Could the near-threshold $XYZ$ states be simply kinematic effects?

Abstract

We demonstrate that the spectacular structures discovered recently in various experiments and named as $X$, $Y$ and $Z$ states cannot be purely kinematic effects. Their existence necessarily calls for nearby poles in the $S$--matrix and they therefore qualify as states. We propose a way to distinguishing kinematic cusp effects from genuine $S$--matrix poles: the kinematic threshold cusp cannot produce a narrow peak in the invariant mass distribution in the elastic channel in contrast with a genuine $S$--matrix pole.