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

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.