Phd in PHYSICS

Thesis title: Search for lepton flavor violation in τ→3μ muons decay with the Run3 data of the CMS experiment at LHC

Lepton flavor conservation is one of the accidental symmetries of the Standard Model 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 τ 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, corresponding to an integrated luminosity of 62.2 /fb. The analysis targets τ 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 τ 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 x 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 the Appendix. 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.