  Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting
quantum chromodynamics (QCD) matter, are routinely produced in heavy nuclei
high-energy collisions. Although the experimental signatures marked a paradigm
shift away from expectations of a weakly coupled QGP, a challenge remains as to
how the locally deconfined state with a lifetime of a few fm can be resolved.
The only colored particle that decays mostly within the QGP is the top quark.
Here we demonstrate, for the first time, that top quark decay products are
identified, irrespective of whether interacting with the medium (bottom quarks)
or not (leptonically decaying W bosons). Using $1.7 \pm 0.1\,\mathrm{nb^{-1}}$
of lead-lead ($A = 208$) collision data recorded by the CMS experiment at a
nucleon-nucleon center-of-mass energy of 5.02 TeV, we report evidence of top
quark pair ($\mathrm{t\bar{t}}$) production. Dilepton final states are
selected, and the cross section ($\sigma_\mathrm{t\bar{t}}$) is measured from a
likelihood fit to a multivariate discriminator using lepton kinematic
variables. The $\sigma_\mathrm{t\bar{t}}$ measurement is additionally performed
considering the jets originating from the hadronization of bottom quarks, which
improve the sensitivity to the $\mathrm{t\bar{t}}$ signal process. After
background subtraction and analysis corrections, the measured
$\sigma_\mathrm{t\bar{t}}$ is $2.02 \pm 0.69\,\rm{(tot)}$ and $2.56\pm
0.82\,\rm{(tot)}\,\mu\mathrm{b}$ in the two cases, respectively, consistent
with predictions from perturbative QCD.
