Rare $B$ decays and $CP$-Violation in the $B$ system with the ATLAS Detector

A search for the rare decay $B_{s}^{0} \to \mu^{+}\mu^{-}$ has been performed with the ATLAS detector at the Large Hadron Collider LHC in proton-proton collisions at a centre of mass energy of $\sqrt{s} = 7\,\text{TeV}$ using $2.4\,\text{fb}^{-1}$ of integrated luminosity. The observed number of signal events is consistent with the background expectations and the resulting upper limit on the branching fraction is set to $\mathcal{B}(B_{s}^{0} \to \mu^{+}\mu^{-}) < 2.2 (1.9) \times 10^{-8}$ at $95\%$ ($90\%$) confidence level. A time dependent measurement of $CP$-violation in the exclusive decay channel ${B_{s}^{0} \to J/\psi(\mu^{+}\mu^{-})\phi(K^{+}K^{-})}$ has been performed with $4.9\,\text{fb}^{-1}$ at an energy of $\sqrt{s} = 7\,\text{TeV}$. The resulting measurements of the $CP$-violation weak mixing phase $\phi_{s} = 0.22 \pm 0.41(\text{stat.}) \pm 0.10(\text{syst.})$ and the decay width difference $\Delta \Gamma_{s}= 0.053 \pm 0.021(\text{stat.}) \pm 0.008(\text{syst.})$ are consistent with the theoretical predictions of the Standard Model and the world average values.