Spectral statistics of chaotic systems with a pointlike scatterer

The statistical properties of a Hamiltonian H-0 perturbed by a localized scatterer are considered. We prove that if H-0 describes a bounded chaotic motion, the universal part of the spectral statistics is not changed by the perturbation. This is done first within the random matrix model. Then it is shown by semiclassical techniques that the result is due to a cancellation between diagonal diffractive and off-diagonal periodic-diffractive contributions. The compensation is a very general phenomenon encoding the semiclassical content of the optical theorem.