  Following recent work of Chernov, Markarian, and Zhang, it is known that the
billiard map for dispersing billiards with zero angle cusps has slow decay of
correlations with rate 1/n. Since the collisions inside a cusp occur in quick
succession, it is reasonable to expect a much faster decay rate in continuous
time. In this paper we prove that the flow is rapid mixing: correlations decay
faster than any polynomial rate. A consequence is that the flow admits strong
statistical properties such as the almost sure invariance principle, even
though the billiard map does not.
  The techniques in this paper yield new results for other standard examples in
planar billiards, including Bunimovich flowers and stadia.
