Correlation satellites in optical and loss spectra

The coupling of excitations leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. We propose a cumulant formulation for neutral electronic excitations which opens the way to describe effects such as double plasmon satellites or exciton-exciton coupling. Our approach starts from the widely used GW plus Bethe-Salpeter approximation to many-body perturbation theory which is based on a quasiparticle picture, and it adds coupling to other excitations through a consistent inclusion of dynamically screened interactions. We show that this requires to consider scattering contributions that are usually neglected. The result is formulated in a way that highlights essential physics, that can be implemented as a post-processing tool in widely used first principles codes, and that suggests which kind of materials and measurements should exhibit strong effects. This is illustrated using a model.