Evidence for Top-quark Production in Nucleus–Nucleus Collisions with the CMS Experiment

Evidence for the production of top quarks in heavy-ion collisions is reported in a data sample of lead–lead collisions recorded in 2018 by the CMS experiment at a nucleon–nucleon center-of-mass energy of \(\sqrt {s_{_{NN}}} = 5.02\) TeV, corresponding to an integrated luminosity of \(1.7\pm 0.1\) nb\(^{-1}\). Top quark pair (\(t\bar {t\,}\)) production is measured in events with two opposite-sign high-\(p_{\mathrm {T}}\) isolated leptons (\(\ell ^\pm \ell ^\mp =\,e^{+} e^{-},\,\mu ^{+} \mu ^{-}\), and \(e^{\pm } \mu ^{\mp }\)). We test the sensitivity to the \(t\bar {t}\) signal process by requiring or not the additional presence of \(b\)-tagged jets, and hence demonstrate the feasibility to identify top quark decay products irrespective of interacting with the medium (bottom quarks) or not (leptonically decaying \(W\) bosons). To that end, the inclusive cross section (\(\sigma _{t\bar {t}}\)) is derived from likelihood fits to a multivariate discriminator, which includes different leptonic kinematic variables with and without the \(b\)-tagged jet multiplicity information. The observed (expected) significance of the \(t\bar {t}\) signal against the background-only hypothesis is 4.0 (5.8) and 3.8 (4.8) standard deviations, respectively, for the fits with and without the \(b\)-jet multiplicity input. After event reconstruction and background subtraction, the extracted cross sections are \(\sigma _{t\bar {t}} = 2.03 ^{+0.71}_{-0.64}\) and \(2.54 ^{+0.84}_{-0.74}~\mu \)b, respectively, which are lower than, but still compatible with, the expectations from scaled proton–proton data as well as from perturbative quantum chromodynamics predictions. This measurement constitutes the first crucial step towards using the top quark as a novel tool for probing strongly interacting matter.