Search for Λ+c → pe±µ∓ decays at LHCb

This thesis presents the search for the lepton-flavour violating decays of the charmed baryon, Λc+ → pe+µ− and Λc+ → pe− µ+ , and their charge conjugates. It is performed using 5.4 fb −1 of the pp collision data, collected at the centre-of-mass energy of 13 TeV by the LHCb experiment. These decays are effectively forbidden within the Standard Model, which makes them sensitive probes for effects beyond the Standard Model. Finding any significant Λ+ c → pe± µ∓ signal would be a clear sign of New Physics. The primary challenge in this study lies in identifying and suppressing background. The dom- inant background stems from frequent Λc+ → pπ + π − decays, with pions misidentified as leptons, and from random pe± µ∓ combinations. Multivariate classification models based on kinematical, topological and particle-identification observables are employed to reduce these backgrounds. Sig- nal decays are identified with the reconstructed mass of the Λ+ c candidates, m(Λc + ). The selection efficiencies for the signal and misidentified decays are determined using the calibrated simulations. Expected yields and shapes of the combinatorial background are estimated through fitting the data in the m(Λc+) sidebands, as well as the data with the wrong-charge pe± µ± combinations. The decay Λc+ → pϕ, with ϕ → µ+ µ− , is chosen as the reference channel, with respect to which the branching fractions of the signal decays are measured. Selection requirements on the multivariate classifier output, and electron and muon identifications are optimised to provide the best (i.e. lowest) upper limits on B(Λc+ → pe± µ∓ ). These limits are calculated using the CLs statistical method. To avoid any experimental bias, the analysis is conducted with the m(Λ+) signal region cblinded until the methods have been established and the final selection optimised. No significant signal has been observed, and the upper limits at 90% confidence level are set as: B(Λ+c → pe+µ−) < 7.7 × 10−8, and B(Λc+→ pe−µ+) < 9.5 × 10−8. They are dominated by the statistical uncertainty, while the largest systematic contribution comes from the branching fraction of the reference channel. The measured limits improve those previ- ously set by the BaBar experiment by about two orders of magnitude. The presented study is one of the most sensitive New Physics searches performed so far in the charm sector. In addition, the thesis also presents a contribution into the development of Magnet Stations, in particular various software and simulation studies. The Magnet Stations subdetector is planned by the LHCb experiment for its Upgrade II, in order to improve the reconstruction of low- momentum particles. It will increase sensitivities of many charm studies, including future searches for Λc+ → pe±µ∓ decays.