The nature of $X(3872)$ from high-multiplicity $pp$ collisions.

The structure of exotic resonances that do not trivially fit the usual quark model expectations has been a matter of strong scientific debate during the last two decade. An excellent way to measure the size of these states is to observe how much they are affected when immersed in QCD matter. Recently, LHCb has measured the relative abundance of the exotic $X(3872)$ over the ordinary $\psi(2S)$. Built on several decades of phenomenological understanding of quarkonia production in hot and dense QCD matter, we employ the comover interaction model to study the yield of the $X(3872)$. To confirm the reliability of the model in large multiplicity $pp$ collisions, we describe the larger suppression of excited over ground $\Upsilon$ states, as well as the enhancement of deuterons over protons, for which coalescence is implemented. With this at hand, we show that the size of the $X(3872)$ is only slightly larger than that of the $\psi(2S)$, that is, it corresponds to a typical hadronic state. This finding clearly supports the $X(3872)$ being a tetraquark state and strongly disfavors the molecular interpretation, that would need a much larger size.