Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Physics and Astronomy


High Energy Physics

First Advisor

Chunhui Chen


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

W boson.

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.

Copyright Owner

Michael Werner



File Format


File Size

120 pages

Included in

Physics Commons