Study of $t\bar{t}$ threshold effects in $e\mu$ differential distributions measured in $\sqrt{s}=13$\,TeV $pp$ collisions with the ATLAS detector

TL;DR

This study analyzes ATLAS $e\mu$ distributions at 13 TeV to investigate $tar{t}$ threshold effects, finding evidence for quasi-bound states with over three sigma significance.

Contribution

It extends previous measurements to include sensitivity to quasi-bound $tar{t}$ states near threshold, comparing data with models with and without these states.

Findings

Data favor models with quasi-bound-state formation.

Evidence for quasi-bound states exceeds three standard deviations.

Measured cross-section aligns with threshold region predictions.

Abstract

The recent ATLAS measurement of the normalised $e\mu$ dilepton invariant mass and azimuthal angle distributions in $\sqrt{s}=13$\,TeV $pp$ collisions at the Large Hadron Collider is extended to study the sensitivity to the formation of quasi-bound states near the $t\bar{t}$ threshold. The measured differential distributions are compared with $t\bar{t}$ models incorporating perturbative QCD predictions for the hard process, with or without the addition of colour-singlet quasi-bound states. The data are better described by the predictions incorporating quasi-bound-state formation. Fits to the dilepton invariant mass distribution show evidence for the latter process with an observed significance exceeding three standard deviations, and a measured cross-section in agreement with dedicated studies of the threshold region.