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 intense scientific debate during the last two decades. A possible way of estimating the size of these states is to study their behavior when immersed in QCD matter. Recently, LHCb has measured the relative abundance of the exotic $X(3872)$ over the ordinary $\psi(2S)$. We use the comover interaction model to study the yield of a compact $X(3872)$. To confirm the reliability of the model in high-multiplicity $pp$ collisions, we describe the suppression of excited over ground $\Upsilon$ states. With this at hand, we show that the size of the compact $X(3872)$ would be slightly larger than that of the $\psi(2S)$. If the $X(3872)$ is instead assumed to be a meson molecule of large size, we argue that its evolution in QCD matter should be described via a coalescence model, as suggested by data on deuteron production. We show that the predictions of this model for the $X(3872)$ are in contrast with data.