The spatial alignment of the Muon Detector for the LHCb experiment

The phenomenology of particle physics is well described by the Standard Model. However, some of the parameters of the model are not predicted and have to be experimentally determined, as for instance, the four parameters of the quark mixing matrix, the Cabibbo-Kobayashi-Maskawa (CKM) matrix. One of these parameters, the phase of the CKM matrix, is responsible of the violation of the CP symmetry that was identified as one of the causes for the asymmetry between matter and anti-matter in the Universe~\cite{bib:Sak}. LHCb is one of the four experiments of particle physics at the Large Hadron Collider accelerator built at CERN. The LHCb experiment is dedicated to high precision measurement of CP violating parameters in the system of $B$ mesons. The large samples of $B-\overline{B}$ pairs that will be produced allow to measure very rare $B$ decays like $B_{s} \rightarrow \mu^{+} \mu^{-}$. The branching ratio predicted by the Standard Model for this decay is of the order of $O(10^{-9})$, but it can receive large contributions by virtual processes involving new particles. The challenge of the LHCb experiment is to efficiently discriminate the events of interest from the background produced by the $p-p$ collisions. For this reason the experiment is equipped not only with specific detectors, but also with an efficient trigger system, in which the Muon Detector plays an important role. The Muon Detector task, in the first stage of the t rigger system, is the detection of muons with high transverse momentum\footnote{Given the large mass of the $B$ mesons, their decay products have a transverse momentum larger than the background particles.}. Moreover, in the high level trigger and in the off-line reconstruction the Muon Detector provides the muon identification. The information on muons in the final states of $B$ meson decays are used by the tagging algorithms for the determination of the $b$ flavour of the decaying $B$ mesons. This procedure is of great relevance in CP violation measurements. Because of the role it plays in the experiment, it is crucial to know the actual position of the Muon Detector, and to monitor it during the time. This is the aim of the software alignment of the detector and the subject of this thesis. The software procedure to align the Muon Detector, locally and globally, has been carried out developing specific algorithms for the track finding and reconstruction. The reconstructed tracks have been expressed in the standard LHCb track format and fitted with the default LHCb track fit method. The Muon Detector software alignment procedure has been tested with three kinds of data: Monte Carlo muons coming from the interaction point, real cosmic rays and Monte Carlo cosmic data. The real cosmic rays, analysed in this work, have been the first experimental data ever acquired by LHCb. This thesis is divided in the following chapters: Chapter 1 describes the CP violation in the Standard Model of particles with the phenomenology of interest in the LHCb experiment. Chapter 2 briefly describes LHC and then discusses all the detectors and sub-detectors that constitute the LHCb experiment. Chapter 3 is entirely dedicated to the Muon Detector for its relevance to this thesis. In Chapter 4 the alignment methods used are described. Chapter 5, after a brief description of the LHCb software organisation, discusses the first test of the global alignment procedure applied to the Muon Detector, and performed with Monte Carlo data of single muons coming from the interaction point. With real cosmic rays a local alignment of the Muon Detector was performed. In addition a preliminary study for a relative alignment of the detector with respect to the Outer Tracker was also carried on. This is the subject of Chapter 6. Chapter 7 describes a Monte Carlo cosmic data sample that has been produced for testing the local alignment performed with real cosmic rays and for studying local systematics arising from the acceptance of the Muon Detector unsuited for the cosmic ray geometry.