Observation of Quenching in Isoscalar and Isovector $0^{+}$ → $1^{+}$ Transitions in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}(p,p^{\prime })$

Excitations of a $T=0\mathrm{}$ and nine $T=1, 1^{+}$ states have been observed in the reaction ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}(p,p^{\prime })$ at 201 MeV. The magnitudes of the measured cross sections of the isoscalar and isovector transitions are, respectively, about 24% and 33% of predictions obtained with fullbasis $\mathrm{sd}$-shell-model wave functions. The large reduction factor in the isoscalar channel, where $\Delta$-isobar effects must be insignificant, suggests that higher-order configuration mixing must be an important source of quenching in both isoscalar and isovector channels.