Electroexcitation of isoscalar and isovector magnetic dipole transitions in [formula omitted]C and isospin mixing

Abstract

The excitation of the Jπ;T=1+;0 and 1+;1 states at Ex=12.71 and 15.11 MeV in 12C has been studied at momentum transfers q<0.5 fm−1 in inelastic electron scattering. The corresponding ground state radiation widths and transition radii for the isoscalar and isovector transition are deduced as Γγ0=0.32(2) eV, Rtr=2.95(9) fm and Γγ0=35.9(6) eV, Rtr=2.63(4) fm, respectively. For the evaluation of Γγ0 and Rtr for the transition to the 15.11 state (e,e′), data of the present work have been combined with older data from other low energy electron accelerators. The results support the strong isospin mixing of the two 1+ states, and in the frame of a two-state model a Coulomb matrix element 〈HC〉=118(8) keV is derived. This value agrees with other experiments using electromagnetic or pionic probes but is much more accurate. Furthermore, the measured difference in the transition radii for the isoscalar and isovector transitions allows for the first time a unique determination of the sign of 〈HC〉.