Prospects for measuring the branching ratio of the rare B0s→μ+μ- decay with the ATLAS experiment

The Large Hadron Collider (LHC) located at the CERN laboratory in Geneva provides p-p collisions at a centre-of-mass energy of √(s)=7 TeV. The study of the rare B0s→μ+μ- decay is among the research topics of ATLAS, one of the main experiments at the LHC. This decay is highly suppressed in the Standard Model of particle physics and may give an indirect evidence for New Physics models. This PhD thesis investigates prospects for measuring the branching ratio of the B0s→μ+μ- decay with the ATLAS experiment. The analysis is based on Monte Carlo data, with p-p collisions generated at a centre-of-mass energy of √(s)=10 TeV. The strategy employed is to calculate the B0s→μ+μ- branching ratio relative to the branching ratio of the B+→ J/ψ(μ+μ-)K+ decay. The dominant background channel is the b anti b→μ+μ-X combinatorial background. True B0s→μ+μ- decay candidates are separated from the much larger amount of combinatorial background events using several discriminating quantities. Upper limits on the B0s → μ+μ- branching ratio are computed using a Bayesian and a frequentist method. The expected precision of the branching ratio measurement is estimated for different values of the integrated luminosity. An expected upper limit on the branching ratio is computed to BR(B0s→μ+μ-)<3.8 x 10-8 at a 95% confidence level for 1 fb-1. The precision of the ATLAS measurement of the branching ratio will reach a level compatible with the best current measurements with about 2-5 fb-1 of data.