## Universal Few-Body Physics

Recent **Physics Today** review and an article in *Physics* highlight the research on few-body physics in ultracold atoms.

Few-body physics is universal when inter-particle interactions are
insensitive to the microscopic details of the short-range
interaction potentials and can be characterized by only one or few
universal parameters. In the limit of zero
collision energy the two-body interactions are determined by a
single parameter, the s-wave scattering length *a*. Universality
requires *a* to greatly exceed the two-body potential range. This
can be achieved by a resonant enhancement of *a*, yielding the
appearance of the peculiar quantum states known as quantum halos
whose wavefunction acquires long-range properties and gives rise to
loosely bound states of size ~*a*. In the case
of three interacting bosons, universality means that the three-body
observables show log-periodic behavior that depends only on the
scattering length *a* and on a three-body parameter which serve as
boundary conditions for the short-range physics. Such a behavior is
associated with so called Efimov physics. In a series of theoretical
papers Vitaly Efimov predicted and characterized an infinite set of weakly
bound triatomic states (Efimov trimers) whose binding energies (in
the limit of infinit *a*) are related in powers of the famous
universal scaling factor ~1/(22.7)^{2}.

See publications for our recent work.