# Top-quark properties at ATLAS

## TL;DR

This paper presents recent ATLAS measurements of top-quark properties, including mass and spin correlations, using proton-proton collision data at 8 TeV and 13 TeV, with results that refine the top-quark mass and reveal higher-than-expected spin correlations.

## Contribution

It provides new measurements of the top-quark mass in different schemes and detailed analysis of spin correlations, comparing experimental data with theoretical predictions.

## Key findings

- Top-quark mass measured as 171.1 GeV (pole scheme)
- Spin correlations observed to be higher than some generator predictions
- Results agree with fixed-order theoretical calculations

## Abstract

Properties of the top-quark are presented, with emphasis on the most recent ATLAS measurements of the mass and $t\bar{t}$ spin correlations, obtained with proton-proton collision data collected at the Large Hadron Collider. Normalised differential distributions are used in both cases. For the extraction of the top-quark mass, $t\bar{t}+1$-jet single-lepton events are selected from the 20.2 fb$^{-1}$ 8 TeV dataset, and the unfolded distribution at parton level is compared with theoretical predictions to obtain $m_t^{\text{pole}} = 171.1^{+1.2}_{-1.1}$ GeV in the pole-mass scheme and $m_t(m_t) = 162.9^{+2.4}_{-1.6}$ GeV in the running-mass scheme. For the measurement of spin correlations in $t\bar{t}$ production, dilepton events are selected using 36.1 fb$^{-1}$ 13 TeV data. The azimuthal opening angle between the two leptons is measured inclusively and as a function of the invariant mass of the $t\bar{t}$ system. The observed degree of spin correlation is significantly higher than predicted by the generators used, but agrees well with the prediction of one of the fixed-order calculations.