for operation in central Pb-Pb and delivers a solution that meets the ALICE 3 PID goals validated by combining simulation studies, laboratory measurements, and beam-test results. Thanks to the studies presented in this thesis, the proposed projective bRICH concept was adopted as the baseline layout in the ALICE 3 Scoping Document.