Inclusive and exclusive spectra of the ${\mathrm{He}}^4$(π,π′p) reaction were measured with ${\mathrm{\pi }}^{+}$ and ${\mathrm{\pi }}^{\mathrm{-}}$ at ${\mathrm{T}}_{\mathrm{\pi }}$=140 MeV and ${\mathrm{\theta }}_{\mathrm{\pi }}$=40°, and at 180 MeV and 30°, 40°, 60°, and 80° using the EPICS system at LAMPF in coincidence with eight plastic scintillators. The inclusive spectra yield cross section ratios ${\mathrm{R}}_{\mathrm{\pi }}$=σ(${\mathrm{\pi }}^{+}$)/σ(${\mathrm{\pi }}^{\mathrm{-}}$)=1.1±0.1 at both ${\mathrm{T}}_{\mathrm{\pi }}$ and all ${\mathrm{\theta }}_{\mathrm{\pi }}$. Exclusive spectra were obtained between ${\mathrm{He}}^{4\mathrm{*}}$ excitation energies ${\mathrm{E}}_{\mathrm{x}}$=21.5 and 45.0 MeV. Angular correlation functions for (π,π′p) were extracted at ${\mathrm{T}}_{\mathrm{\pi }}$=180 MeV at the four pion angles for selected regions of ${\mathrm{E}}_{\mathrm{x}}$. The (${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^{+\prime }$p) angular correlations show a distinct peak near the free proton knockout angle; however, the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{\mathrm{-}\prime }$p) data show either a flat distribution or a minimum near that angle. Calculations, using a distorted wave impulse approximation (DWIA) code which models the (π,π′p) reaction as a pion-included proton knockout, predict peaks in the angular correlation functions in the quasifree knockout direction and reproduce quite well the (${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^{+\prime }$p) data above ${\mathrm{E}}_{\mathrm{x}}$=30 MeV. However, these calculations do not resemble the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{\mathrm{-}\prime }$p) data at any excitation energy. Above ${\mathrm{E}}_{\mathrm{x}}$=30 MeV, the ratios ${\mathrm{R}}_{\mathrm{\pi }\mathrm{p}}$=σ(${\mathrm{\pi }}^{+}$,${\mathrm{\pi }}^{+\prime }$p)/σ(${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{\mathrm{-}\prime }$p) were found to be unexpectedly large (up to ${\mathrm{R}}_{\mathrm{\pi }\mathrm{p}}$=50) near the free proton knockout angle and very small (${\mathrm{R}}_{\mathrm{\pi }\mathrm{p}}$=0.3) in the opposite direction in contrast to DWIA predictions of 8 and 5, respectively. These discrepancies are evidence that strong interference occurs between the quasifree proton knockout and another process, especially in the (${\mathrm{\pi }}^{\mathrm{-}}$,${\mathrm{\pi }}^{\mathrm{-}\prime }$p) reaction. Near the ${\mathrm{He}}^4$→p+t breakup threshold, where the $2^{\mathrm{-}}$, T=0 state in ${\mathrm{He}}^4$ (${\mathrm{E}}_{\mathrm{x}}$=21.8 MeV) is known to exist, the ratio ${\mathrm{R}}_{\mathrm{\pi }\mathrm{p}}$ was found to be between 1 and 2 at all pion angles which is in reasonable agreement with the expected value of 1 for the excitation and decay of a state of good isospin.

• Received 4 March 1992

DOI:https://doi.org/10.1103/PhysRevC.46.52