Resonance width distribution for open quantum systems

Recent measurements of resonance widths for low-energy neutron scattering off heavy nuclei show large deviations from the standard Porter-Thomas distribution. We propose a new resonance width distribution based on the random matrix theory for an open quantum system. Two methods of derivation lead to a single analytical expression; in the limit of vanishing continuum coupling, we recover the Porter-Thomas distribution. The result depends on the ratio of typical widths $\Gamma$ to the energy level spacing $D$ via the dimensionless parameter $\kappa=(\pi\Gamma/2D)$. The new distribution suppresses small widths and increases the probabilities of larger widths.