Search for $B → \mu^{+}\mu^{−}$ Decays with the Full Run I Data of The ATLAS Experiment

Rare $B_s$ and $B_d$ meson decays into two muons are forbidden at the tree level of the Standard Model. They offer the opportunity to perform genuine probes of Yukawa interactions or Electroweak precision tests and play very important role to find signatures of physics beyond the Standard Model. A brief insight into the theoretical foundations of these decays is given in the introductory section, followed by a section with the ATLAS experiment description, none of which is directly a result of author's own work. The ATLAS Collaboration has been searching for $B_{(s)}^0 \to \mu^{+}\mu^{-}$ decays using merged 2011 $\sqrt{s} = 7$ TeV and 2012 $\sqrt{s} = 8$ TeV Full Run I Data sample ($\approx 25~fb^{-1}$). The analysis procedure has been firmly established and unblinding of the search region of $B_{(s)}^0 \to \mu^{+}\mu^{-}$ is imminent. A sensitivity of the analysis to the $B_s^0 \to \mu^{+}\mu^{-}$ signal is estimated to be $4.7~\pm~1.0~\sigma$. The description of the whole ATLAS $B_{(s)}^0 \to \mu^{+}\mu^{-}$ analysis procedure is given in the third section, where a summary of author's contributions is described as well. The final three sections describe in great detail explicitly author's contributions to the analysis and beyond. The algorithm for the rejection of the ATLAS mis-identified hadrons (as muons) has proven to be very useful not only to separate the signal from the almost indistinguishable peaking background for the $B_{(s)}^0 \to \mu^{+}\mu^{-}$ analysis, but found use also in other physics analysis. Secondly, the branching ratio $\cal{BR}~(B_{(s)}^0 \to \mu^{+}\mu^{-})$ measurement on ATLAS is performed with respect to a reference channel decay $B^\pm \to J/\psi K^\pm$, the yield of which was extracted with a very good accuracy. Finally, as a natural outcome from the reference channel yield extraction, a measurement of $\Gamma(B^{\pm} \to J/\psi \pi^{\pm})/\Gamma( B^{\pm} \to J/\psi K^{\pm})$ has been found competitive with other measurements and performed in parallel to the main $B_{(s)}^0 \to \mu^{+}\mu^{-}$ analysis on the Full Run I Data with the result of $3.9 \pm 0.4^{stat.} \pm 1.0^{syst.}\%$