Nucleonic versus nuclear spin-isospin polarization: A study of the ⁴⁸Ca and ⁸⁸Sr M1 form factors

We compare standard nuclear polarization mechanisms, Δ-hole-polarization and meson-exchange-current effects in the q-dependent quenching of isovector spin transitions. Calculations are performed for the M1-transition form factors of the 1⁺ states in ⁴⁸Ca (10.23 MeV) and ⁸⁸Sr (3.48 MeV). We obtain a satisfactory description of both form factors if the repulsive part of the residual interaction in the Δ-hole channel is of similar strength to that in the nucleon-hole channel. Meson-exchange currents lead to an enhancement of M1 transitions by an amount which is small in general, but sensitive to the particular nuclear state involved.