The ALICE muon spectrometer: trigger detectors and quarkonia detection in p-p collisions

This work was carried out in the context of the optimisation of the performances of the muon spectrometer of the forthcoming ALICE experiment at the Large Hadron Collider (LHC, CERN). The aim of ALICE is the study of nuclear matter at the highest energy densities ever accessed experimentally. More in detail, the focus is on the expected phase transition to a deconfined phase of matter where the degrees of freedom are those of quarks and gluons: the Quark-Gluon Plasma. The conditions for QGP formation are expected to be achieved in highly relativistic heavy ion collisions. The energy in the centre of mass of Pb-Pb collisions at the LHC will be 5.5 TeV per nucleon pair. The ALICE physics program also includes data-taking in p-p collisions at the centre-of-mass-energy of 14 TeV. The ALICE muon spectrometer has been designed for the detection of heavy quarkonia through their muon decay: both theoretical predictions and experimental data obtained at SPS and RHIC indicate that the production of these resonances should be strongly affected by the nature of the medium formed in the collision. A high resolution measurement of the charmonium and bottomonium spectra, with significant statistics, is expected to provide crucial information to test different theoretical predictions. The work presented in this thesis consists of two different items. The first item is the testing of the final production of the trigger detectors for the spectrometer. The trigger system will perform a selection of muon tracks according to their transverse momentum. It consists of 72 Resistive Plate Chambers, arranged in two stations of two detection planes each. The RPCs have been tested in Torino with a dedicated test station, providing a complete characterisation (including high granularity efficiency maps) of all produced detectors and selective criteria for the validation of the final RPCs, which are now installed in ALICE. The second item consists in the development of two different strategies for the normalisation of quarkonia yield in Pb-Pb collisions with respect to the p-p yield at the same centre-of-mass energy (5.5 TeV). This is a crucial issue for quarkonia suppression studies. The first strategy is the direct measurement of quarkonia in p-p at 5.5 TeV: the performance of the spectrometer and the J/psi and Upsilon statistics that can be obtained in a 1 month run have been evaluated by means of simulation. The second strategy is the rescaling to 5.5 TeV of the yields measured at 14 TeV, according to parametrisations for the total and differential cross sections. In this case, the uncertainties related to the extrapolation need to be carefully evaluated. The phenomenological framework adopted for the analysis of both strategies is the Color Evaporation Model for quarkonia production. The thesis is organised as follows: - In Chapter 1 an introduction is given to the Quark-Gluon Plasma and to the expected suppression of quarkonia resonances in the deconfined medium. An overview on quarkonia phenomenology and on the experimental results obtained by SPS and RHIC experiments on J/psi suppression is also given. - In Chapter 2 the ALICE experiment is described, with particular regard to the muon spectrometer and its trigger system. - In Chapter 3 the general characteristics of Resistive Plate Chambers are described, as well as the specific options adopted for the muon trigger detectors by the ALICE collaboration in order to meet the requirements for both heavy ion and p-p data-taking. - In Chapter 4 the tests carried out in Turin are described in detail; an analysis of the result is presented, together with the criteria adopted for the validation of the final detectors. - In Chapter 5 the results of the efficiency tests of detectors which had previously undergone a long-term test under constant irradiation are presented. The aim of the tests is to detect any permanent effects of ageing on the detectors. - In Chapter 6 the physics performance of the ALICE muon spectrometer for J/psi and Upsilon detection in a p-p run of 1 monthat the centre-of-mass energy of 5.5 TeV, as obtained from Monte Carlo simulation, are presented. The results obtained for the scaling factors of p-p data at 14 TeV down to 5.5 TeV are also presented, together with an estimation of the theoretical uncertainties arising from the choice of Parton Distribution Function set.