Searching for direct probe of the neutrino mass generation mechanism and luminosity determination in pp collisions for elastic scattering measurements with the ATLAS detector at the LHC

This thesis presents two analyses performed with data collected by the ATLAS detector at CERN. The first analysis investigates the origin of neutrino masses through a search for right-handed heavy neutrinos predicted in the framework of the Left-Right Symmetric Model. The search exploits proton-proton collision data at a centre-of-mass energy of √s = 13 TeV, corresponding to an integrated luminosity of 140 fb⁻¹ collected during the LHC Run 2 (2015–2018). The analysis targets lepton-number-violating final states with same-sign leptons, where backgrounds from Standard Model processes and detector effects are controlled with simulation, dedicated data-driven techniques, and machine learning methods for signal discrimination. As no significant excesses with respect to Standard Model processes are observed, exclusion limits are set on the production cross-section of the Beyond-the-Standard-Model heavy neutrinos and Δ boson, as a function of their masses. The second analysis focuses on precision luminosity determination, a key input for many ATLAS physics measurements. Two datasets collected in 2018 at √s = 900 GeV and √s = 13 TeV are analysed. The resulting integrated luminosity measurements contribute to the evaluation of the total proton-proton cross-section. Looking toward the High-Luminosity LHC upgrade, this thesis also contributes to studies of a silicon-based luminometer prototype. The sensors characterisation and the data-taking performance in 2025 are presented. The efforts described in this work demonstrate the dual role of ATLAS in extending the search for new phenomena beyond the Standard Model and delivering the precision measurements required to support them.