  We carry out non-perturbative calculations of the single-particle excitation
spectrum in strongly interacting neutron matter. These are microscopic quantum
Monte Carlo computations of many-neutron energies at different densities as
well as several distinct excited states. As input, we employ both
phenomenological and chiral two- and three-nucleon interactions. We use the
single-particle spectrum to extract the effective mass in neutron matter. With
a view to systematizing the error involved in this extraction, we carefully
assess the impact of finite-size effects on the quasiparticle dispersion
relation. We find an effective-mass ratio that drops from 1 as the density is
increased. We conclude by connecting our results with the physics of ultracold
gases as well as with energy-density functional theories of nuclei and
neutron-star matter.
