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

TL;DR

This paper argues that recently observed $XYZ$ states are genuine particles, not just kinematic effects, by analyzing their invariant mass distributions and the necessity of nearby poles in the $S$-matrix.

Contribution

It introduces a method to distinguish between kinematic cusp effects and genuine $S$-matrix poles in experimental data.

Findings

$XYZ$ states require nearby $S$-matrix poles, confirming their particle nature.

Kinematic cusps cannot produce narrow peaks in elastic channels.

Proposed criterion effectively differentiates between kinematic effects and true states.

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.