Effects ofS-wave thresholds

The opening of a new $S$-wave threshold is frequently accompanied by an abrupt dip in the magnitude of an amplitude for an already-open channel. One familiar example is the behavior of the $I=0$ $S$-wave $\pi \pi$ scattering amplitude at $K\overline{K}$ threshold. Numerous other examples of this phenomenon in recent data are noted, and a unified description of the underlying dynamics is sought.