Search for lepton flavor violation in τ→3μ decay with the Run~3 data of the CMS experiment at LHC

Lepton flavor conservation is one of the accidental symmetries of the SM that has been proven to be non-fundamental following the observation of neutrino oscillations. Among the possible lepton flavor–violating (LFV) processes, the decay of the tau lepton into three muons (τ→3μ) provides a particularly sensitive probe of new physics scenarios involving for instance heavy mediators or an extended Higgs sector. This thesis presents a search for the LFV decay τ→3μ using proton–proton collision data, collected with the CMS detector at a center-of-mass energy of 13.6 TeV during 2022 and 2023, and corresponding to an integrated luminosity of 62.2 /fb. The analysis targets tau leptons produced in vector boson decays. Candidate events are reconstructed from three muons originating from a common vertex in the detector and are selected using multivariate techniques. The search performs a bump hunt in the invariant mass distribution of the three muons, which would exhibit a narrow resonance around the tau lepton mass (1.777 GeV) in the presence of a LFV signal. At the time of writing, the analysis is undergoing final internal review within the CMS Collaboration. Therefore, the results remain blind: data in the signal region are not used, and only the expected sensitivity, derived from simulated signal samples, is reported. The expected upper limit on the branching ratio is Br(τ→3μ) <7.2 1e-8 at 90% confidence level, obtained from the combination of the 2022 and 2023 datasets. The search for the τ→3μ process is complemented by performance studies of the upgraded readout electronics for the CMS electromagnetic calorimeter barrel in preparation for the High-Luminosity LHC phase, which are documented in Appendix A. Beam test measurements demonstrate that the new front-end electronics achieves an energy resolution better than 0.6 % and a time resolution below 30 ps for 50 GeV electrons, fully meeting the design specifications.