Search for dark matter in events containing jets and missing transverse momentum using ratio measurements

• Main Author: Christodoulou, V.
• Published: University College London (University of London) 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.747102

This thesis presents a measurement of missing transverse momentum in association with jets at √s = 13 TeV with 3.2 fb−1 of proton-proton collisions at the LHC, collected in 2015 using the ATLAS detector. In the Standard Model of particle physics, this is the experimental signature of Z boson production in association with jets, where the Z boson decays to neutrinos, however it could also be the signature of dark matter production in association with jets. A ratio can be formed using events containing oppositely charged same-flavour lepton pairs in association with jets, consistent with the decay of a Z/γ* boson. Detector inefficiencies can be accounted for by defining a correction factor and applying it to the ratio in order to recover the lost events. The detector-corrected ratio is measured differentially with respect to four variables in two jet phase spaces. The measured ratios are consistent with the Standard Model prediction and the data are used to place limits on the production of dark matter in proton-proton collisions at the LHC on three models, an effective field theory model, a simplified model with an axial-vector mediator, and the production of an invisibly decaying Higgs boson. In addition, the ATLAS trigger system has been upgraded for the 2015 data taking and a new jet reconstruction algorithm has been developed for the updated jet trigger software. Diagnostic algorithms have been developed to test the new software and its validation has been automated using a new jet trigger validation package. The new jet triggers perform as expected and their performance has been evaluated using the full 2015 dataset.