The thesis focuses on Higgs boson physics, specifically one of its rare and yet unobserved decay channels: the decay into a Z boson and a photon (Zgamma channel). This decay not only complements our understanding of the Higgs boson but also uniquely involves all currently known neutral bosons (Higgs, Z, photon) and is sensitive to potential processes beyond the Standard Model. The final state of the analysis consists of the two lepton decay products from the Z boson (muons or electrons for this study) and a photon. Background events produced by other Standard Model processes that contain two leptons and a photon (or misidentified particles) form the background of the analysis. With all data gathered during LHC Run 2 (2015-2018) and Run 3 (2021-2026), it is possible to have evidence of this decay, that is to observe it with a statistical significance exceeding three standard deviations.

In addition, the thesis includes an instrumental part focused on optimizing the time resolution of the CMS electromagnetic calorimeter (ECAL). Although designed for precise energy measurements, the ECAL also shows excellent timing resolution for photons and electrons (approximately 150 ps in collisions, 70 ps in test beam conditions). In a final state populated by photons from multiple overlapping events (pileup), the arrival time of a photon helps to verify its compatibility with the Higgs boson decay vertex. This will be crucial during the high-luminosity phase of the LHC (2029 onward), when the number of overlapping events is expected to be about three times greater than today. A new readout electronics for the ECAL is being developed and will be installed in the ECAL and CMS during the duration of the thesis. The new electronics achieves a timing resolution of 30 ps for high-energy photons and electrons. This performance was tested in ideal beam conditions (no magnetic fields, no tracker material in front of ECAL, no pileup). The thesis aims to develop algorithms to maintain this performance within CMS.

The thesis work is a continuation of the ongoing Z? analysis within the CMS group at CEA Saclay and the timing performance analysis of the ECAL, where the Saclay group is a leader. Simple, robust, and efficient analysis tools written in modern C++ and leveraging the ROOT analysis framework allow to understand and contribute to every stage of the analysis, from raw data to published results. The CMS Saclay group has leading responsibilities in CMS since its construction, including deep expertise in Higgs physics, electron and photon reconstruction, detector simulation, and machine learning and artificial intelligence techniques.

Regular trips to CERN are proposed for presenting the results of this work to the CMS collaboration and for participating in laboratory tests planned for the new ECAL electronics, as well as for participating to its installation.