Pursuing forbidden beauty: Search for the lepton-flavour violating decays B0 → e± μ∓ and Bs0 → e± μ∓ and study of electron-reconstruction performance at LHCb

Physicists have developed a successful theory, called the Standard Model, describing all known fundamental particles and interactions, except gravity, up to high energies and small length scales. Despite its success, it is when extrapolating to astronomical length and time scales that the theory fails. To alleviate these issues, physicists search for more fundamental laws of physics by exploring higher energies. One such approach is the study of heavy and therefore energetic particles called beauty mesons.

Since interactions of higher energies mediating decays of particles are relatively suppressed, decays of particles that are rare or forbidden according to the Standard Model are of particular interest, as such interactions can have a relatively large effect. At the particle accelerator LHC at CERN, beauty mesons are produced by the trillions a year. Hence, rare and forbidden beauty decays can be searched for and studied to great precision.

This dissertation covers the search for the in the Standard Model forbidden decays of neutral beauty mesons to an electron and muon (a heavier sibling of the electron) with the data collected in 2011 and 2012 by the LHCb detector operating at the LHC. No such decays were found, but its increased precision of the upper limit of its probability is used to constrain new models. In addition, a study of the performance of the reconstruction of electrons at the LHCb experiment is presented, which will allow to determine and reduce systematic uncertainties in future analyses of beauty decays with electrons in the final
state.