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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

Published version
Peer-reviewed

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Authors

Aad, G 
Abbott, B 
Abbott, DC 
Abed Abud, A 
Abeling, K 

Abstract

jats:titleAjats:scbstract</jats:sc> </jats:title>jats:pA simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in jats:italict</jats:italic>-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 fbjats:supjats:italic−</jats:italic>1</jats:sup>, 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 jats:italicb</jats:italic>-tagged. Stringent selection requirements are applied to discriminate jats:italict</jats:italic>-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 jats:inline-formulajats:alternativesjats:tex-math$$ {P}{x^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:mix</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = 0jats:italic.</jats:italic>01 jats:italic±</jats:italic> 0jats:italic.</jats:italic>18, jats:inline-formulajats:alternativesjats:tex-math$$ {P}{y^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:miy</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = jats:italic−</jats:italic>0jats:italic.</jats:italic>029 jats:italic±</jats:italic> 0jats:italic.</jats:italic>027, jats:inline-formulajats:alternativesjats:tex-math$$ {P}{z^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:miz</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = 0jats:italic.</jats:italic>91 jats:italic±</jats:italic> 0jats:italic.</jats:italic>10 and for the top-antiquark event sample they are jats:inline-formulajats:alternativesjats:tex-math$$ {P}{x^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:mix</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = jats:italic−</jats:italic>0jats:italic.</jats:italic>02 jats:italic±</jats:italic> 0jats:italic.</jats:italic>20, jats:inline-formulajats:alternativesjats:tex-math$$ {P}{y^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:miy</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = jats:italic−</jats:italic>0jats:italic.</jats:italic>007 jats:italic±</jats:italic> 0jats:italic.</jats:italic>051, jats:inline-formulajats:alternativesjats:tex-math$$ {P}{z^{\prime }} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:msub mml:miP</mml:mi> mml:msup mml:miz</mml:mi> mml:mo′</mml:mo> </mml:msup> </mml:msub> </mml:math></jats:alternatives></jats:inline-formula> = 0jats:italic.</jats:italic>79 jats:italic±</jats:italic> 0jats:italic.</jats:italic>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 jats:inline-formulajats:alternativesjats:tex-math$$ \mathcal{O} $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> mml:miO</mml:mi> </mml:math></jats:alternatives></jats:inline-formula>jats:subjats:italictW</jats:italic></jats:sub> operator in the framework of an effective field theory. The obtained bounds are jats:italicC</jats:italic>jats:subjats:italictW</jats:italic></jats:sub> ∈ [jats:italic−</jats:italic>0jats:italic.</jats:italic>9jats:italic,</jats:italic> 1jats:italic.</jats:italic>4] and jats:italicC</jats:italic>jats:subjats:italicitW</jats:italic></jats:sub> ∈ [jats:italic−</jats:italic>0jats:italic.</jats:italic>8jats:italic,</jats:italic> 0jats:italic.</jats:italic>2], both at 95% confidence level.</jats:p>

Description

Keywords

Hadron-Hadron Scattering, Top Physics

Journal Title

Journal of High Energy Physics

Conference Name

Journal ISSN

1029-8479
1029-8479

Volume Title

Publisher

Springer Science and Business Media LLC