Freeze-out at constant energy density and spin polarization in heavy-ion collisions

Using the local equilibrium density operator, we develop a geometry-informed linear response theory that takes into account the parametrization of the freeze-out hypersurface before the gradient expansion is carried out. Assuming local equilibrium on an iso-energy density hypersurface, we study the Λ hyperon spin polarization, and compute corrections to the isothermal case due to finite density. We argue that corrections to isothermal freeze-out should be small, and that even in heavy ion collisions with energy as low as sNN=11.5 GeV they constitute at most a 10% effect. They may, however, become relevant for local polarization observables.