Search for Dark Matter in Invisible Higgs Boson Decays with the ATLAS Detector at the LHC

Abstract

The nature of dark matter is one of the biggest open questions in modern physics. While dark matter accounts for roughly 25 % of the energy density in the Universe, its particle nature remains unknown so far. In this thesis a search for dark matter at the Large Hadron Collider with the ATLAS experiment is presented using proton-proton collision data collected at a center-of-mass energy of 13 TeV. Furthermore, the absolute Monte Carlo based jet energy scale calibration of particle flow jets is presented.

The dark matter search targets vector boson fusion Higgs boson production, which is expected to be the most sensitive channel. For the full Run II data set, the background estimates with a focus on multijet processes, the event categorisation, limit setting as well as the interpretation of the result are presented.

In absence of a signal excess over the background-only hypothesis, an upper limit is set on the invisible Higgs boson branching fraction. The observed (expected) 95 % CL limit is 0.145 (0.103), which is the best limit ever achieved so far for invisible Higgs boson decays. The result is interpreted in terms of Higgs portal models to set upper limits on the WIMP-nucleon cross section. Moreover, also limits on potential other scalar mediators than the Higgs boson of the Standard Model are set.