Evidence for Two-Nucleon Processes inA(ppol, π−)A+1

Possible signatures of two-nucleon pion production processes in reactions $A(p_{\mathrm{pol}}, {\pi }^{-})A+1\mathrm{}$ near threshold are identified: a dependence of the analyzing power on the total angular momentum, and a simple scaling of the cross section with subshell occupancy for the struck target neutron. Measurements for ${}_{}{}^{12,13,14}{}_{}{}^{}\mathrm{C}(p_{\mathrm{pol}}, {\pi }^{-})$ exhibit these expected features, supporting the view that the fundamental $\mathrm{NN}\to \mathrm{NN}\pi$ processes dominate in nuclear pion production.