  Statistical equilibration of energies in a slow-fast system is a fundamental
open problem in physics. In a recent paper, it was shown that the equilibration
rate in a springy billiard can remain strictly positive in the limit of
vanishing mass ratio (of the particle and billiard wall) when the frozen
billiard has more than one ergodic components [Proc. Natl. Acad. Sci. USA 114,
E10514 (2017)]. In this paper, using the model of a springy Sinai billiard, it
is shown that this can happen even in the case where the frozen billiard has a
single ergodic component, but when the time of ergodization in the frozen
system is much longer than the time of equilibration. It is also shown that as
the size of the disc in the Sinai billiard is increased from zero, thereby
leading to a decrease in the time required for ergodization in the frozen
system, the system undergoes a smooth phase transition in the equilibration
rate dependence on mass ratio.
