Near-threshold (p→,π−) reactions on calcium isotopes

Cross section and analyzing power angular distribution measurements are reported for the ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$(p→,${\pi }^{\mathrm{-}}$ ${)\mathrm{}}^{49}$Ti reaction at two bombarding energies ($T_{\mathrm{p}=166\mathrm{}}$ and 205 MeV) and for ${}_{}{}^{42}{}_{}{}^{}\mathrm{Ca}$(p→,${\pi }^{\mathrm{-}}$ ${)\mathrm{}}^{43}$Ti at $T_{\mathrm{p}=205\mathrm{}}$ MeV. Also presented for comparison are single-angle spectra for (p,${\pi }^{\mathrm{-}}$) on other $f_{7/2}$-shell targets and for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ti}$(p,${\pi }^{+}$ ${)\mathrm{}}^{49}$Ti. The (p, ${\pi }^{\mathrm{-}}$) spectra show a strong selectivity for population of high-spin (${\mathrm{}(15/2)\mathrm{}}^{\mathrm{-}}$ and ${\mathrm{}(19/2)\mathrm{}}^{\mathrm{-}}$) 2p-1h [(π$f_{7/2}$ ${)}^2$(ν$f_{7/2}$ ${)}^{\mathrm{-}1}$] states, which are not prominent in the (p,${\pi }^{+}$) spectrum. Supporting evidence for the high-spin identification comes from comparisons with simple (p,${\pi }^{\mathrm{-}}$) model calculations and the results of other reaction studies. Characteristic features of the data for the high-spin states and for the continuum portions of the (p,${\pi }^{\mathrm{-}}$) spectra are discussed in the context of a two-nucleon (NN→NNπ) pion production mechanism. All of the main systematic features observed are at least qualitatively consistent with expectations for such a mechanism.