The excitation spectrum of a Bose gas with an impurity in the Gross-Pitaevskii regime

Abstract: We study a dilute system of $N$ interacting bosons coupled to an impurity particle via a pair potential in the Gross--Pitaevskii regime. We derive an expansion of the ground state energy up to order one in the boson number, and show that the difference of excited eigenvalues to the ground state is given by the eigenvalues of the renormalized Bogoliubov--Fr\"ohlich Hamiltonian in the limit $N\to \infty$.