Practical Parametrization for Line Shapes of Near-Threshold States

Numerous quarkonium(like) states lying near S-wave thresholds are observed experimentally. Wepropose a self-consistent approach to these near-threshold states compatible with unitarity and analyticity.The underlying coupled-channel system includes a bare pole and an arbitrary number of elastic andinelastic channels treated fully nonperturbatively. The resulting analytical parametrization is ideally suitedfor a combined analysis of the data available in various channels that is exemplified by an excellent overalldescription of the data for the charged Zb(10610)and Zb(10650) states