Evidence for $\chi_{c1}$(3872) in PbPb collisions and studies of its prompt production at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}=5.02$ TeV

The first evidence for $\chi_{c1}$(3872) in relativistic heavy ion collisions is reported. Prompt $\chi_{c1}$(3872) production is studied in PbPb collisions at a center-of-mass energy of $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = 5.02$ TeV per nucleon pair, using the decay chain $\chi_{c1}$(3872)$\rightarrow {\rm J}/\psi\pi^+\pi^-\rightarrow \mu^+\mu^-\pi^+\pi^-$. The data were recorded by the CMS detector in 2018 and correspond to an integrated luminosity of $1.7\pm 0.1$nb$^{-1}$. The measurement is performed in a kinematic range in which the $\chi_{c1}$(3872) candidates have a transverse momentum $15 < p_{\mathrm{T}} < 50 \,\mathrm{Ge\hspace{-.08em}V\hspace{-0.16em}/\hspace{-0.08em}}c$ and rapidity $|y|<1.6$. The statistical significance of the $\chi_{c1}$(3872) signal is in excess of 3 standard deviations. The production and survival of the prompt $\chi_{c1}$(3872) in the quark-gluon plasma is expected to depend upon its internal structure. The ratio of the $\chi_{c1}$(3872) and $\psi$(2S) yields is found to be $R = 1.10 \pm 0.51\;\mathrm{(stat.)}\: \pm 0.53\;\mathrm{(syst.)}$. This results is compared to the similar ratio measured in pp collisions at $\sqrt{s} = 7$ and 8 TeV. An indication of larger $R$ in PbPb collisions with respect to the pp reference is reported.