From the line shape of the X(3872) to its structure

From a study of the line shape of the X(3872), the LHCb collaboration measures a sizeable negative effective range. This cannot be reconciled with a shallow D (D) over bar* bound state hypothesis. Based on Weinberg's compositeness criterion, together with a theorem by Smorodinsky, it follows that the X has to have a compact hidden charm structure interacting with unbound D (D) over bar* pairs via short-distance color forces. This conclusion is strengthened by the general pattern recently emerging from exotic mesons.