Isospin breaking in nuclear physics: The Nolen-Schiffer effect

Using the QCD sum rules we calculate the neutron-proton mass difference at zero density as a function of the difference in bare quark mass md-mu. We confirm results of Hatsuda, Hoegaasen and Prakash that the largest term results from the difference in up and down quark condensates, the explicit C (md-mu) entering with the opposite sign. The quark condensates are then extended to finite density to estimate the Nolen-Schiffer effect. The neutron-proton mass difference is extremely density dependent, going to zero at roughly nuclear matter density. The Ioffe formula for the nucleon mass is interpreted as a derivation, within the QCD sum rule approach, of the Nambu-Jona-Lasinio formula. This clarifies the Nc counting and furthermore provides an alternative interpretation of the Borel mass. We compare calculations in the constituent quark model treated in the Nambu-Jona-Lasinio formalism with ours in the QCD sum rule approach. (orig.)