intensity starting from the time that the beam intensity (which was reduced during polarization reversals) was restored. When coupled with a rate-dependent dead time, this effect introduced a deviation from zero inThe customary reliance on one-photon-exchange calculations in electron-proton scattering makes it important to study those processes which could only arise from higher-order effects. A measurement of nonzero proton polarization in elastic electron-proton scattering would be evidence for a two-photon-exchange amplitude, since the polarization must vanish for pure onephoton exchange. The interference between onephoton-exchange and two-photon-exchange amplitudes is expected to be smaller than the one-photon-exchange contribution by an order of a, but it may be enhanced due to the presence of some resonance process. 1 In electron-proton elastic scattering, one-photon exchange leads to the Rosenbluth formula 2 for the differential cross section. Higher-order effects, which could show up as deviations from the Rosenbluth form, have not been observed so far. 3 The interference between the one-photon amplitude and the real part of some two-photon amplitudes can be obtained by comparing electron-proton and positron-proton elastic scattering. These measurements 4 (after allowing for radiative losses 5 ) have shown no evidence of two-photon effects, to an accuracy of about the order a, up to four-momentum-transfers squared of 5.0 (GeV/
cf.Information relating to the imaginary part of a different combination of two-photon-ex change amplitudes can be measured by performing a poone of the test asymmetries. However, this produced a negligible correction to the real data. 19 To Powell et al. t following Letter [Phys. Rev. Letters 24, 753 (1970)]. larization experiment. Two kinds of experiments are possible. One can measure the polarization P of the recoiling nucleon in the elastic scattering of unpolarized electrons from an unpolarized proton target. Alternatively (as in the present experiment), one can measure the asymmetry A in the scattering of electrons from a polarized proton target, defined as where a* and aj denote the cross sections on hydrogen polarized parallel and antiparallel to the normal {ft) to the electron scattering plane. The quantity e is the asymmetry in the raw counts from the polarized target, and the factors P T and H F allow for the target proton polarization and the fraction of hydrogen counts present in the data, respectively. We define ft as (Pin x Poutl where p in and p out are the momenta of the initial and final electron, respectively.The asymmetry A is related to the polarization P. If only one photon is exchanged, then A =P = 0 because Hermiticity and current conservation combine to prohibit any polarization of the recoil proton or, equivalently, any dependence of the cross section on the initial proton spin direction. 6 If T invariance holds, then A ~P to all orders in the electromagnetic interaction. FourWe have measured the asymmetry in the elastic scattering of electrons from a po...