Measurement of the Polarization in Elastic Electron-Proton Scattering

Abstract

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...

The structure of the nucleon: Elastic electromagnetic form factors

“…The earliest polarization experiments, measuring the polarization of the recoil proton [34], or measuring the asymmetry using a polarized proton target [35] with unpolarized electron beams, were done to search for two photon effects.…”

Measurement of the Target-Normal Single-Spin Asymmetry in Quasielastic Scattering from the ReactionHe3↑(e,Zhang1, Long2, Mihovilovič3 et al. 2015Phys. Rev. Lett.352361View full textAdd to dashboardCite

“…However, there are no predictions in the kinematic range of this experiment. The only existing measurement was made on the proton at SLAC in 1970 [13] …”

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

“…1. For a long time the TPE effects have eluded direct experimental searches [6][7][8]. The situation changed when a striking discrepancy between the Rosenbluth separation [9,10] and the polarization transfer [11?…”