Prospects for Measuring the Branching Ratio of the Rare B$^{0}$${_s}$ $\rightarrow$ µ+µ− Decay with the TLAS Experiment

The Large Hadron Collider (LHC) located at the CERN labora- tory in Geneva provides p−p collisions at a centre-of-mass energy of √s = 7 TeV. The study of the rare Bs0 → 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 bran- ching ratio of the Bs0 → 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 Bs0 → µ+µ− bran- ching ratio relative to the branching ratio of the B+ → J/ψ(µ+µ−)K+ decay. The dominant background channel is the b¯b → µ+µ−X combinatorial background. True Bs0 → µ+µ− decay candidates are separated from the much larger amount of combina- torial background events using several discriminating quantities. Upper limits on the Bs0 → µ+µ− branching ratio are com- puted using a Bayesian and a frequentist method. The expected precision of the branching ratio measurement is estimated for dif- ferent values of the integrated luminosity. An expected upper limit on the branching ratio is computed to BR(Bs0 → µ+µ−) < 3.8 × 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.