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| Book/Report | FZJ-2018-03649 |
1991
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/19057
Report No.: Juel-2540
Abstract: In charge-exchange reactions and in $\pi$-absorption at intermediate energies the $\Delta$ peak position in nuclei is shifted downwards in energy compared to the $\Delta$-excitation of the nucleon. For electromagnetic probes the experimental data do not show such a shift. We describe the excitation of the $\Delta$ in nuclei consistently for all these different probes using the isobar-hole-model ($\Delta$N$^{-1}$-model). The shift of the $\Delta$ peak position in chargeexchange reactions and in $\pi$-absorption is due to an attractive $\Delta$N$^{-1}$-residual interaction in the spin-longitudinal ($\overrightarrow{\sigma} \cdot \overrightarrow{q}$) channel. A $\Delta$N$^{-1}$-residual interaction consisting of the $\pi$-exchange potential alone is too attractive in the spin-longitudinal channel and shifts the $\Delta$ peak position for $\pi$-absorption to too low energies. In order to reproduce the $\Delta$ peak position we have to include short range correlations in the $\Delta$N$^{-1}$-residual interaction. This is done either by adding a $\delta$-force, parametrized through the Landau-Migdal-parameter g$^{'}_{\Delta \Delta}$, to the $\pi$-exchange potential, or by multiplying the $\pi$-exchange potential with a correlation-function. This leads to a repulsive $\Delta$N$^{-1}$-residual interaction in the spin-transverse ($\overrightarrow{\sigma}$ x $\overrightarrow{q}$) channel, which shifts the $\Delta$ peak position in the calculations for photo-absorption to higher energies compared to the photo-absorption on the nucleon.
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