Measurements of top quark spin observables in $t\bar{t}$ events using dilepton final states in $\sqrt{s} = 8$ TeV $pp$ collisions with the ATLAS detector

TL;DR

This paper reports precise measurements of top quark spin observables in $t\bar{t}$ events from 8 TeV proton-proton collisions, using the ATLAS detector, comparing results to Standard Model predictions with no significant deviations found.

Contribution

First measurements of 10 top quark spin observables in dilepton final states at 8 TeV, corrected to parton and stable-particle levels, and compared to Standard Model predictions.

Findings

All measured observables agree with Standard Model predictions.

Ten observables are measured for the first time.

No significant deviation from the Standard Model is observed.

Abstract

Measurements of top quark spin observables in $t\bar{t}$ events are presented based on 20.2 fb$^{-1}$ of $\sqrt{s} = 8$ TeV proton-proton collisions recorded with the ATLAS detector at the LHC. The analysis is performed in the dilepton final state, characterised by the presence of two isolated leptons (electrons or muons). There are 15 observables, each sensitive to a different coefficient of the spin density matrix of $t\bar{t}$ production, which are measured independently. Ten of these observables are measured for the first time. All of them are corrected for detector resolution and acceptance effects back to the parton and stable-particle levels. The measured values of the observables at parton level are compared to Standard Model predictions at next-to-leading order in QCD. The corrected distributions at stable-particle level are presented and the means of the distributions are compared to Monte Carlo predictions. No significant deviation from the Standard Model is observed for any observable.