  In recent years extensive theoretical and experimental studies of universal
few-body physics have led to advances in our understanding of universal Efimov
physics [1]. The Efimov effect, once considered a mysterious and esoteric
effect, is today a reality that many experiments in ultracold quantum gases
have successfully observed and continued to explore [2-14]. Whereas theory was
the driving force behind our understanding of Efimov physics for decades,
recent experiments have contributed an unexpected discovery. Specifically,
measurements have found that the so-called three-body parameter determining
several properties of the system is universal, even though fundamental
assumptions in the theory of the Efimov effect suggest that it should be a
variable property that depends on the precise details of the short-range two-
and three-body interactions. The present Letter resolves this apparent
contradiction by elucidating unanticipated implications of the two-body
interactions. Our study shows that the three-body parameter universality
emerges because a universal effective barrier in the three-body potentials
prevents the three particles from simultaneously getting close to each other.
Our results also show limitations on this universality, as it is more likely to
occur for neutral atoms and less likely to extend to light nuclei.
