Spin-flip excitations in a schematic model

Several recent experiments using a proton beam have shown that nuclear-spin excitations in light nuclei appear relatively suppressed at low excitation energy but surprisingly enhanced at higher excitation. We present here a calculation based on energy-weighted sum rules which yields results consistent with spin-flip data in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The calculation is further useful in showing the importance of 2p-2h coupling in producing asymmetric width as well as in demonstrating the effects of meson exchange currents. The prediction of an increase in spin-flip strength at high energy with increasing momentum transfer should be a useful test of the model.