Measuring the coupling constant of polarized fermions via sound wave spectra

The system of hydrodynamic equations is generally limited to the continuity and Euler equations. However, taking higher moments of the distribution function into account improves the description of kinetic properties. We derive the pressure tensor evolution equation (PTEE) for spin-polarized degenerate fermions. We find that the pressure tensor enters the term describing the interaction that generalizes the $p$-wave interaction in the Euler equation. Hence, calculating the interaction for the PTEE allows describing the interaction in the Euler equation more accurately. The proposed model is applied to small-amplitude bulk collective excitations in homogeneous fermions and trapped fermions, yielding a method for experimental measurements of the coupling constant of polarized fermions. It is demonstrated that the anisotropy in the momentum space, manifesting itself in the difference of pressures in the anisotropy direction and the perpendicular directions, leads to a method for determining the coupling constant.