Leaving No Matter Unturned -- Analysing existing LHC measurements and events with jets and missing transverse energy measured by the ATLAS Experiment insearch of Dark Matter

Abstract

Various astrophysical observations point towards an as-of-yet unexplained, mainly gravitationally interacting type of matter. If this matter, called Dark Matter, is an elementary particle, it could be produced in particle collisions at the Large Hadron Collider. Given its weak interaction with ordinary matter, however, it would not be directly observable with the general-purpose detectors at the Large Hadron Collider. Its production would therefore manifest as events in which detector-visible objects recoil against the detector-invisible Dark Matter, giving rise to missing transverse energy. This thesis focuses on final states in which these visible objects are jets. A measurement of the final state of large missing transverse energy and at least one jet in 139 fb$^{-1}$ of proton-proton collisions at 13 TeV recorded with the ATLAS detector at the Large Hadron Collider is performed in this thesis. Good agreement between measured data and Standard-Model prediction is found in a statistical fit, corresponding to a reduced chi-square of 1.37. The measurement is corrected for detector effects to facilitate later reinterpretation. Measurements prepared in such a way can, for example, be exploited by the CONTUR toolkit to set constraints on new theories. Both, the results of the measurement and the CONTUR toolkit making use of existing measurements at the Large Hadron Collider, are employed to set exclusion limits on a model able to explain Dark Matter, the two-Higgs-doublet model with a pseudoscalar mediator to Dark Matter. At $\tan\beta=1$, masses of the pseudoscalar $A$ up to 425 GeV and larger than 1600 GeV are excluded at 95 % confidence level. At $m_H\equiv m_A\equiv m_{H^\pm}=$ 600 GeV, masses of the pseudoscalar $a$ up to 550 GeV and values of $\tan\beta$ up to 1.5 as well as larger than 20 are excluded at 95 % confidence level.