QuasifreeC12(π+,p) reaction and the πNamplitude in nuclei

Protons knocked out of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ by quasifree pion scattering at ${\mathit{P}}_{\mathrm{\pi }}$∼500 MeV/c were measured at forward angles. The effective nucleon number of the reaction was derived from the energy-integrated cross section divided by the p(${\mathrm{\pi }}^{+}$,p)${\mathrm{\pi }}^{+}$ reaction cross section measured simultaneously by using a ${\mathrm{CH}}_2$ target. The effective nucleon numbers obtained are roughly three times larger than the distorted-wave impulse approximation calculation. It is shown that the difference is due to the change of the πN amplitude in a nucleus because there nucleons are bound and off-mass shell.