  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 $\sqrt{s_{_{\mathrm{NN}}}} =$
5.02 TeV, corresponding to an integrated luminosity of
$1.7\pm0.1\,\mathrm{nb}^{-1}$. Top quark pair ($\mathrm{t\bar{t}}$) production
is measured in events with two opposite-sign high-$p_\mathrm{T}$ isolated
leptons ($\ell^\pm\ell^\mp =\,\mathrm{e}^{+} \mathrm{e}^{-},\,\mu^{+}
\mu^{-},\,\mathrm{and}\,\mathrm{e}^{\pm} \mu^{\mp}$). We test the sensitivity
to the $\mathrm{t\bar{t}}$ 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 ($\sigma_\mathrm{t\bar{t}}$) 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 $\mathrm{t\bar{t}}$ 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 $\sigma_\mathrm{t\bar{t}} = 2.03 ^{+0.71}_{-0.64}$ and $2.54
^{+0.84}_{-0.74}\,\mu\mathrm{b}$, 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.
