Implications of the experimental results on the photodisintegration ofHe4

The photoproton and photoneutron cross sections for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are evaluated in the light of several recent measurements, and are found to be substantially different in the energy region below 30 MeV. Because these cross sections are dominated by $\Delta S=0\mathrm{}$, $\Delta T=1\mathrm{}$ $E1\mathrm{}$ transitions, this result implies strong isospin mixing in the four-nucleon system. Furthermore, since available continuum shell-model calculations including Coulomb effects predict only small differences in the photoproton and photoneutron cross sections except near threshold, this result suggests that a nonzero charge asymmetry is present in the nuclear force.