The essence of rare beauty: Studying B0(s) → μ+μ− decays with the LHCb experiment

The model developed by particle physicists which describes our Universe up to very high energies and to the smallest scales, called the Standard Model, has performed excellently to explain the results of measurements up to now. The Standard Model is built on symmetries, matter and interactions; a curious property of the matter in the Standard Model is that it appears in three generations. Particles from different generations are almost identical, but differ greatly in their couplings to the Higgs particle, and the structure of the couplings is unexplained; this is called the flavour puzzle. For example, their masses differ greatly: third-generation particles are at least a thousand times heavier than those from the first generation. The third-generation particles called beauty quarks are especially sensitive to possible differences between the generations.

Extra sensitive to such effects are decays of beauty quarks that occur seldomly according to the Standard Model, for which even small effects from particles beyond the Standard Model can be significant. A special class of these decays are B decays to two muons (siblings of the electron), which occur at the level of one time every billion B decays; they are extra sensitive to new Higgs particles or dark matter particles. At the Large Hadron Collider, trillions of beauty quarks per year are produced and even such very rare decays can be tested.

This dissertation describes the study of B decays to two muons with data collected by the LHCb experiment from 2011 to 2016, resulting in the discovery of the decay of a strange beauty particle or "Bs meson" to two muons and the first measurement of the lifetime of this decay, the strongest limit on the decay of the "Bd meson" to two muons. By itself, the measurement is consistent with the Standard Model. Combined with similar measurements, hints are found of deviations from the Standard Model. Future measurements are needed to shed light on this possibility to solve the flavour puzzle.