  Ultrarelativistic heavy ion collisions recreate in the laboratory the
thermodynamical conditions prevailing in the early universe up to 10$^{-6}$
seconds, thereby allowing the study of the quark-gluon plasma (QGP), a state of
quantum chromodynamics (QCD) matter with deconfined partons. The top quark, the
heaviest elementary particle known, is accessible in nucleus-nucleus collisions
at the CERN LHC, and constitutes a novel probe of the QGP. Here, we report the
first-ever evidence for the production of top quarks in nucleus-nucleus
collisions, using lead-lead collision data at a nucleon-nucleon centre-of-mass
energy of 5.02 TeV recorded by the CMS experiment. Two methods are used to
measure the cross section for top quark pair production
($\sigma_\mathrm{t\bar{t}}$) via the decay into charged leptons (electrons or
muons) and bottom quarks. One method relies on the leptonic information alone,
and the second one exploits, in addition, the presence of bottom quarks. The
measured cross sections, $\sigma_\mathrm{t\bar{t}} = $ 2.54 $^{+0.84}_{-0.74}$
and 2.03 $^{+0.71}_{-0.64}$ $\mu$b, respectively, are compatible with
expectations from scaled proton-proton data and QCD predictions.
