Symmetry energy
In nuclear physics, the symmetry energy reflects the variation of the binding energy of the nucleons in the nuclear matter depending on its neutron to proton ratio as a function of baryon density. Symmetry energy is an important parameter in the equation of state describing the nuclear structure of heavy nuclei and neutron stars.
Definition
Let and be the number density of protons and neutrons in nuclear matter, and. Let be the binding energy per nucleon in symmetric matter, with equally many protons as neutrons, as a function of density. The binding energy per nucleon of non-symmetric matter is then a function that also depends on the isospin asymmetry,so to lowest order the energy per baryon is
where is the symmetry energy. There are no odd powers of in the expansion because the nuclear force acts the same between two protons as between two neutrons. At saturation density, the symmetry energy is.