Search for Heavy Neutral Leptons in three prompt leptons final states with at least one hadronic tau at the CMS Experiment

The Standard Model (SM) of particle physics provides a highly successful description of the known fundamental particles and their interactions. Nevertheless, several experimental observations remain unexplained within the SM framework, such as the small but nonzero neutrino masses, the nature of dark matter and the observed matter--antimatter asymmetry in the Universe.

Extensions of the SM that introduce heavy neutral leptons (HNLs), often referred to as sterile or Majorana neutrinos, offer a compelling mechanism to address these open questions.
This dissertation presents a search for the production of HNLs in final states containing three prompt charged leptons, with at least one hadronically decaying tau lepton ($\tau_h$). The analysis targets promptly decaying Majorana HNLs that could give rise to lepton number violating signatures. It is based on the full RunII dataset of proton--proton collisions recorded by the CMS experiment at the CERN Large Hadron Collider between 2016 and 2018, corresponding to an integrated luminosity of 138fb$^{-1}$ at a center-of-mass energy of $\sqrt{s} = 13$TeV.
The search explores a mass range from 30GeV to 1~TeV and is optimized for HNL masses above the $W$ boson mass. Dedicated signal regions are defined according to the lepton flavor composition, and background contributions are estimated using a combination of simulation and data-driven methods. The search uses a combination of cut-based search regions and deep neural network to extract signal.

The analysis sensitivity reaches unprecedented levels in the parameter space involving couplings to tau neutrinos, with expected limits representing the most stringent results to date at the Large Hadron Collider in the mass range from $m_W$ to 800~GeV.