Quasideuteron mechanism and the photodisintegration ofHe4at intermediate energies

The ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,p){}_{}{}^3{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,n){}_{}{}^3{}_{}{}^{}\mathrm{He}$ reactions from $E_{\gamma }\gtrsim 60$ MeV to $E_{\gamma }\lesssim 170$ MeV are considered, along with the ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,pnd)$ reaction from threshold up to intermediate energies. The absorption of photons in the intermediate energy range up to pion threshold is expected to occur predominantly via the quasideuteron mechanism as is indicated from the reaction ${}_{}{}^4{}_{}{}^{}\mathrm{He}(\gamma ,pnd)$. For the ($\gamma ,pnd$) amplitude, the breakup of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ into a deuteron and quasideuteron is calculated using a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ Gaussian ground-state wave function fitted to the form factor, a quasideuteron $S$-wave scattered state, and a deuteron effective range wave function. The photobreakup of the quasideuteron can be expressed in terms of the amplitude for the photobreakup of the deuteron as shown by Levinger. The photoamplitude for the ($\gamma ,pnd$) reaction is used to obtain the photoamplitude for the ($\gamma ,n$) and ($\gamma ,p$) reactions. The contribution to the ($\gamma ,p$) and ($\gamma ,n$) reactions from the quasideuteron mechanism is calculated as a rescattering of a nucleon-deuteron pair to form a triton. For both nucleon-triton reactions, this yields angular distributions with forward peaking in agreement with the forward asymmetry obtained experimentally. The cross section at intermediate energy appears to be dominated by the quasideuteron effect. Finally, the alternative quasideuteron calculation using nucleon-nucleon short-range correlations is shown to be equivalent to the present quasideuteron calculation.