Search for deeply bound pionic states inPb208using thePb208(n,p) reaction atTn=418 MeV

A search for deeply bound ${\mathrm{\pi }}^{\mathrm{-}}$ states in heavy nuclei was undertaken using the pion-transfer ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$(n,p) reaction at the TRIUMF CHARGEX facility. Monoenergetic 418-MeV neutrons from the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$(p,n) reaction were used to induce (n,p) reactions on a segmented ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ target. The spectrum between 100- and 180-MeV excitation energy was measured with an energy resolution of 1.15 MeV (FWHM). In the region of 120–140 MeV, where the deeply bound pionic states were expected, the observed spectrum exhibited a smooth continuumlike behavior with a cross section of ≊0.8 mb/sr MeV. This resulted in an upper limit of 0.35 mb/sr for the production cross section of ${\mathrm{\pi }}^{\mathrm{-}}$ bound states in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. This cross section is substantially smaller than the estimate based on the plane-wave impulse approximation, showing the importance of the effect of distortion. The distortion effect is calculated and discussed.