Study of $b \to sll$ decays at ATLAS

The study of flavour-changing neutral current (FCNC) gives access to important tests of the Standard Model (SM) and allows to search for hints of beyond the SM phenomena. We present here the study of the very rare decays $B^0\to \mu^+\mu^-$ and $B_S^0\to \mu^+\mu^-$ using data corresponding to an integrated luminosity of 25 fb$^-1$ of $7$~TeV and $8$~TeV proton--proton collisions collected with the ATLAS detector during the LHC Run 1. For B0, an upper limit on the branching fraction is set at BR$(B^0\to\mu^+\mu^-)<4.2\times 10^{-10}$ at 95\% confidence level. For $B^0_s$, the branching fraction BR$(B^0_s\to\mu^+\mu^-)=(0.9^{+1.1}_{-0.8})\times10^{-9}$ is measured. The results are consistent with the SM expectation with a p-value of 4.8\%, corresponding to 2.0 standard deviations. Another study sensitive to possible new physics contributions in $b \to sll$ decays is the angular analysis of the decay $B^0\to K^*\mu^+\mu^-$. Here we present the results obtained using proton-proton collisions at $\sqrt{s}=8$~TeV from LHC data collected with the ATLAS detector. The study is based on 20.3 fb$^{-1}$ of integrated luminosity collected during 2012. Measurements of the $K^∗$ longitudinal polarisation fraction and a set of angular parameters obtained for this decay are presented. The results are compared to a variety of theoretical predictions and found to be compatible with them.