Publication: Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at $\sqrt {s_{NN}}$=5.02 TeV

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The first evidence for X(3872) production in relativistic heavy ion collisions is reported. The X(3872) production is studied in lead-lead (Pb-Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ per nucleon pair, using the decay chain $\mathrm{X}(3872)\to J/\psi {\pi }^{+}{\pi }^{-}\to {\mu }^{+}{\mu }^{–}{\pi }^{+}{\pi }^{-}$. The data were recorded with the CMS detector in 2018 and correspond to an integrated luminosity of $1.7{\mathrm{number}}^{–1}$. The measurement is performed in the rapidity and transverse momentum ranges $|y|<1.6$ and $15<p_{\mathrm{T}}<50\mathrm{GeV}/c$. 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 ${\rho }^{\mathrm{Pb}\text{–}\mathrm{Pb}}=1.08\pm 0.49(\mathrm{stat})\pm 0.52(\mathrm{system})$, 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.