Pursuing forbidden beauty - Search for the lepton-flavour violating decays $B^0\to e^{\pm} \mu^{\mp}$ and $B^0_s\to e^{\pm} \mu^{\mp}$ and study of electron-reconstruction performance at LHCb

This dissertation covers the search for the in the Standard Model forbidden (lepton-flavour violating) decays of neutral beauty mesons to an electron and muon 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.