Beyond the binary collision approximation for the large- q response of liquid 4 He

We discuss corrections to the linear response of a many-body system beyond the binary collision approximation. We first derive for smooth pair interactions an exact expression of the response $\ensuremath{\propto}{1/q}^{2},$ considerably simplifying existing forms and present also the generalization for interactions with a strong, short-range repulsion. We then apply the latter to the case of liquid ${}^{4}\mathrm{He}.$ We display the numerical influence of the ${1/q}^{2}$ correction around the quasielastic peak and in the low-intensity wings of the response, far from that peak. Finally we resolve an apparent contradiction in previous discussions around the fourth order cumulant expansion coefficient. Our results prove that the large-$q$ response of liquid ${}^{4}\mathrm{He}$ can be accurately understood on the basis of a dynamical theory.