Analysis of the D0 → K-π+e-e+ decay with the LHCb Upgrade I detector

Rare charm decays, such as D0 → K-K+e-e+ and D0 → π-π+e-e+, provide a sensitive probe in the search for New Physics. In particular, a measurement of a branching fraction that significantly deviates from the Standard Model prediction could constitute a first indication of physics beyond the Standard Model. In this framework, the decay D0 →K-π+e-e+ is of particular importance, as it serves as the natural normalization channel for the aforementioned rare decays. In this thesis, the yields of the normalization channel are determined for the first time using data collected with the upgraded LHCb detector. The analysis is based on proton–proton collision data recorded during Run 3 of the LHC at a center-of-mass energy of √s = 13.6 TeV. The signal yields are extracted from unbinned maximum-likelihood fits to the invariant mass distributions of D0 candidates. The selection and fit strategies are optimized to account for differences between decays with zero or at least one reconstructed bremsstrahlung photon. The background shapes used in the fit model are studied with RapidSim, a fast simulation framework for heavy-hadron decays, following a dedicated validation presented in this thesis. The measured yields of the signal channel are 7389±172 and 1130±78 for the cases in which at least one reconstructed photon is associated with the decay, or no photon is associated, respectively. The quoted uncertainties are statistical only, as determined from the fits. This study will be extended within the LHCb experiment in the ongoing search for rare charm decays.