Isospin mixing in the electroexcitation of the Ex = 10.84 MeV, Jπ; T = 1−; 0 state in 12C at low momentum transfers

Electroexcitation of the isospin-forbidden transition to the broad Jπ; T = 1−; 0 state at 10.84 MeV in 12C has been studied for momentum transfers q < 0.6 fm−1. The longitudinal form factor exhibits a sensitivity to the interference of small T = 1 admixtures. The isospin breaking Coulomb matrix element is determined in a two-state model to 〈HC〉 = 145(80) keV. A shell-model calculation using a new effective psd-shell interaction qualitatively agrees with the shape of the form factor but underpredicts its magnitude by a factor of about two.