Forward Compton Scattering from Hydrogen and Deuterium at 8 and 16 GeV

Compton-scattering cross sections from hydrogen ($\gamma p\to \gamma p$) and from deuterium have been measured at four-momentum transfer $t$ in the range $0.014<~-t<~0.17$ Ge${\mathrm{V}}^2$ and photon energies of 8 and 16 GeV. Fits to our proton data of the form $\frac{d\sigma }{\mathrm{dt}}={\mathrm{Ae}}^{\mathrm{Bt}}$ give $B\approx 7.8$ Ge${\mathrm{V}}^{-2\mathrm{}}$ and an intercept $A$ which is in agreement with the optical point. Both coherent scattering from deuterons and incoherent scattering from neutrons and protons are seen from deuterium. A small difference between the neutron and proton cross sections is seen, indicating the presence of about a 3% isovector $t$-channel exchange amplitude in addition to the predominant isoscalar amplitude. The vector-dominance model predicts lower cross sections (by at least 20%) for both the hydrogen and deuterium cases.