  The study of flavour-changing neutral currents (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
$B^0\rightarrow \mu^+\mu^-$ and $B_s^0\rightarrow \mu^+\mu^-$ 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 $B^0$, an upper limit on the branching fraction is set at
BR$(B^0 \rightarrow \mu^+\mu^-) < 4.2\times 10^{-10}$ at 95\% confidence level.
For $B^0_s$, the branching fraction BR$(B^0_s \rightarrow
\mu^+\mu^-)=(0.9^{+1.1}_{-0.8})\times 10^{-9}$ 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 $b \rightarrow s\ell\ell$ decays is the angular analysis of
the decay $B^0 \rightarrow K^*\mu^+\mu^-$. 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.
