Search for light long-lived neutral particles that decay to collimated pairs of leptons or light hadrons in $pp$ collisions at √s = 13 TeV with the ATLAS detector

The Standard Model (SM) of particle physics provides an incredibly accurate description of a wide range of physics phenomena. However the scientific community is well aware that the SM is not the ultimate, complete theory we are hoping for. A key role in the quest for new physics is covered by searches for new phenomena, such as the one described in this thesis. This search has been performed using events selected in 139 inverse fb of √s = 13 TeV of proton-proton collision data provided by the Large Hadron Collider (LHC) and collected by the ATLAS experiment. The search aims to identify new physics particles, referred-to as dark-photons, that are neutral, have a long lifetime and travel macroscopic distances inside the detector. In particular, the analysis targets the production of dark-photons arising in the decay of a Higgs boson, produced via gluon-gluon fusion or in association with a W boson, and exploits simplified models where the existence of a hidden sector, weakly coupled to the SM, is assumed. The search aims to identify light neutral particles that decay outside the innermost region of the detector, involving the production of collimated bundles of SM fermions in the final state. This experimental signature, referred to as Dark Photon Jets (DPJs), requires the use of dedicated triggers, custom object reconstruction algorithms, and sophisticated background rejection techniques involving deep-learning-based classifiers. The observed event yields are consistent with the expected background and are used to set constraints on benchmark models predicting the existence of long-lived particles. A Higgs boson branching fraction above 1% is excluded at 95% confidence level for a Higgs boson decaying into two dark-photons, for dark-photon mean proper decay lengths between 10 mm and 250 mm, and dark-photons with masses between 0.4 GeV and 2 GeV. Finally, this thesis describes also a study that has been carried out in the context of the upgrade of the tracking system of the ATLAS experiment. This study involves the validation of a new framework for the reconstruction of testbeam data.