Six quark cluster effects and binding energy differences between mirror nuclei

The computed mass difference between six quark clusters formed by two neutrons and two protons is less than that (2.6 MeV) of free nucleons in both the nonrelativistic and MIT bag quark models. This is because quark-quark Coulomb and hyperfine interactions have an increased magnitude in the six-quark system. This makes a neutron-rich nucleus slightly more bound than its proton-rich mirror. The magnitude of the effect is of about the right size to account for the missing Coulomb energy problem (Nolen-Schiffer anomaly) for nuclei with A=3, 13, 17, 29, 33, and 41.