Study of Rare Beauty Decays with ATLAS Detector at LHC and MDT Chamber Perfomances

The Large Hadron Collider (LHC) is a proton-proton collider that will operate at a center of mass energy of $14~TeV$ and at a maximum luminosity of $L=10^{34}cm^{-2}s^{-1}$. The LHC will reproduce interactions similar to those which existed when the universe was only $\sim 10^{-12}s$ old, conditions which have not been achieved in any previous collider. The primary goals of the LHC project are to discover the origin of particle masses, to explain why different particles have different masses and to search for new phenomena beyond the Standard Model. Also heavy quark systems and precision measurements on Standard Model parameters will be subject of LHC physics studies. ATLAS (A Toroidal LHC ApparatuS) is one of the two LHC general purpose experiments. The guiding principle in optimizing the ATLAS experiment has been maximizing the discovery potential for New Physics such as Higgs bosons and supersymmetric particles, while keeping the capability of high precision measurements of known objects such as heavy quarks and gauge bosons. The rate of $B$-hadron production at the LHC is enormous thanks to the large cross-section for $b$-quark production and the high luminosity of the machine ($L=10^{33}cm^{-2}s^{-1}$ even at initial low luminosity). About $1\%$ of collisions produce a $b$-quark pair. An important range of $B$-Physics studies has therefore been planned for the ATLAS experiment. An important aim of the $B$-Physics work is to test the Standard Model through precision measurements of $B$-hadron decays that together will over-constrain the $CKM$ matrix, to give indirect evidence for New Physics. This program includes: precise measurements of $CP$ violation in $B$-meson decays, precise measurements of the periods of flavour oscillations in $B^0_s$ as well as $B^0_d$ mesons, searches and measurements of very rare decays. Certain rare decays, for which the decay products themselves provide a distinctive signature that can be used in the trigger, will be studied very effectively in ATLAS. These so-called ``self-triggering'' modes include decays of the type $B \to \mu^+ \mu^- (X)$. Such decays involve flavour changing neutral currents and are strongly suppressed in the Standard Model, which predicted branching ratios are typically in the range $10^{-5}-10^{-9}$. New Physics might result in significant enhancements compared to the Standard Model predictions and thus their measurement provide an indirect search for New Physics. This thesis presents a study on simulated data for the two semileptonic decays $B \to K^{+} \mu^+ \mu^-$ and $B \to K^{*+} \mu^+ \mu^-$. The goal of the study has been to evaluate the ATLAS experiment sensitivity to New Physics discover through precise measurements of these rare semileptonic beauty decays. The ATLAS sensitivity to their branching ratios has thus been assessed. Moreover, the signal reconstruction and the background rejection strategies for the next ATLAS data taking have been outlined. Also two studies, on the ATLAS Muon Spectrometer performance and on the ageing and tracking capability in a high rate background of Monitored Drift Tube (MDT) precision chambers, are presented. The Muon Spectrometer defines the overall dimensions of the ATLAS detector. The outer chambers of the barrel are at a radius of about $11~m$ and the third layer of the forward muon chambers is located about $23~m$ from the interaction point. The effort of the INFN-Cosenza group to the spectrometer realization and test was enormous. The test on H8 beam line at CERN during 2004 has given the possibility to test and validate not only the performance of the single spectrometer subsystem but also their integration. The analysis presented in this thesis is to the evaluation of the intrinsic spatial resolution of the tracking system. The beam momentum has been also measured, for the first time, by means of the Muon Spectrometer. In addition, a comparison of experimental results with a {\sc Geant4}-based simulation has constituted an important validation test of the official ATLAS simulation software. The precision tracking chambers of the Muon Spectrometer have to operate for more than 10 years in the harsh LHC background, due mainly to low energy neutrons and photons. Ageing effects and difficulties in tracking can appear, moreover in view of the upgrade to Super-LHC. Neutron and gamma irradiation tests of the muon system precision chambers have been performed during 2005 at Enea Casaccia research center, in order to study the MDT behaviour after massive irradiation and in a high rate background environment. Analysis on accumulated charge spectra for these tests are here presented.\\ {\bf Chapter~1} is to the physics of rare semileptonic decays of beauty hadrons. The effective Hamiltonian, in the heavy quark limit, is introduced and the methods to compute the perturbative short-distance and the non-perturbative long-distance contributions are discussed. Results obtained in some supersymmetric models are shown pointing out differences with respect to Standard Model predictions. Current experimental results from beauty factories are also shown. {\bf Chapter~2} provides a general overview of the experimental facilities. The LHC and the ATLAS experiment are described. A section has been to the description of the ATLAS trigger requirements for $B$-physics. {\bf Chapter~3} describes the official ATLAS software, the simulation and the reconstruction of beauty events. Also the LHC Computing Grid project is briefly mentioned. Grid facilities have been extensively used for this work. In {\bf Chapter~4} the analysis results on simulated rare $B \to K^{+} \mu^+ \mu^-$ and $B \to K^{*+} \mu^+ \mu^-$ decays are shown. Signal reconstruction and characterization, background rejection and estimation are described in great detail. Moreover, the ATLAS sensitivity to their branching ratio measurements and the possibility to discover New Physics signals are outlined. {\bf Chapter~5} is completely to the analysis of the experimental H8 test beam data of the summer 2004. The experimental setup is described and results of the tracking system intrinsic resolution measurements are given and compared with {\sc Geant4} simulation. In {\bf Chapter~6}, after an overview of expected background rate in ATLAS, the neutron and gamma ageing and tracking in a high rate environment tests are described. Accumulated charge spectra analysis strategy and results are shown. Finally, in the last part, some general conclusions and considerations are given.