A search for the B_0(s) to mu mu (gamma) decays at LHCb

Feasibility studies are conducted to test if the B(s) → µµγ branching fraction can be measured using a partial reconstruction technique, using the B(s) → µµ analysis with no optimisation for the radiative counterpart. An improved measurement of the decay B_s → µµ and searches for the decays B → µµ and Bs → µµγ are performed at the LHCb experiment using data collected in proton-proton collisions at √s = 7, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2 and 6 fb−1, respectively. The B_s → µµ branching fraction is measured to be Br(B_s → µµ) = 3.09 (+0.46 -0.43) (+0.15 - 0.11) × 10^−9, where the uncertainty includes both statistical and systematic contributions. No significant signal is found for B → µµ and the upper limit of Br(B0 → µµ) < 2.6 × 10^−10 at 95% confidence level is determined. Positive results from the feasibility studies led to the first search for B_s → µ µγ decays using partial reconstruction, and an upper limit of Br(B_s → µµγ) < 2.0 × 10^−9 is set at 95% confidence level, limited to the range m(µµ) > 4.9 GeV/c^2. Additionally, the ratio between the B_s → µµ and B0 → µµ branching fractions is measured to be R(µ+µ−) < 0.095 at 95% confidence level. The results are all in agreement with the Standard Model predictions.