Observation of T(t)over-Bar Production in Pb Plus Pb Collisions at √snn=5.02 TeV with the ATLAS Detector

Abstract

Top-quark pair production is observed in lead-lead (Pb+Pb) collisions at root s(NN)=5.02 TeV at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of 1.9 nb(-1). Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross section is sigma(t (t) over bar) =3.6 (+1.0)(-0.9)(stat)(-0.5) (+0.8)(syst) mu b, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the preequilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early Universe.