Dynamical polarization in pionic atoms

Abstract

Dynamical nuclear polarization occurs in pionic atoms when a nuclear excitation of appropriate multipolarity is nearly degenerate with de-excitation of a pion atomic level. This phenomenon has been studied in several nuclei, one goal being to test the pion optical potential for pion atomic states normally "hidden" because of pion absorption. We find that, in addition to Coulomb mixing of the atomic and nuclear levels; strong interaction mixing and nuclear excitations above the lowest collective quadrupole mode are important for understanding the experimental results. All cases except $^{110}\mathrm{Pd}$ can then be understood. For $^{110}\mathrm{Pd}$, additional nuclear structure information is needed to determine whether or not the conventional pion optical potential will suffice again. We discuss the sensitivity of dynamical polarization measurements to the parameters of the optical potential and to various aspects of nuclear structure. In particular, we find that pionic $^{150}\mathrm{Sm}$ provides a test of the interacting boson model and that the difference in neutron and proton radii predicted by Hartree-Fock calculations affects the mixing appreciably.NUCLEAR REACTIONS Dynamical nuclear polarization in pionic atoms $^{48}\mathrm{Ti}$, $^{104}\mathrm{Ru}$, $^{110}\mathrm{Pd}$, $^{112}\mathrm{Cd}$, $^{150}\mathrm{Sm}$; nuclear structure effects.