Self-gravitational dephasing of quasiclassical Stern-Gerlach trajectories

The nonlinear Schrödinger-Newton equation, a prospective semiclassical alternative to a quantized theory of gravity, predicts a gravitational self-force between the two trajectories corresponding to the two $z$-spin eigenvalues for a particle in a Stern-Gerlach interferometer. To leading order, this force results in a relative phase between the trajectories. For the experimentally relevant case of a spherical particle with localized wave function, we present a rederivation of that phase which is both rigorous in its approximations and concise, allowing for simple but accurate experimental predictions.