Experimental study of pionic Ca and Ti and examination of the pion-nuclear potential for low-Z nuclei

We report the measurement of the strong interaction shifts and widths of the 3d → 2p transitions in pionic ^{40,42,43,44,48}Ca and ^46,48,50Ti. Using these new data in combination with earlier low-Z pionic data (6 ≦ Z ≦ 16), we have studied, in a purely phenomenological way, the pion-nuclear optical potential. Employing nuclear-structure information from Hartree-Fock calculations and measured charge densities, we have fitted the pion-nuclear potential parameters to the pionic atom data. The effect of adding an isovector dependence to the s- and p-wave two-nucleon terms has been investigated. We have also explored the sensitivity of these data to the value of the Lorentz-Lorenz parameter ξ. A small but definite sensitivity to the latter parameter has been observed with the best fit value of ξ being significantly greater than 1. We have used the phenomenological potentials to determine the neutron radii of the Ca and Ti isotopes and, in fact, find reasonable agreement with Hartree-Fock predictions and with the results of other hadronic probes.