Investigation of $\Delta$-medium effects using the ${}^4\mathrm{He}(\gamma ,{\pi }^{+}{n)}^3\mathrm{H}$ and ${}^4\mathrm{He}(\gamma ,{\pi }^{+}p)nnn$ reactions

Measurements of the ${}^4\mathrm{He}(\gamma ,{\pi }^{+}{n)}^3\mathrm{H}$ and ${}^4\mathrm{He}(\gamma ,{\pi }^{+}p)nnn$ reactions in quasifree $\pi$ production kinematic regimes have been performed using unpolarized and linearly polarized tagged photons in conjunction with large solid angle $\pi$ and n detectors at the MAMI-B accelerator, Mainz. Differential cross sections are presented for photon energies spanning the $\Delta \mathrm{}\left(1232\right)\mathrm{}$ excitation region. These data combined with previously presented results for the ${}^{12}\mathrm{C}(\gamma ,{\pi }^{+}{n)}^{11}\mathrm{B}$ reaction provide an extensive data set for future investigations of $\Delta$-medium effects in nuclei. The ${}^4\mathrm{He}(\gamma ,{\pi }^{+}{n)}^3\mathrm{H}$ cross sections are significantly smaller than theoretical predictions based on the free proton amplitude for $\Delta$ excitation with a crude correction for final state interactions, and exhibit different trends, which could possibly be a consequence of $\Delta$-medium effects. From a comparison with published ${}^{12}\mathrm{C}(\gamma ,{\pi }^{+}{p)}^{11}\mathrm{Be}$ data, it is concluded that the ${}^4\mathrm{He}(\gamma ,{\pi }^{+}p)nnn$ reaction mechanisms are probably two-step processes, although the possibility that part or all of the cross section arises from interactions with preexisting ${\Delta }^{++}$ particles cannot be ruled out.