Performance study of the RICH at COMPASS experiment for hadron identification in SIDIS physics

Hadron production in semi-inclusive measurements of deep-inelastic lepton nucleon scattering (SIDIS) is one of the most powerful tools to investigate the hadron structure. Particle IDentification (PID) is essential to study the processes where inclusive DIS is not able to provide all the information. For instance to study the role played by the different flavours in SIDIS processes. In the high momenta regime ($\textbf{p}$ $\geq$ 20 $GeV/c$) the Ring Imaging Cherenkov technique is a unique tool to perform hadron identification in a wide momentum range. In particular in the COMPASS experiment at CERN pion-kaon-proton identification is obtained thanks to a $\approx$ 80 $m^3$ $C_4F_{10}$ gaseous radiator focusing RICH capable of 2$\sigma$ $\pi~K$ separation up to 50 $GeV/c$. The COMPASS RICH has $5.6 ~m^{2}$ of active photon detector (PD) surface, composed of three different detection technologies, MultiAnode PMTs ($\approx$1.4$m^2$) in the central part, gaseous detectors with CsI photocathodes in the peripheral part: MultiWire Proportional Chambers ($\approx$ 2.8 $m^2$) and new photon detectors based on THick Gaseous Electron Multiplers (THGEMs) and Micromegas covering the remaining $\approx$1.4~$m^2$. The first chapters describe SIDIS physics, the COMPASS spectrometer, the principle of PID and COMPASS PDs. Chapter 5, 6 and 7 of this thesis will describe and discuss the work have been done by me. Chapter 5 will elaborate the characterization of both the newly installed detector and the RICH-1 performance. For this purpose a dedicated pion beam data taking was requested to the COMPASS collaboration and a dedicated trigger was implemented. An illustration of the methodology to obtain the the number of photons of the newly installed detector, single photon resolution along with the operating gain and stability will be described together with the main results. Chapter 6 and 7 describe the study of the non uniform RICH-1 response, the procedure for curing it and the results obtained after the final tuning. The analysis of the detector position survey and the corrections of the mirror VUV wall position is described. This thesis presents the technique developed and adopted to extract the refractive index of the radiator from the data, which greatly reduces the contamination from electrons and positrons tracks and the consequently improves the RICH-1 PID performance. PID is obtained both via Cherenkov ring angle based on $\chi^{2}$ analysis and using the single photon information in an extended likelihood algorithm on a track by track basis. A detailed study of the likelihood algorithm and the consistency with the $\chi^{2}$ analysis are presented. With the updated RICH-1 detector parameters, the performance figure for COMPASS RICH-1 in terms of PID efficiency and purity is extracted from selected meson decays. The result of this work is also an essential element for the extraction of DIS hadron multiplicities, namely the differential cross section for identified hadron production normalised to the differential inclusive DIS cross section.