The search for new high-mass resonances in the dilepton final state using the full Run-2 ATLAS dataset of 13 TeV proton-proton collisions

Synopsis

There are good reasons to anticipate that the Standard Model (SM), successful as it is, will nonetheless give way to a more complete description of particle interactions. New physics scenarios beyond the SM often address its key shortcomings through extended gauge sym-metries with the implied existence of additional massive bosons. The dilepton final state is a particularly clean experimental channel to explore for new heavy resonances, as was the case for SM discoveries of the 1970’s and 80’s. This thesis presents a search for high-mass dilepton resonances using the full dataset collected by the ATLAS detector during Run 2 of the LHC. The analysis targets the invariant mass spectrum of an inclusive selection of dielectron and dimuon events with some of the highest energies reported by ATLAS. Prompted by the unprecedented statistical precision of the dataset, the results are based on a new approach for estimating the SM backgrounds using a functional form fit. While no significant excess is observed, the resulting upper limits on the cross section for production of a dilepton resonance are used to constrain the parameter space of models that predict new bosons coupled to leptons. For example, the existence of a sequential standard model Z0 boson is excluded for pole masses below 5.1 TeV. Updated bounds are also derived for the mass and leptonic couplings of a hypothetical spin-1 dark matter mediator.

Keywords: particle physics, LHC, high-mass resonance, grand unification theory, dark matter