Effective range expansion for narrow near-threshold resonances

Abstract

We discuss some general features of the effective range expansion, the content of its parameters with respect to the nature of the pertinent near-threshold states and the necessary modifications in the presence of coupled channels, isospin violations and unstable constituents. As illustrative examples, we analyse the properties of the ${\chi }_{c1}(3872)$ and $T_{cc}^{+}$ states supporting the claim that these exotic states have a predominantly molecular nature.

Comments: 15 pages, 1 figure