Observation of top-quark pair production in lead-lead collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV with the ATLAS detector

TL;DR

This paper reports the first observation of top-quark pair production in lead-lead collisions at the LHC, confirming the presence of all quark flavors in the quark-gluon plasma at high energy densities.

Contribution

It provides the first measurement of top-quark pair production in heavy-ion collisions at 5.02 TeV with the ATLAS detector, demonstrating the process's significance in such environments.

Findings

Top-quark pair production observed with 5.0 sigma significance.

Measured cross-section of 3.6 μb with 31% total uncertainty.

Results are consistent with theoretical predictions.

Abstract

Top-quark pair production is observed in lead-lead (Pb+Pb) collisions at $\sqrt{s_\mathrm{NN}}=5.02$ 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 $\sigma_{t\bar{t}} = 3.6\;^{+1.0}_{-0.9}\;\mathrm{(stat.)}\;^{+0.8}_{-0.5}\;\mathrm{(syst.)} ~\mathrm{\mu b}$, 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 pre-equilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early universe.