Search for displaced dark photons in vector boson fusion Higgs events with the ATLAS detector

Abstract

A range of promising models predicting new physics beyond the Standard Model have been proposed to explain the nature of Dark Matter through the introduction of a new Dark Sector. In these models, the Dark Sector interacts very weakly with the Standard Model, with the interaction mediated only through selected portals such as the dark photon, a neutral massive particle that can mix kinetically with the Standard Model photon.

This thesis presents the search for a long-lived dark photon produced via Higgs boson decays to the Dark Sector with the ATLAS detector at the Large Hadron Collider. Due to the very weak interaction between the Dark Sector and the Standard Model, the dark photons travel a macroscopic distance before decaying back to Standard Model fermions in the form of collimated electron, hadron or muon pairs. These decay products are reconstructed from energy deposits in the ATLAS detector hadronic calorimeter and muon spectrometer as jet-like structures known as displaced Dark Photon Jets. This analysis searches for dark photons produced from Higgs boson decays, focusing on vector boson fusion Higgs production, in which sensitivity is gained by exploiting the specific initial state topology of two highly energetic forward jets.

This analysis uses the full 139 fb-1 of the ATLAS Run 2 dataset collected during proton-proton collisions √s=13 TeV. The number of observed events are found to be consistent with the background expectation, and upper limits are placed on the branching ratio of the Higgs boson to two dark photons as a function of the proper decay length of the dark photon, providing stringent constraints in a large region of m(γd) vs. ɛ parameter space. This search is combined with the previous ATLAS searches in the gluon-gluon fusion and WH production modes, excluding a branching fraction above 10% at 95% CL for a 125 GeV Higgs boson decaying into two dark photons for a dark photon mean proper decay length between 173 mm and 1296 mm and a mass of 10 GeV.

Work on beam-test analysis of pixel modules for the High-Luminosity upgrade of the ATLAS Inner Tracker is also presented. In particular, a comparison of the latest hybrid silicon pixel chips under consideration for the new ATLAS Inner Tracker is made and an overall chip efficiency reported for each device under test.