Evidence for X(3872) in PbPb collisions and studies of its prompt production at $\sqrt{s_{\mathrm{N}\mathrm{N}}}=$ 5.02 TeV

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 $\sqrt{s_{\mathrm{N}\mathrm{N}}}=$ 5.02 TeV per nucleon pair, using the decay chain X(3872) $\to$ J$\mathrm{/}\psi {\pi }^{+}{\pi }^{-}\to$ ${\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-}$. 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 $\mathrm{\mid }y\mathrm{\mid }$ $<$ 1.6 and 15 $<$ $p_{\mathrm{T}}$ $<$ 50 GeV$\mathrm{/}c$. 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 ${\rho }^{\mathrm{P}\mathrm{b}\mathrm{P}\mathrm{b}}=$ 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.