CMS pixel module qualification and search for $B^{0}_{s} \to \mu^{+}\mu^{-}$

This work is divided into two parts. The rst part focuses on the qualication and performance optimisation of the barrel pixel detector hardware in the CMS experiment. The barrel part of the pixel system holds 768 detector modules on three cylindrical layers around the interaction region. In the period of April 2006 to March 2008, 971 fully assembled detector modules were produced at the Paul Scherrer Institute (PSI) in Villigen, Switzerland. To meet the performance and lifetime requirements of the CMS pixel system, an elaborate test procedure was developed and a grading system was established. From the total of 813 modules that qualied for use in the CMS pixel system, 75 % were attested an excellent performance and 25 % held deciencies with only minor impact on the detector performance. The remaining 158 modules exhibited serious aws and were rejected. The second part of this work is on the development of a physics analysis aiming at the measurement of the rare decay B0 s → µ +µ −. This decay mode provides good sensitivity to tan β, a central parameter of the Minimal Supersymmetric extension of the Standard Model (MSSM). The MSSM and many other Standard Model extensions predict a (very) large increase of the branching fraction expected in the Standard Model. This rare decay mode therefore oers an excellent opportunity to explore new physicspossible with only a small data sample gathered from the very rst running period at the LHC. With the rst 1 fb−1 of integrated luminosity, an upper limit on the branching fraction of 1.3 × 10−8 at the 90% C.L. is expected. In this analysis, the most eective selection criteria to discriminate the large number of background events from the signal events are based on the long lifetime of the B mesons. The pixel detector allows a precise determination of the displaced vertices and will also therefore play a crucial role in this part of thesis.