Measurement of the $W$ boson polarisation in $t\bar{t}$ events from $pp$ collisions at $\sqrt{s}$ = 8 TeV in the lepton+jets channel with ATLAS

TL;DR

This paper measures the polarization states of W bosons from top quark decays in proton-proton collisions at 8 TeV, providing the most precise results to date and setting limits on anomalous Wtb couplings.

Contribution

It presents the most precise measurement of W boson polarization fractions in top decays and introduces a method using b-jet tagging and lepton analysis for improved accuracy.

Findings

Measured polarization fractions: F0=0.709, FL=0.299, FR=-0.008.

Achieved the most precise W boson polarization measurements to date.

Set limits on anomalous Wtb couplings.

Abstract

This paper presents a measurement of the polarisation of $W$ bosons from $t\bar{t}$ decays, reconstructed in events with one high-$p_{\mbox{T}}$ lepton and at least four jets. Data from $pp$ collisions at the LHC were collected at $\sqrt{s}$ = 8 TeV and correspond to an integrated luminosity of 20.2 fb$^{-1}$. The angle $\theta^{*}$ between the $b$-quark from the top quark decay and a direct $W$ boson decay product in the $W$ boson rest frame is sensitive to the $W$ boson polarisation. Two different $W$ decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying $W$ boson, respectively. The most precise measurement of the $W$ boson polarisation via the distribution of $\cos{\theta^{*}}$ is obtained using the leptonic analyser and events in which at least two of the jets are tagged as $b$-quark jets. The fitted fractions of longitudinal, left- and right-handed polarisation states are $F_{\mathrm{0}}= 0.709 \pm {0.019}$, $F_{\mathrm{L}}= 0.299 \pm {0.015}$ and $F_{\mathrm{R}}= -0.008 \pm {0.014}$, and are the most precisely measured $W$ boson polarisation fractions to date. Limits on anomalous couplings of the $Wtb$ vertex are set.