Mead, James 
ORCID: 0000-0003-0875-2533
(2021)
A measurement of top quark production
at √s = 13TeV with LHCb data.
PhD thesis, University of Liverpool.
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Abstract
The LHCb experiment provides unique detector coverage of the highest energy proton-proton interactions ever produced. Designed to study b-& c-hadron physics at the LHC, the detector is fully instrumented in the forward region, 2.0 < η < 4.5, with excellent tracking, vertex resolution and particle identification. The increased centre of mass energy in RunII gives rise to 3-fold increased inclusive top cross-section over RunI at the LHC, corresponding to a 10-fold increase within the LHCb acceptance. The top quark is the heaviest fundamental particle and is expected to play a special role in new physics scenarios. Higher order interference mechanisms, sensitive to physics beyond the reach of current colliders, result in a charge asymmetry in the relative angular distributions of tt pairs. The LHCb acceptance offers greater sensitivity to AttC due to reduced dilution from gluon-gluon fusion. Top quarks are identified through the presence of a high pT muon and b-jet in the final state. Forward production was first observed with RunI data at LHCb in this channel. Top pairs may be identified with an additional opposite-sign lepton or b-jet. The increase in available statistics with RunII, as well as improved signal to background ratio, enables differential measurements of heavy flavour tagged W+jet yields in muon pseudorapidity. New running conditions necessitated re-optimisation of jet input selection for reconstruction as well as renewal of heavy flavour tagging algorithms, achieved using deep learning techniques. Together these provide the first full RunII top cross-section in the μ +b channel at LHCb and the first top asymmetry measurement in the forward region. Each use data corresponding to an integrated luminosity of 5.4fb−1 (5% systematic). σ(t) [13 TeV] = 0.89 ± 0.06 (stat) ± 0.18 (syst) pb , σ(¯t) [13 TeV] = 0.66 ± 0.05 (stat) ± 0.17 (syst) pb . AtopC [13 TeV] = 0.14 ± 0.05 (stat) ± 0.05 (syst) . While the latter measurement was inconclusive with respect to the t ¯t asymmetry, the combined asymmetry was observed to 2.1σ above zero. Differential cross-sections were found to be within 1σ per bin of NLO standard model predictions. The precision of both sets of measurements are systematically limited, with largest contributions from heavy flavour yields (∼ 20%) and SV-tagging efficiencies (10%) despite at least partially cancelling in δAC.
| Item Type: | Thesis (PhD) |
|---|---|
| Divisions: | Faculty of Science and Engineering > School of Physical Sciences |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 14 Sep 2021 15:23 |
| Last Modified: | 18 Jan 2023 21:28 |
| DOI: | 10.17638/03136872 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3136872 |
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