Identification and measurement of low energy electrons and the decay $B_{s}^{0} \to J/\psi \phi$ at CMS

The Compact Muon Solenoid (CMS) is a general purpose detector that will take advantage of the rich physics potential of the Large Hadron Collider (LHC) at the European Centre for Particle Physics (CERN). LHC will provide a copious source of b-quarks and enables a comprehensive b-physics programme. An important aspect of this programme is the ability to reconstruct J/psi resonances from their decay to electrons. This will require measurement of electrons with energies of a few GeV and will be achieved by exploiting the precision electromagnetic calorimetry provided by the CMS lead tungstate crystal calorimeter (ECAL). This thesis examines the performance of a CMS Endcap ECAL prototype calorimeter using beams of high energy (5 GeV to 180 GeV) electrons. A parameterisation of the energy resolution is presented that allows stochastic and constant contributions to be obtained. These are found to have values that meet the design specification. The contribution to the energy resolution that arises from the performance of individual components comprising the calorimeter is also considered. The difficulties associated with accurate reconstruction of low-momentum electrons are discussed. These difficulties arise due to the high probability of bremsstrahlung in the tracker material combined with the large magnetic field in CMS. A reconstruction method has been developed which combines tracker and ECAL information and provides a transverse momentum resolution for 5 GeV electrons varying between 1.5% at a central pseudorapidity to 6% at a pseudorapidity of 2.4. The method has been applied to simulated J/psi decays in the channel B sub s -> J/psi(ee)phi(KK) and is shown to provide improved simulated B sub s mass resolution. A simulated untagged transversity analysis is shown to separate the two B sub s CP eigenstates allowing the mixing induced width difference, DELTA GAMMA, between these to be obtained from angular distributions of the decay products. The width difference has then been related to the mixing parameter X sub s and the X sub s reach of CMS using this method studied.