  Efimov physics is renowned for the self-similar spectrum featuring the
universal ratio of one eigenenergy to its neighbor. Even more esoteric is the
numerically unveiled fact that every Efimov trimer is accompanied by a pair of
tetramers. Here we demonstrate that this hierarchy of universal few-body
clusters has a topological origin by identifying the numbers of universal 3-
and 4-body bound states with the winding numbers of the renormalization-group
limit cycle in theory space. The finding suggests a topological phase
transition in mass-imbalanced few-body systems which should be tested
experimentally.
