Limit on $B_S^0\to\mu^+\mu^-$ Branching Fraction

A search for the decay Bs -> mu+mu- has been performed using proton-proton collisions recorded with the ATLAS detector at the LHC. The Standard Model predicts the time-integrated branching fraction for the decay to be extremely small. CLs tests method is used in extracting upper limit of the branching fraction from the experimental data. This report describes the two test methods used, the counting method and the mass fit method, and compares them.