Date of Award
Doctor of Philosophy
Physics and Astronomy
High Energy Physics
The standard model fails to fully explain several observations about the universe, one of the
prominent ones is the existence of dark matter. Many different models have been proposed for
dark matter candidates, but it is possible that the dark matter itself will remain elusive. Dark
sector models contain both dark matter and a new force which acts upon it. The force-carrying
boson of this dark sector may be more easily detected than dark matter itself. These models
contain a number of free parameters, some combinations of which result in the dark sector
particle being long-lived but unstable, resulting in a unique experimental signature.
A search is presented for the production of an uncharged long-lived particle in association
with a standard model Z boson. The search is based on 36.01 fb −1 of proton-proton collision
data at s = 13 TeV recorded in 2015-2016 with the ATLAS detector at the Large Hadron
Collider. Data are analyzed in a dilepton + jets region which is characterized by two high
transverse momentum electrons or muons and at least one jet with a very large fraction of its
energy deposited in the hadronic calorimeter. Background rates for such jets are estimated
using a single lepton + jets region consistent with a lepton from the decay of a standard model
No significant excess is observed and 95% confidence level upper limits are derived on the
cross section times the branching ratio as a function of the particle’s proper lifetime.
Werner, Michael, "Search for new long-lived hadronically-decaying particles produced in association with a Z boson with the ATLAS detector at the LHC" (2018). Graduate Theses and Dissertations. 16691.