Measurement of theC12(π,2π) reactions and possible evidence of a double-Δ excitation

Cross sections, pion momentum spectra, and angular distributions for the reactions ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$) and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,2${\mathrm{\pi }}^{\mathrm{-}}$) at a bombarding energy of 292 MeV were measured wth a 360° spectrometer. An enhancement of the cross-section ratio of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,2${\mathrm{\pi }}^{\mathrm{-}}$) to ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$) relative to that of the elementary cross sections was observed, and is discussed in terms of various nuclear-medium effects including a double-Δ production channel.