Decoding the density dependence of the nuclear symmetry energy

The large imbalance in the neutron and proton densities in very neutron rich systems increases the nuclear symmetry energy so that it governs many aspects of neutron stars and their mergers. Extracting the density dependence of the symmetry energy therefore constitutes an important scientific objective. Many analyses have been limited to extracting values for the symmetry energy, S-0, and its "derivative", L, at saturation density rho(0) approximate to 2.6 x 10(14) g/cm(3) approximate to 0.16 nucleons/fm(3), resulting in constraints that appear contradictory. We show that most experimental observables actually probe the symmetry energy at densities far from rho(0), making the extracted values of S-0 or L imprecise. By focusing on the densities these observables actually probe, we obtain a detailed picture of the density dependence of the symmetry energy from 0.25 rho(0) to 2.0 rho(0). From this experimentally derived density functional, we extract L-01 = 54 +/- 6 MeV corresponding to the symmetry pressure of P-01 = 1.8 +/- 0.2 MeV/fm(3) at rho approximate to 0.10 fm(-3), a neutron skin thickness for Pb-208 of R-np = 0.23 +/- 0.03 fm, and symmetry pressure at saturation density of P-0 = 4.4 +/- 1.3 MeV/fm(3). The extrapolated symmetry pressure at high density is consistent with results from recent measurements of neutron star radii from NICER and deformability from LIGO. (C) 2022 The Author(s). Published by Elsevier B.V.