Isospin Mixing In The Electroexcitation Of The E(X)=10.84-Mev, J(Pi), T=1(-), 0 State In C-12 At Low Momentum-Transfers

Electroexcitation of the isospin-forbidden transition to the broad J(pi);T = 1(-);0 state at 10.84 MeV in C-12 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 [H-C] = 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.