  We analyze low-energy scattering for arbitrary short-range interactions plus
an attractive 1/r^6 tail. We derive the constraints of causality and unitarity
and find that the van der Waals length scale dominates over parameters
characterizing the short-distance physics of the interaction. This separation
of scales suggests a separate universality class for physics characterizing
interactions with an attractive 1/r^6 tail. We argue that a similar
universality class exists for any attractive potential 1/r^{alpha} for alpha >=
2. We also discuss the extension to multi-channel systems near a magnetic
Feshbach resonance. We discuss the implications for effective field theory with
attractive singular power law tails.
