Development of the HiDRa highly granular dual-readout calorimeter prototype and search for heavy neutral leptons with the ATLAS experiment

This thesis presents the work carried out during my Ph.D. at the University of Pavia and INFN, divided into two main lines of research. The first describes the development of a highly granular dual-readout calorimeter prototype, named HiDRa. The second outlines a search for beyond the Standard Model particles called Heavy Neutral Leptons (HNLs) with the ATLAS experiment at the LHC. The HiDRa project is part of the detector R&D programme for future lepton colliders such as FCC-ee (CERN) and CEPC (China). At both future collider facilities, precise measurements of Higgs boson properties, most interestingly in completely hadronic final states, require calorimeters with excellent energy resolution and powerful particle identification. The dual-readout technique is particularly interesting for hadron energy measurements, as the combination of two independent signals for the measurement of the same hadron shower allows to significantly improve the energy resolution of the detector. HiDRa combines the dual-readout technique with very fine lateral granularity, achieved through Silicon Photomultipliers (SiPMs) reading out scintillation and Cherenkov light produced in two sets of optical fibres. In my thesis, the most relevant contributions to this project are outlined. I firstly contributed to the Geant4 simulation studies guiding the detector design, studying the detector response with different physical parameters. The construction and laboratory commissioning of the prototype are then described, together with the analysis of data collected during tests with particle beams to which I participated and contributed. These studies demonstrate the potential of HiDRa-like calorimeters for linear response to hadrons, improved resolution, and also particle identification capabilities, with direct implications for the IDEA detector concept at FCC-ee. In parallel, I worked in the ATLAS Collaboration on a search for HNLs using Run 2 proton-proton collision data. HNLs are motivated by several open problems in particle physics, including the generation of neutrino masses, dark matter, and the matter-antimatter asymmetry. The analysis targets lepton-number-violating signatures that would be possible if neutrinos were Majorana particles. These signatures comprise three charged leptons, including a same-sign same-flavour lepton pair, which guarantees final states with low Standard Model background. Among my contributions to this analysis were the optimisation of signal regions, validation of the expected backgrounds, implementation of statistical fits with the TRExFitter software framework, and, since no significant excesses over the expected background were found, the extraction of exclusion limits on the HNL mixing parameters.