A search for invisibly decaying Higgs Bosons in the fully hadronic final state in the ttH and VH production modes

Abstract

For as much as we know about the universe, substantial portions of it remain tantalisingly out of our grasp. The discovery of the Higgs Boson was a landmark achievement of particle physics, but progress since has been slower than many may have hoped.

Dark matter may be the key to furthering our understanding of the universe. Although the evidence of its existence is overwhelming, we have no direct proof of its existence. However, it is natural to posit that dark matter, which might interact very weakly with standard model particles but experiences the effects of gravity, may have acquired its mass through the same mechanism that standard model particles did: the Higgs mechanism.

This thesis presents new upper limits on the branching fraction of the Higgs boson to particles undetected by the Large Hadron Collider. For the first time for the CMS experiment, it focuses on the rare and challenging production mode of the Higgs where two top quarks produce an invisibly decaying Higgs boson, ttH, as well as production in association with a vector boson, VH. Observed (expected) limits on the Higgs to invisible branching fraction of 0.50 (0.53), 0.69 (0.52), and 0.48 (0.39), for ttH, VH, and the combination, respectively, are achieved.

Simplified models of dark matter are also explored, where dark matter is formed through a topology almost identical to that of the Higgs boson production modes that are studied, with dark matter masses above 200GeV excluded for both scalar and pseudoscalar mediators at the 95% CL. Finally, upper limits on the branching fraction of the Higgs boson to various forms of dark matter are also set as a function of dark matter mass and nucleon coupling, in the context of state of the art dark matter direct detection experiments, excluding these models at the 90% CL for masses above roughly 10GeV and setting new limits for masses of a few GeV.

Date of Award	9 May 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Henning U Flaecher (Supervisor) & Jim Brooke (Supervisor)