Measurement of the polarisation of single top quarks and antiquarks produced in the t-channel at √s = 13 TeV and bounds on the tWb dipole operator from the ATLAS experiment

ATLAS Collaboration, Paul Newman

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Abstract

A simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in t-channel single-top-quark production is presented. This analysis is based on data from proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb−1, collected with the ATLAS detector at the LHC. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being b-tagged. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from the background contributions. The top-quark and top-antiquark polarisation vectors are measured from the distributions of the direction cosines of the charged-lepton momentum in the top-quark rest frame. The three components of the polarisation vector for the selected top-quark event sample are Px′=0.01±0.18, Py′=−0.029±0.027, Pz′=0.91±0.10 and for the top-antiquark event sample they are Px′=−0.02±0.20, Py′=−0.007±0.051, Pz′=−0.79±0.16. Normalised differential cross-sections corrected to a fiducial region at the stable-particle level are presented as a function of the charged-lepton angles for top-quark and top-antiquark events inclusively and separately. These measurements are in agreement with Standard Model predictions. The angular differential cross-sections are used to derive bounds on the complex Wilson coefficient of the dimension-six OtW operator in the framework of an effective field theory. The obtained bounds are CtW∈ [−0.9,1.4] and CitW∈[−0.8,0.2], both at 95% confidence level.
Original languageEnglish
Article number40
Number of pages62
JournalJHEP
Volume2022
DOIs
Publication statusPublished - 9 Nov 2022

Bibliographical note

57 pages in total, author list starting page 41, 11 figures, 7 tables, submitted to JHEP. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-10/

Keywords

  • hep-ex
  • Hadron-Hadron scattering
  • Top physics

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