Measurement of the Reference Channel $B^{\pm} \to J/\psi K^{\pm}$ for the Rare Decay $B_{s}^{0} \to \mu^{+}\mu^{-}$ with the ATLAS Experiment

The ATLAS experiment is one of the four major experiments at the Large Hadron Collider (LHC) of CERN, Geneva. The experiment addresses the open questions of particle physics. The decay of the $B_{s}^{0}$ meson to two muons is strongly suppressed in the Standard Model (SM). It is a rare decay with an expected branching fraction of $3.56\times10^{-9}$. The $B_{s}^{0} \to \mu^{+}\mu^{-}$ branching fraction is measured relative to a reference channel, $B^{\pm} \to J/\psi K^{\pm}$. The subject of this thesis is the measurement of the $B^{\pm}$ yield. The analysis uses $4.9\,fb^{-1}$ of ATLAS data collected in the year 2011 at $7\,TeV$ center-of-mass energy. The $B^{\pm}$ candidate selection is performed using a multivariate (MVA) technique. The yield is estimated using a two-dimensional unbinned maximum likelihood fit that accounts for the per-event uncertainties. The result is used for the computation of the upper limit on the $B_{s}^{0} \to \mu^{+}\mu^{-}$ branching fraction published by the ATLAS collaboration [1,2].