Precision Comparison of Inelastic Electron and Positron Scattering from Hydrogen

Using 13.5-GeV beams at Stanford Linear Accelerator Center, we have compared electron and positron inelastic scattering over the range $1.2<\left|q^2\right|<\mathrm{}3.3\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$, $2<\mathrm{}\nu <9.5\mathrm{}$ GeV for the four-momentum and energy transfers, respectively. We find the ratio of the cross sections to be $\frac{e^{+}}{e^{-}}=1.0027\mathrm{}\pm 0.0035$ (including statistical and systematic effects), with no significant dependence on $q^2$ or $\nu$. This result has appreciably smaller errors than previous attempts to find two-photon-exchange effects in electron or muon scattering.