Experimental search for coherent subthreshold pion production

An experiment has been performed to search for coherent pion production due to the excitation and decay of collective Δ-hole states in the peripheral ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${[}^{12}$C${,}^{12}$C($1^{+}$,T=1; 15.1 MeV)] reaction at 95 MeV per nucleon. We derive an upper limit for the cross section of 60 nb. This value is consistent with present theoretical predictions.