Elastic photon scattering fromHe4: Charge symmetry problem

Elastic photon scattering cross sections have been measured on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in the region of the giant dipole resonance (GDR) using the tagged-photon technique. These data place stringent constraints on the total photoabsorption ${\mathrm{\sigma }}_{\gamma }$ at the peak of the GDR, which to very good approximation is the sum of the (γ,p) and (γ,n) cross sections. We find the peak cross section to be ${\mathrm{\sigma }}_0$=2.86±0.12 mb. This result is inconsistent with the sum of the most recent measurements of the (γ,p) and (γ,n) cross sections. Thus any conclusion about charge symmetry based on the ratios of these cross sections is suspect.