Nambu<i>–</i>Jona-Lasinio model and charge independence

For different up and down current quark masses, charge independence follows in the Nambu--Jona-Lasinio model from chiral-symmetry breaking; however, chiral symmetry is restored at high densities. The dependence of the constituent quark masses and the neutron-proton mass difference on the density are examined. The effect of the up-down-quark mass difference on the neutron-proton mass difference is large and in the right direction to explain the Nolen-Schiffer anomaly.

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