Thesis defense of Maik Becker

This thesis presents two measurements probing the Standard Model of particle physics with data recorded by the LHCb experiment in Run 1 and Run 2 of the Large Hadron Collider.

The first measurement is a precise branching fraction determination of the purely leptonic Bs→μ⁺μ⁻ and Bd→μ⁺μ⁻ decays. The Bs→μ⁺μ⁻ branching fraction is determined with a relative uncertainty of 15%, making it the most precise single-experiment result available at publication time. The Bd→μ⁺μ⁻ decay is not observed. An upper limit on its branching fraction is calculated to be of O(1e-10) at a confidence level of 95%. These published results strongly constrain theoretical models of new physics effects in the flavour sector.

The second analysis is the first search for the semi-baryonic Bs→ppμ⁺μ⁻ and Bd→ppμ⁺μ⁻ decays. It is the first time that decays of Bs mesons via flavour-changing neutral b→sl+l- transitions into a final state with baryons are studied. The current state of the measurement and preliminary results are presented. Expected upper limits on the branching fractions are determined to be of O(1e-9) at a confidence level of 95%.