Testing the new physics scale in the B^{0} system at ATLAS

Weak decays that are naturally suppressed in the Standard Model, such as processes with flavour-changing neutral-currents, are perfect for indirect searches of new physics. These decays allow us to investigate higher energy ranges with respect to direct searches thus representing a fundamental complementary tool. Results on the ATLAS analysis of the $B_s^0 \rightarrow \mu^+\mu^-$ rare decay will be presented. The ATLAS study of $B_d^0 \rightarrow K^* \mu^+\mu^-$ is also reported where the parameters $A_{FB}$ and $F_L$ are extracted from the angular distribution of the final state. The measurement of the $\phi_s$ phase in the $B_s^0 \rightarrow J/\psi \,\phi$ system, that can provide probes for physics beyond the SM, is showed too.