Aspects of hadronic final states and a charming search for new physics with the ATLAS detector

This thesis presents a collection of studies related to the reconstruction of hadronic final states and a search for new physics with the ATLAS detector. This includes an investigation on ambiguities between jet and object reconstruction using traditional distance-based approaches and in the paradigm of Global Particle Flow; an alternative parametrisation of the expected energy deposited by charged particles in the calorimeters in the context of particle flow reconstruction; and the potential consequences of increased pile-up conditions at the High-Luminosity Large Hadron Collider. The search presented pertains to the pair production of charm or top squarks, each decaying to a charm quark and the lightest supersymmetric particle ($\tilde{\chi}^0_1$) to produce a final state containing missing transverse momentum and jets originating from charm quarks. This was conducted using the full Run 2 dataset of $\sqrt{s}=13$ TeV proton–proton collisions collected by the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$ . No significant excess of events over the expected Standard Model backgrounds is observed, and the pair production of the supersymmetric particles with masses up to 900 GeV is excluded in scenarios where the mass of the $\tilde{\chi}^0_1$ is below 50 GeV. The production of leptoquarks with masses up to 900 GeV is also excluded for the scenario where up-type leptoquarks decay into a charm quark and a neutrino.