Measurement of top quark polarisation in t-channel single top quark production

TL;DR

This paper reports the first measurement of top quark polarization in t-channel single top production at 8 TeV, using angular distributions and data fitting to compare with Standard Model predictions.

Contribution

It introduces a novel measurement of top quark spin asymmetry in t-channel single top events, employing a differential cross section analysis with data-driven background estimation.

Findings

Measured top quark spin asymmetry of 0.26 ± 0.03 (stat) ± 0.10 (syst)

Result compatible with Standard Model prediction of 0.44

Demonstrated the effectiveness of angular observables in polarization studies

Abstract

A first measurement of the top quark spin asymmetry, sensitive to the top quark polarisation, in $t$-channel single top quark production is presented. It is based on a sample of pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.7 $\mathrm{fb^{-1}}$. A high-purity sample of $t$-channel single top quark events with an isolated muon is selected. Signal and background components are estimated using a fit to data. A differential cross section measurement, corrected for detector effects, of an angular observable sensitive to the top quark polarisation is performed. The differential distribution is used to extract a top quark spin asymmetry of $0.26 \pm 0.03 \textrm{(stat)} \pm 0.10 \textrm{(syst)}$, which is compatible with a p-value of $4.6\%$ with the standard model prediction of 0.44.