Differential cross sections and a complete set of polarization observables have been measured for quasielastic $(\vec{p},\vec{n})$ reactions on ${}^{12}\mathrm{C}$ and ${}^{40}\mathrm{Ca}$ at a bombarding energy of $345\text{MeV}$. The laboratory momentum transfers are $q_{\text{lab}}\simeq 1.2$, 1.7, and $2.0{\text{fm}}^{-1}$ for ${}^{12}\mathrm{C}$ and $q_{\text{lab}}\simeq 1.7{\text{fm}}^{-1}$ for ${}^{40}\mathrm{Ca}$. In these momentum transfer regions, the isovector spin-longitudinal interaction is attractive where the one-pion exchange is dominant. The spin-longitudinal and spin-transverse polarized cross sections, $I{D}_q$ and $I{D}_p$, are deduced. The theoretically expected enhancement in the spin-longitudinal mode is observed at $q_{\text{lab}}\simeq 1.7$ and $2.0{\text{fm}}^{-1}$. The observed $I{D}_q$ is consistent with the pionic enhanced $I{D}_q$ evaluated in distorted wave impulse approximation (DWIA) calculations employing random phase approximation (RPA) response functions. The enhanced $I{D}_q$ implies the existence of a precursor to pion condensation in nuclei. On the other hand, the theoretically predicted quenching in the spin-transverse mode is not observed. The observed $I{D}_p$ is not quenched, but rather enhanced relative to that predicted via the $\mathrm{DWIA}+\mathrm{RPA}$ calculations. Two-step contributions are responsible in part for the enhancement of $I{D}_p$.

• Received 14 January 2004

DOI:https://doi.org/10.1103/PhysRevC.69.054609