Detection of muons in the LHCb experiment: the aging of RPC detectors and the study of $Z^0 \to \mu^+ \mu^-$

The LHCb experiment will take place at the future LHC accelerator at CERN and will start in 2007. It is a single arm spectrometer dedicated to precision measure- ments of CP violation and rare decays in the b quark sector . Recent experimental results have shown that CP violation is large in this sector . LHCb is designed with a robust and exible trigger , in order to extensively gain access to a wide spread of different physical processes involving the beauty particles. This will allow to over-constrain the Standard Model predictions about CP violation, and to discover any possible inconsistency , which would reveal the presence of ìNew Physicsî. The work presented in this thesis has two main parts: the development of the Resistive Plate Chambers, proposed for the detection of muons in LHCb, and the study of the detector performances in the physical channel Z 0 -> u + u -. The LHCb experiment has nished its R&D at the end of 2002. We have par- ticipated in the development of a suitable detector for the outer part of the Muon system. Because of the large surface to cover (more than 200 m ) and the high time resolution requested for triggering, the Resistive Plate Chambers (RPC) ap- peared to represent a valid technology , and their industrial implemented produc- tion would have allowed to reduce the costs. The efforts have then been concen- trated on the aging effects, to verify that the RPC detectors are able to satisfy the LHCb requirements for a period of ten years working. The results of a compre- hensive aging test carried through three years are presented in the rst part of the thesis. As we shall see the detectors have showed important aging effects and have been abandoned. The design values of the center-of -mass energy and luminosity at LHC repre- sent new limits, never reached by hadron colliders. As a result, high production cross-sections will be attained and high statistics data samples are foreseen to be collected, for a large variety of processes. In the second part of the thesis is reported a complete study of the the process pp -> Z 0 -> u + u -, a marginal channel respect to main LHCb physics program. However it has recently gained interest, since its the theoretical cross-section has been determined with a good accuracy . New physics results can thus arise from the comparison of the predicted value and a precise experimental measurement. A particular focus has been put on the detection performances of LHCb, and on off - line and on-line event selections. Three possible physics elds of interest have been individuated and discussed: the application as an absolute luminosity monitor , the efcacy in a precise calibration of the LHCb spectrometer , and the study of the proton structure functions.