Measurement of the polarisation of $W$ bosons produced in top-quark decays using dilepton events at $\sqrt{s}=13$ TeV with the ATLAS experiment

Abstract

A measurement of the polarisation of $W$ bosons produced in top-quark decays is presented, using proton-proton collision data at a centre-of-mass energy of $\sqrt{s}=13$ TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139 fb${}^{-1}$. The measurement is performed selecting $t\bar{t}$ events decaying into final states with two charged leptons (electrons or muons) and at least two $b$-tagged jets. The polarisation is extracted from the differential cross-section distribution of the $\cos {\theta }^{\ast }$ variable, where ${\theta }^{\ast }$ is the angle between the momentum direction of the charged lepton from the $W$ boson decay and the reversed momentum direction of the $b$-quark from the top-quark decay, both calculated in the $W$ boson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the $\cos {\theta }^{\ast }$ angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be $f_0=0.684\pm 0.005(stat.)\pm 0.014(syst.)$, $f_{\mathrm{L}}=0.318\pm 0.003(stat.)\pm 0.008(syst.)$ and $f_{\mathrm{R}}=-0.002\pm 0.002(stat.)\pm 0.014(syst.)$, in agreement with the Standard Model prediction.

41 pages in total, author list starting page 23, 3 figures, 3 tables, published in PLB. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-28/

References in corpus (11)

• The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations
• An Introduction to PYTHIA 8.2
• Herwig++ Physics and Manual
• Parton distributions in the LHC era: MMHT 2014 PDFs
• A parton shower algorithm based on Catani-Seymour dipole factorisation
• Two-loop soft anomalous dimensions for single top quark associated production with a W- or H-
• Muon reconstruction and identification efficiency in ATLAS using the full Run 2 $pp$ collision data set at $\sqrt{s}=13$ TeV
• ATLAS data quality operations and performance for 2015-2018 data-taking
• Performance of the ATLAS muon triggers in Run 2
• Effective-Field-Theory Approach to Top-Quark Production and Decay
• O new physics, where art thou? A global search in the top sector

Cited by in corpus (7)

• Polarized and unpolarized off-shell $H^{\ast }\to ZZ\to 4\ell$ decay above the $2m_Z$ threshold
• $CP$ violation in the $HZZ$ vertex and left-right asymmetries
• Staying on Top of SMEFT-Likelihood Analyses
• Non-diagonal contributions to $Z\gamma V^{\ast }$ vertex, polarizations and bounds on $Z\overline{t}q$ couplings
• Top-Quark Decay at Next-to-Next-to-Next-to-Leading Order in QCD
• W-boson helicity fractions in top decay as probes of dimension-6 and dimension-8 SMEFT operators
• $CP$-odd form factor in the $HWW$ vertex