A search for electroweakly-interacting dark matter with the ATLAS experiment

This thesis presents a calibration of the ATLAS calorimeter response to jets, a detector-corrected measurement of missing transverse momentum in association with jets, and the first model-independent beyond-the-Standard-Model interpretation of that detector-corrected measurement. The calibration of the jet energy resolution in the forward region of the ATLAS calorimeter is performed using dijet events in 36.1 fb−1 of $\sqrt{s}= 13$~TeV proton-proton collision data collected in 2015 and 2016. The dijet-balance methodology, used in this measurement, is described for events produced in the ATLAS detector forward region ($0.8 < |\eta| < 3.2$) relative to a central region ($|\eta| < 0.8$). Dark matter is motivated by cosmological experiments and remains one of the great mysteries of the Universe. Among the possible approaches to investigate this elusive type of matter are searches for missing transverse momentum at particle colliders. This thesis describes one such search. The ratio of the fiducial cross-sections of $p_{T}^{miss}$+jets to $l^+l^−$+jets is measured at $\sqrt{s}= 13$~TeV with 3.2 fb$^{-1}$ of proton-proton collision data collected by the ATLAS detector in 2015. This study is conducted using variables corrected for detector effects in two kinematic regions and four differential distributions. The consequence of using the ratio is the cancellation of a number of significant systematic uncertainties and detector effects. Expected and observed exclusion limits at the 95% confidence level are set on three dark matter models using these distributions and the correlations between bins of the measured distributions.