The GEM detectors for the innermost region of the forward muon station of the LHCb experiment

The LHCb experiment will take place at the LHC accelerator at CERN and will start in 2008. It is dedicated to precision measurements of CP violation and rare decays in the b quark sec- tor. The apparatus is a single arm spectrometer and it is designed with a robust and flexible trigger in order to extensively gain access to a wide spread of differ ent physical processes involving beauty particles. This will allow to over-constrain the Standard M odel predictions about CP violation, and to discover any possible inconsistency, whi ch would reveal the presence of “New Physics” beyond the Standard Model. This thesis reports the work performed on two aspects of the L HCb experiment: the main contribution is the development and the construction of a de tector based on Gas Electron Multiplier (GEM) technology for the instrumentation of the high irradiated region around the beam pipe of the forward Muon Station; in the second part t he possibility of the search of the rare D 0 → + − decay at the LHCb experiment is investigated, demonstratin g that the experiment can found application also in the charm secto r physics. The triple-GEM detectors equipping the inner region (R1) of the first muon station (M1) of the LHCb experiment was proposed, developed and realized by our group at the Laboratori Nazionali di Frascati (LNF) of Istituto Nazionale di Fisica Nucleare (INFN) in collaboration with the INFN section of Cagliari (Ca-INFN). The main task of such a detector is the muon p T measurement for the LHCb trigger, which requires high time performance. The use of a triple-GEM dete ctor as a triggering device is certainly a novelty in the field of high energy physics. The fir st application of GEM detectors was in the COMPASS experiment, where they are currently used as a trackers, while a little interest was devoted so far to the optimization of the time re sponse of GEM detectors. In the LHCb trigger logic a critical issue is the high efficiency in t he bunch-crossing identification, so a high detector time resolution is the main constraint. Since the time resolution of a triple-GEM detector operated with an Ar/CO 2 (70/30) gas mixture is about 10 ns (r.m.s.), an intense R&D activity on GE M detectors was required. The use of fast CF 4 and isobutane based gas mixtures, together with an optimiza tion of the geometry and the electric fields of the detector, has allowed to improve the time resolution of the single detector down to 5 ns (r.m.s.), largely fulfilling the requirements of the experiment ( σ t ≤ 3 ns is achieved by two logically OR-ed detector layers, as re quired in the LHCb muon stations). The final design of the detector has taken into account the tig ht space constraints around the beam pipe. We defined detailed construction procedures as we ll as severe quality controls for the production of 12 fully instrumented detectors requi red to cover the ∼ 1 m 2 area of M1R1, plus 6 additional spare detectors. The performance of one of such detectors, fully integrated w ith front-end electronics and all the other components, has been measured in a recent test at th e SPS beam at CERN, where the LHCb running condition has been simulated by means of a si milar bunch crossing time structure and the adoption of the official LHCb DAQ chain. The results largely fulfilled the experiment requirements and gave us useful information s for the future commissioning phase. In addition we have demonstrated that the detector is robust from the point of view of dis- charges as well as ageing processes, and can tolerate the rad iation dose foreseen in 10 years of operation in the region M1R1 of the LHCb experiment. All the detectors are now at CERN, ready for the installation on the M1 station, foreseen for the first months of 2008. The charm production at the LHCb experiment is even greater t han the beauty production, thanks to the about seven times higher expected cross sectio n (3.5 mb) with respect to the beauty cross section (0.5 mb). Recent works demonstrated some inte rest for rare decays in the charm sector, that can gives complementary information with respect to the K or B – mesons physics and constrain the parameters of several exte nsion of the Standard Model. In this framework, the results of a preliminary study of the r are D 0 → + − decay at LHCb experiment is presented in the second part of the thesis. Usi ng the simulation and the analysis software realized for the experiment, a selection algorith m has been written, obtaining a first estimation of the upper limit on the branching ratio achieva ble at LHCb that is about two order of magnitude lower than the current one.