Search for new charged bosons and dark matter in final states with one lepton and missing transverse energy with the ATLAS detector at the LHC

The Standard Model (SM), the current theory of elementary particles and interactions, has been extremely successful in predicting and describing experimental results. The prediction of the electron's anomalous magnetic moment served as an early triumph of quantum electrodynamics, and one success after another has followed, including the discovery of the weak interaction gauge bosons $W^\pm$ and $Z^0$, and more recently the discovery of the Higgs boson at CERN's Large Hadron Collider (LHC) in 2012. In spite of the success of the theory, though, there are phenomena which it does not explain, such as the dark matter and dark energy making up most of the universe. Extensions of the SM aiming to address its shortcomings typically predict observable deviations from the theory. Although theories predicting significant deviations from the SM in the energy regime so far explored can be immediately excluded, theories that predict deviations at higher, unexplored energies are still viable. Therefore, exploring physics at such energies is crucial in order to improve our understanding of nature at the most fundamental level. Currently, experimental data at the energy frontier are provided by the LHC experiments. In this thesis, we present a search for new physics in final states with one lepton and missing transverse energy using data from the ATLAS detector. No significant deviations from SM predictions are observed in the transverse mass distribution. The search is interpreted in terms of the production of hypothetical heavy, charged bosons, and also in terms of the production of dark matter particles in association with a leptonically decaying $W$ boson. Limits on the cross sections and relevant mass scales of these processes are presented. In the case of new charged boson signal, the Sequential Standard Model (SSM) and the $W^*$ reference model are considered. The SSM is a reference model widely used to represent new gauge bosons, of which the charged ones are usually denoted $W^\prime$, related to hypothetical symmetries of nature and the associated interactions. The $W^*$ boson is a common occurence in theories addressing the unreasonably large radiative corrections to the Higgs boson mass, and differs significantly from the new gauge bosons in its interactions with the SM fermions. We find that new charged bosons are excluded at $95\%$ CL for masses up to $3.28\,\mathrm{TeV}$ in the case of the SSM $W^\prime$ boson and $3.21\,\mathrm{TeV}$ in the case of the $W^*$ reference model. As a search for dark matter particles, the analysis is found to be competitive with ATLAS searches in other final states, in particular in the case of constructive interference in the so-called D5 effective field theory (EFT). Limits at $90\%$ CL on the suppression scale in the effective coupling between dark matter particles and quarks are presented. For low dark matter particle masses, where collider searches are particularly sensitive, the limits extend to $1.4\,\mathrm{TeV}$ in the D9 EFT and $1.2\,\mathrm{TeV}$ in the D5 EFT in the case of constructive interference.