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

Compton-scattering cross sections from hydrogen ($\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\gamma}p$) and from deuterium have been measured at four-momentum transfer $t$ in the range $0.014l~\ensuremath{-}tl~0.17$ Ge${\mathrm{V}}^{2}$ and photon energies of 8 and 16 GeV. Fits to our proton data of the form $\frac{d\ensuremath{\sigma}}{\mathrm{dt}}={\mathrm{Ae}}^{\mathrm{Bt}}$ give $B\ensuremath{\approx}7.8$ Ge${\mathrm{V}}^{\ensuremath{-}2}$ 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.