  The first evidence for X(3872) production in relativistic heavy ion
collisions is reported. The X(3872) production is studied in lead-lead (PbPb)
collisions at a center-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV per
nucleon pair, using the decay chain X(3872) $\to$ J$/\psi\, \pi^+\pi^- \to$
$\mu^+\mu^-\pi^+\pi^-$. The data were recorded with the CMS detector in 2018
and correspond to an integrated luminosity of 1.7 nb$^{-1}$. The measurement is
performed in the rapidity and transverse momentum ranges $|y|$ $\lt$ 1.6 and 15
$\lt$ $p_\mathrm{T}$ $\lt$ 50 GeV$/c$. The significance of the inclusive
X(3872) signal is 4.2 standard deviations. The prompt X(3872) to $\psi$(2S)
yield ratio is found to be $\rho^\mathrm{PbPb} = $ 1.08 $\pm$ 0.49 (stat) $\pm$
0.52 (syst), to be compared with typical values of 0.1 for pp collisions. This
result provides a unique experimental input to theoretical models of the
X(3872) production mechanism, and of the nature of this exotic state.

High Energy Physics - Experiment;Nuclear Experiment