  In systems of ultracold atoms, pairwise interactions can be resonantly
enhanced by a new mechanism which does not rely upon a magnetic Feshbach
resonance. In this mechanism, interactions are controlled by tuning the
frequency of an oscillating parallel component of the magnetic field close to
the transition frequency between the scattering atoms and a two-atom bound
state. The real part of the resulting s-wave scattering length $a$ is
resonantly enhanced when the oscillation frequency is close to the transition
frequency. The resonance parameters can be controlled by varying the amplitude
of the oscillating field. The amplitude also controls the imaginary part of $a$
which arises because the oscillating field converts atom pairs into molecules.
The real part of $a$ can be made much larger than the background scattering
length without introducing catastrophic atom losses from the imaginary part.
For the case of a shallow bound state in the scattering channel, the
dimensionless resonance parameters are universal functions of the dimensionless
oscillation amplitude.
