The ratio of the elastic e + p to e − p scattering cross sections has been measured precisely, allowing the determination of the two-photon exchange contribution to these processes. This neglected contribution is believed to be the cause of the discrepancy between the Rosenbluth and polarization transfer methods of measuring the proton electromagnetic form factors. The experiment was performed at the VEPP-3 storage ring at beam energies of 1.6 and 1.0 GeV and at lepton scattering angles between 15 • and 105 • . The data obtained show evidence of a significant two-photon exchange effect. The results are compared with several theoretical predictions.

An internal-target technique was used to make the first measurements of the tensor analyzing power 7^20 of electron-deuteron elastic scattering in the four-momentum-transfer range of 2-3 fm _l . Polarized deuterium atoms were confined within a storage cell in the VEPP-3 electron storage ring in Novosibirsk to achieve a total target thickness of 3x 10 12 cm ~2, 15 times greater than was previously possible with an atomic-beam target alone. The results for T20 are in agreement with reasonable models of the deuteron wave function.

This paper describes a new multipurpose event generator, ESEPP, which has been developed for the Monte Carlo simulation of unpolarized elastic scattering of charged leptons on protons. The generator takes into account the lowest-order QED radiative corrections to the Rosenbluth cross section including first-order bremsstrahlung without using the soft-photon or ultrarelativistic approximations. ESEPP can be useful for several significant ongoing and planned experiments.

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We report an absolute measurement of the tensor analyzing powers T 20 and T 22 in elastic electrondeuteron scattering at a momentum transfer of 1.6 fm 21 . The novel approach of this measurement is the use of a tensor polarized 2 H target internal to an electron storage ring, with in situ measurement of the polarization of the target gas. Scattered electrons and recoil deuterons were detected in coincidence with two large acceptance nonmagnetic detectors. The techniques demonstrated have broad applicability to further measurements of spin-dependent electron scattering.[ S0031-9007(96) Measurements of spin-dependent electron scattering have the potential to greatly enhance our understanding of nucleon and nuclear structure. For example, spin observables in elastic, quasielastic, and deep-inelastic scattering from polarized deuterium are predicted to provide important information on the effects of D-wave components in the ground state of 2 H [1], the largely unknown charge form factor of the neutron [2], and the neutron spin structure functions [3]. This has prompted development of both polarized 2 H targets for use with internal [4] or external beams [5] and polarimeters for measuring the polarization of recoiling hadrons [6]. Indeed the first round of measurements of spin-dependent e-2 H scattering has been carried out at Novosibirsk [7,8], Bonn [9], MITBates [10,11], and SLAC [12].The measurement of analyzing powers and spincorrelation parameters in spin-dependent electron scattering from polarized nuclei is optimally performed by scattering electrons from a pure and highly polarized target. Polarized internal gas targets in electron storage rings have the advantage that spin-dependent scattering from chemically and isotopically pure atomic species of high polarization can be realized. They offer rapid polarization reversal and flexible orientation of the nuclear spin direction by using low magnetic holding fields, a low thickness at high luminosity which allows for the detection of low-energy recoiling hadrons, and access to a broad kinematic range by using large acceptance detectors. For polarized deuterium one has the additional ability to reverse the tensor polarization, P zz , at fixed vector polarization, P z , and vice versa. Subsequently, small systematic errors can be expected.The first pioneering measurements [7,8] with a polarized deuterium internal target have been carried out at VEPP-3 in Novosibirsk. They realized a target with a thickness limited to about 3 3 10 11 atoms cm 22 [8] as viewed by their detectors. Recently, this was increased by an order of magnitude [13]. Since many mechanisms can depolarize the target nuclei in the storage cell, and no polarimeters were available to measure the target polarization in situ, they normalized one datum to a theoretical prediction, setting the scale for the other data points [8].The electron spin-averaged cross section for elastic electron-deuteron scattering can be expressed [1] as s s 0

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5-11 and -t=2-7 GeV2 with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0+/-0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.