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

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 ${\sigma }_{t\bar{t}}=3.6{}_{-0.9}^{+1.0}(stat.){}_{-0.5}^{+0.8}(syst.)\mu \mathrm{b}$, 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.

32 pages in total, author list starting page 15, 2 figures, 0 tables, submitted to Phys. Rev. Lett. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2022-10/