A short review on the compositeness of the X(3872)

The X(3872) could be a shallow DD¯∗ bound state, a compact four-quark state, or a partially composite particle, i.e. a superposition of the two. We will review how these hypotheses could be tested experimentally, examining especially the cases in which the X is a pure bound state or a pure compact tetraquark. Data on X→DD¯π decays are compared with the analysis of the X lineshape. The pure bound state hypothesis corresponds to a well-defined region in parameter space defined by the width of the D∗ versus the binding energy of the X. As for the X lineshape, we observe that the currently available experimental analysis tests the compatibility with the compact hypothesis for the X. We propose how to extend the analysis to examine the molecular or the partially composite hypotheses. We also review the analysis on the radiative decays of the X including pion corrections confirming some conclusions reached in the literature on the use of the universal wave function description for the molecular X.