Search for dark Higgs boson production in pp events at √s = 13 TeV with the ATLAS detector at the LHC

Abstract

This thesis presents the design of a signal region, as well as two control regions to constrain the tt ̄ background, in the search for the theorized dark sector process qq ̄ → Z′∗ → Z′s with s → W W → qq ̄lν and Z′ → χχ(invisible). In this process, dark matter is produced as a new heavy vector boson, Z′, together with a scalar “dark Higgs” boson, s, that decays to a pair of W bosons with a semileptonic final state. In this model, a new Higgs mechanism associated with s generates the masses of the dark sector particles, and the mixing of s with the Standard Model Higgs boson allows it to decay to Standard Model particles and be observable over a large range of model parameter space at the Large Hadron Collider. This search uses 139 fb−1 of proton-proton collision data collected by the ATLAS experiment at a center-of-mass energy of 13 TeV. Using the designed signal and control regions, in this search we will be able to exclude Z′ masses up to a range between approximately 1200 to 2200 GeV, and s masses from approximately 150 GeV up to a range between approximately 290 to 360 GeV.