Symmetry energy from QCD sum rules

We review the recent attempts to calculate the nuclear symmetry energy from QCD sum rules. Calculating the difference between the proton and neutron correlation function in an isospin asymmetric nuclear matter within the QCD sum rule approach, the potential part of the nuclear symmetry energy can be expressed in terms of local operators. We find that the scalar (vector) self-energy part gives negative (positive) contribution to the nuclear symmetry energy, consistent with the results from relativistic mean-field theories. Moreover, the magnitudes are consistent with phenomenological estimates. In terms of the operators, we find that an important contribution to self-energies contributing to the symmetry energy comes from the twist-4 matrix elements, whose leading density dependence can be extracted from deep inelastic scattering experiments. Our result also extends an early success of the QCD sum rule method in understanding the symmetric nuclear matter in terms of QCD variables to the asymmetric nuclear matter case. (orig.)