Search for CP violation in the charmless decay B0 → pp ̄K+π− using triple product asymmetries at LHCb and feasibility studies of a SiPM-based readout system for the Upgrade II RICH detector

The subject of CP symmetry and its violation is often referred to as one of the least understood in particle physics. Indeed, CP symmetry violation is an expected consequence of the Standard Model with three quark generations, but calculations within this framework show that the predicted amount of violation is too small to generate the matter-anti-matter imbalance observed in our universe. So far, direct CP violation has been established in K, B and D meson decays and the experimental measurements are in good agreement with the Standard Model expectations. However, CP violation is yet to be confirmed in heavy baryon decays and also in B decays with half-spin particles in the final state, where sizeable asymmetries are predicted as well. In particular, baryonic B-meson decays mediated dominantly through internal W emission are believed to be promising processes. In the first part of this thesis a search for CP violation in the charmless baryonic meson decay B0 → pp ̄K+π− using data collected by the LHCb experiment is reported. Multi-body decays of this kind are a good place to search for CP violation because, due to their rich resonant structures, different amplitudes may interfere and cause local CP violation effects to appear in regions of the phase space. This analysis makes use of triple product correlations, a model- independent method with complementary sensitivity with respect to the direct CP asymmetry. The analysis uses data corresponding to an integrated luminosity of 9 fb^−1 collected during Run 1 and Run 2 from 2011 to 2018 and represents the first measurement of this kind in this channel. In order to assign the correct mass to the charged hadrons in the final state and combine them to form the B0 → pp ̄K+π− candidate, the particle identification information (PID) provided by the RICH detector are necessary. The two LHCb RICH (Ring Imaging Cherenkov) detectors exploit the Cherenkov light emitted by charged particles traversing a gas to distinguish efficiently π, K and p in the momentum range 1-100 GeV/c. During Run 1 and Run 2 the two RICH detectors operated continuosly at the luminosity of ∼ 4×10^32 cm−2s−1 and provided an excellent PID. In order to be able to operate at the new luminosity of ∼ 2 × 10^33 cm−2s−1 from the start of Run 3 (start of the Upgrade I phase) in 2021 the two RICH detectors must be upgraded; the former HPD (Hybrid Photon Detectors) will be replaced by commercial MaPMTs, the optics of the upstream RICH will be modified and the electronics will be upgraded to cope with the challenges of the 40 MHz readout rate. In the second part of this thesis, the development of the detector control system (DCS) for the Upgrade I RICH detector is presented. The DCS partition is the control domain responsible for all the slow control equipment, i.e. control and monitoring of the low voltage (LV), temperature, humidity, pressure etc. The last part of this thesis concerns the feasibility studies for the Upgrade II LHCb RICH detector. The start of the HL-LHC phase in 2027 provides an opportunity to increase the luminosity up to ∼ 10^34 cm−2s−1. To be able to operate the RICH detectors in this much harsher environment a further upgrade in the front-end electronics and a new photo-detector system will be mandatory. I investigated the possibility of using SiPMs (Silicon Photomultipliers) as photo-detectors in the Upgrade II phase by characterizing a device manifactured by Hamamatsu at low temperatures (down to -40 degree celsius) in the lab. The work done in this thesis is still preliminary, but the results obtained in the lab represent valuable inputs towards the design of a new detector prototype.