Beam density functions in van der Meer scans and leptonically decaying off-shell Z bosons in association with two hadronic jets

  • Yaadav Bhauruth

Student thesis: Phd


At present, high energy particle colliders, like the Large Hadron Collider (LHC), are used to investigate the nature and interactions of the elementary particles that make up matter. The precise determination of an interaction cross section at the LHC relies heavily on a robust determination of the delivered luminosity over a given time period. Two analyses are documented in this thesis. The first analysis comprised an investigation of the LHC beam density functions during van der Meer (vdM) scans. vdM scans are used by the ATLAS experiment to calibrate the visible interaction rates measured by a number of luminosity monitoring detectors to the absolute luminosity scale. The vdM scan methodology assumes that the beam density functions factorise in the horizontal and vertical directions. An investigation was carried out to test this assumption by using experimentally determined luminous region parameters, together with data from luminosity monitors in ATLAS, to constrain beam models defined as the sum of two or three Gaussian functions. The impact of the vertex-position resolution on this analysis was also investigated. The vdM scans analysed were performed during the 2012 proton-proton and 2013 proton-lead and lead-proton data-taking runs of the LHC. The second analysis reported is a feasibility study centred on the extraction of the cross section of the electroweak production of a leptonically decaying high mass off-shell Z boson in association with two hadronic jets (Zjj). The study used a dataset corresponding to $32.9 \,\mathrm{fb^{-1}}$ collected by ATLAS during the proton-proton data-taking run in 2016. The analysis was carried out in two channels, comprising Z boson decays to either a pair of muons or to a pair of electrons. The analysis first measured the on-shell electroweak Zjj cross section in the muon and electron channels using Zjj events in which the dilepton invariant mass was close to the rest mass of the Z boson. The analysis techniques were then extended to Zjj events in which the invariant mass of the dilepton pair was higher than the Z boson rest mass. The signal significance of high mass electroweak Zjj production was found to be around five sigma in the combined muon and electron channels for the dataset analysed. The study therefore demonstrated how, and to what degree of precision, the high mass electroweak Zjj production cross section could be measured using the dataset analysed and future LHC datasets.
Date of Award31 Dec 2018
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorTerence Wyatt (Supervisor) & Christian Schwanenberger (Supervisor)

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