The high voltage system of the RICH and the multiplicity study with LHCb data

This thesis is about the work done during my PhD at the Univerità degli studi di Milano–Bicocca. During these three years I was involved in the RICH group of the LHCb experiment. LHCb is one of the four main experiments at CERN. It uses proton–proton collisions to study CP violation and to search for new physics beyond the Standard Model in the $b$ decays. One of the main feature of the experiment is the particle identification, performed with the Ring Imaging Cherenkov (RICH) technology. The RICH detects Cherenkov rings via photons emitted by charged particles traversing radiator materials. The photon detection system used consists of pixel Hybrid Photon Detectors (HPDs). They convert photons into photoelectrons which are then accelerated by means of a field generated by three electrodes, biased with high voltages (-18 kV, -17.7 kV and -14.8 kV), onto the silicon pixel anode. The energy released in the pixel is converted into a binary output. My work can be divided in two main parts: the hardware and the data analysis. The hardware activity is focused on the High Voltage (HV) supply and distribution systems. I optimized the voltage measurement system reducing the systematics determining improvement in the voltage resolution. This result was obtained using measurements taken during the characterization tests performed in Milano. The strong requirement of a good stability implies a continuous monitoring and analysis of the commissioning data. The HV supply system is placed about 40 m from the experiment and voltages must remain stable until they reach the HPDs. The study of the noise evolution and the discovery of its cause determined the substitution of a component inside the supply modules. The company after this evaluation decided to replace a component with a new material in all the new HV modules produced. All the analysis contributed largely to the stability of the RICH system for all the operations of data taking. For future operations, LHCb aims at collecting data at a LHC luminosity of 10$^33$ cm$^-2$s$^-1$, to improve the statistics of the main rare channels studied. For the RICH, this implies a new readout system. The Milano group decided to design a new front–end electronics system and is studying the performance of commercial Multi anode Photo Multiplier (MaPMT) produced by Hamamatsu. I participated to the test of the H9500 tube. We obtained a complete characterization of this tube with respect to the single photon electron response. The main issues were to study noise and cross–talk effects. These are important because one of the RICH requirement is the single photon detection. The tests done in Milano confirmed the cross–talk percentage at the level of 10% with an amplitude of the order of 30% due to the loss of photoelectrons in the first stages of the multiplication chain. The important aspect is that this measurement was o btained using a single photon light source instead of a uniform light source used by the company. All these tests concluded that this MaPMT can be a good candidate for the upgrade, but other photodetectors are under test and the choice will be done in the near future. In the second part of my work I analyzed the first data taken by LHCb with proton–proton collisions at $\sqrt{s}$ = 7 TeV. I used the minimum bias sample collected with a very loose trigger in the very first part of 2010. Minimum bias events are important for understanding the detector and its response. They are due to inelastic processes: these processes are related to the distribution of quarks and gluons inside the proton. These distributions are extrapolated from experiments at lower energies up to $\sqrt{s}$ = 7 TeV. LHC data will be therefore ultimately important to validate the QCD calculation of the evaluation of the structure function. The analysis concerned a first study of the track distributions in the detector, followed by the study of a possible detector charge asymmetry. Data were compared with Monte–Carlo simulations, which describe data satisfactory. This study was done to obtain the most reliable detector description in view of measuring CP asymmetry in the $b$ sector, where charge asymmetry of the detector response would be a dangerous contribution to the systematic error.