Particle Identification with the Cherenkov imaging technique using MPGD based Photon Detectors for Physics at COMPASS Experiment at CERN

Abstract

A novel technology for the detection of single photons has been developed and implemented in 2016 in the Ring Imaging Cherenkov (RICH) detector of the COMPASS Experiment at CERN SPS. Some basic knowledge in the field of particle identification and RICH counters, Micro Pattern Gaseous Detectors (MPGDs) in general and their development for photon detection applications are provided. The characteristics of the COMPASS setup are summarized and the COMPAS RICH-1 detector is described and shown to provide hadron identification in the momentum range between 3 and 55 GeV/c. The THGEM technology is discussed illustrating their characterization as gas multipliers and as reflective photocathodes: large gains and efficient photodetection collections are achieved when using optimized parameters and conditions (hole diameter = THGEM thickness = 0.4 mm; hole pitch = 0.8 mm and no rim; CH4-rich gas mixtures and electric field values > 1 kV/cm at the CsI surface). The intense R\&D program leading to the choice of a hybrid THGEM + Micromegas architecture for the novel detectors is summarized: prototypes construction and test results are presented. The beam test performed at CERN with two 300 mm $\times$ 300 mm active area hybrid prototypes validated this new technology and allowed to demonstrate efficient detection of Cherenkov photons. The optimal design of the detector, consisting in two layers of THGEMs, the first of which is coated with 300 mm thick layer of CsI, couple with a Micromegas on a pad segmented anode with an original design of capacitive – resistive readout is presented. All aspects of the construction, test and assembling from the raw material selection to the procedures applied for the quality assessment are described in detail. The challenges encountered during the detectors assembly, the test in the laboratory and the transportation of the chambers to CERN for the assembly and mounting are then illustrated. The production of the photocathode and the final assembly of the four hybrid detectors, covering an active area of 1.4 m$^2$ are presented. The adventurous installation of the combined Hybrid PDs and Multi Anode Photo Multiplier Tubes with fused silica lenses onto COMPASS RICH-1 and the work for equipping the new detectors with all needed services are presented. The description of the HV control system and of the other services is illustrated and the APV25-based frontend electronics is described together with the studies performed to understand the electronic and physical noises of the new chambers. A preliminary on-line analysis of the detector response and of the performance are presented: an indication that the average number of photons is larger than the neighboring traditional MWPC-based PDs is obtained. The success of the first MPGD-base