Are the XYZ states unconventional states or conventional states with unconventional properties?

Abstract

We discuss three possible scenarios for the interpretation of mesons containing a heavy quark and its antiquark near and above the first threshold for a decay into a pair of heavy mesons in a relative [Formula: see text]-wave. View I assumes that these thresholds force the quark potential to flatten which implies that while in these energy ranges molecular states may be formed, there should not be any quark–antiquark states above these thresholds. View II assumes that the main part of the interaction between two mesons is due to the poles which originate from the [Formula: see text] interaction. The properties of the [Formula: see text] mesons are strongly influenced by opening thresholds but the number of states is given by the quark model. In View III, both types of mesons are admitted also near and above the open flavor thresholds: [Formula: see text] mesons and dynamically generated mesons. Experimental consequences of these different views are discussed.