Search for New Neutral High-Mass Resonances Decaying into Muon Pairs with the ATLAS Detector

Abstract

The question of physics beyond the Standard Model remains as crucial as it was before the discovery of a Higgs boson at the Large Hadron Collider, as the theoretical and experimental shortcomings of the Standard Model remain unresolved. Indeed, theoretical problems such as the hierarchy of energy scales, the Higgs mass fine-tuning and the large number of postulated parameters need to be addressed, while the experimental observations of dark matter, dark energy and neutrino masses are not explained by the Standard Model. Many hypotheses addressing these issues predict the existence of new neutral high-mass resonances decaying into muon pairs. This dissertation documents a search for this process using 25.5 inverse femtobarns of proton-proton collision data collected by the ATLAS experiment in Run‑I of the Large Hadron Collider. After evaluating the performance of the detector for reconstructing muons at very high momentum, the event yields observed as a function of the invariant mass of muon pairs are compared with expected values from Standard Model processes. The observed yields are found to be in good agreement with Standard Model predictions, and no significant excess of events is found. New gauge bosons with couplings to fermions equal to these of the Standard Model Z boson and with masses lower than 2.53 TeV are therefore excluded at 95% confidence level. A statistical combination with the results of the search for the same particle decaying into electron pairs yields a lower mass limit of 2.90 TeV at 95% confidence level. Limits are also placed in the context of two classes of models inspired by Grand Unification Theories: gauge theories with the $E_6$ symmetry group, as well as Minimal Z' Models.