Search for new physics in the B meson decays: $B^{0}_{(s)} \to \mu^{+}\mu^-$

The rare decays B0 s → μ+μ − and B0 → μ+μ − are among the most promising investigation channels for the search of new physics. Their tiny branching fractions and their small theoretical relative uncertainties make these decays very sensitive to many new physics scenarios. I present the first successful reconstruction of the rare decay B0 s → μ+μ − , the measurement of its branching fraction, and the most recent upper limit for the branching fraction of the B0 → μ+μ − decay at CMS, performed with all data acquired at the LHC Run I