Evidence of X(3872) production in PbPb collisions with CMS experiment

<!--HTML-->The structure of the exotic meson $\chi_{c1}(3872)$, also known as $X(3872)$, is still under debate. The similarity of the $\chi_{c1}(3872)$ mass and the $D-\bar{D}^{*}$ mass threshold inspired the interpretation that $\chi_{c1}(3872)$ is a $D-\bar{D}^{*}$ “molecule” with small binding energy. Another explanation is that this meson is a tetra-quark, consisting of a di-quark and di-antiquark. Relativistic heavy-ion collisions produce an extremely hot and strongly interacting medium, which provides a new environment in which to study the nature of multi-quark states. Because of the dramatically different radii of a $D-\bar{D}^{*}$ “molecule” and a tetra-quark, the interactions of these two proposed states are expected to interact differently with the medium. Therefore, the yield of $\chi_{c1}(3872)$ in heavy-ion collisions can provide insight into its structure. The ratios of production cross-section of fully reconstructed $\chi_{c1}(3872)$ over $\psi(2S)$ in PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV with the CMS detector are presented.