Form factor of the M1 transition to the 10.23 MeV state in ⁴⁸Ca and the role of the Δ(1232)

The form factor of the 10.23 MeV 1⁺ state in ⁴⁸Ca has been measured by inelastic electron scattering up to a momentum transfer of 1.4 fm^-1. The state is known to have a relatively simple shell-model structure. A shell-model calculation is, however, in severe disagreement with the data. We have attempted to analyse this discrepancy in terms of non-nucleonic degrees of freedom. The mechanism of Δ-hole polarization is important for the reduction of the form factor at low q if the Δ-hole interaction is sufficiently strong. The remaining quenching may be attributed to nucleonic core polarization while corrections from meson-exchange currents turn out to be small.