The virial equation of state of low-density neutron matter

We present a model-independent description of low-density neutron matter based on the virial expansion. The virial equation of state provides a benchmark for all nuclear equations of state at densities and temperatures where the interparticle separation is large compared to the thermal wavelength. We calculate the second virial coefficient directly from the nucleon-nucleon scattering phase shifts. Our results for the pressure, energy, entropy and the free energy correctly include the physics of the large neutron-neutron scattering length. We find that, as in the universal regime, thermodynamic properties of neutron matter scale over a wide range of temperatures, but with a significantly reduced interaction coefficient compared to the unitary limit.