I. A. Rachek

Measurement of the Two-Photon Exchange Contribution to the Elastic e ± p Scattering Cross Sections at the VEPP-3 Storage Ring

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.

Internal polarized deuterium target with cryogenic atomic beam source

Abstract Description of the polarized deuterium gas target used at the VEPP-3 electron storage ring for experiments on elastic and inelastic ed scattering is given. Superconducting sextupole magnets with the pole tip magnetic field up to 4.8 T are used in atomic beam source (ABS) to focus atoms. The flux of polarized atoms injected into the storage cell was measured to be 8.2×10 16 at / s for deuterium and 7.9×10 16 at / s for hydrogen. The measured target thickness 8×10 13 at / cm 2 is consistent with the thickness calculated from the measured beam intensity. The effective tensor polarization of the deuterium target during the experiment was found to be P zz =0.397. Further improvements of the target and possible limitation of the beam intensity from ABS are discussed.

Compton-scattering cross section on the proton at high momentum transfer.

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.

Measurement of tensor analyzing powers in deuteron photodisintegration

A new accurate measurement of the tensor analyzing powers T20, T21, and T22 in deuteron photodisintegration has been performed. Wide-aperture nonmagnetic detectors allowed broad kinematic coverage in a single set up: E(gamma)=25 to 600 MeV, and theta(p)(cm)=24 degrees to 48 degrees and 70 degrees to 102 degrees . The new data provide a significant improvement over the few existing measurements. The angular dependency of the tensor asymmetries in deuteron photodisintegration is extracted for the first time.

A polarized gas internal target using a storage cell in an electron storage ring

Abstract The first experiment using a storage cell to increase the thickness of an internal polarized gas target in an electron beam storage ring was performed at the VEPP-3 facility. We describe the storage cell technique as applied in this measurement of elastic and inelastic electron scattering from tensor polarized deuterium. An analysis of electron-beam-induced depolarization of the target was performed and experimental tests were carried out which verify the effect. Other effects causing depolarization of the target are discussed as well as the means by which they are overcome. The effective p zz of the target, shown to be stable over 8 months, was 0.57±0.05; the total target thickness was increased over that of a jet target by a factor of fifteen.

A new event generator for the elastic scattering of charged leptons on protons

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.

High field superconducting sextupole magnets

Abstract A magnet system for cryogenic Atomic Beam Source (ABS) having five superconducting sextupoles has been designed, manufactured and tested to get high intensity of polarized hydrogen/deuterium atomic beams. The beams will be used to fill the storage cell of the internal polarized target at the VEPP-3 electron storage ring in Novosibirsk. The geometry of the magnet system, results of the computations and measurements of the magnetic field are presented. A 4.8 T pole tip magnetic field has been measured for the magnet with an aperture of 44mm.

Measurement of the two-photon exchange contribution in elastic ep scattering at VEPP-3

We report on the status of the Novosibirsk experiment on a precision measurement of the ratio R of the elastic e + p and e − p scattering cross sections. Such measurements determine the two-photon exchange effect in elastic electron-proton scattering. The experiment is conducted at the VEPP–3 storage ring using a hydrogen internal gas target. The ratio R is measured with a beam energy of 1.6 GeV (electron/positron scattering angles are θ = 55 ÷ 75 ° and θ = 15 ÷ 25 ° ) and 1 GeV ( θ = 65 ÷ 105 ° ). We briefly describe the experimental method, paying special attention to the radiative corrections. Some preliminary results are presented.

Tensor Analyzing Powers for Quasielastic Electron Scattering from Deuterium

We report on a first measurement of tensor analyzing powers in quasi-elastic electron-deuteron scattering at an average three-momentum transfer of 1.7 fm{sup -1}. Data sensitive to the spin-dependent nucleon density in the deuteron were obtained for missing momenta up to 150 MeV/c with a tensor polarized {sup 2}H target internal to an electron storage ring. The data are well described by a calculation that includes the effects of final-state interaction, meson-exchange and isobar currents, and leading-order relativistic contributions.

An active storage cell for a polarized gas internal target

Abstract The first experiment using an active storage cell for polarized atoms to increase the target thickness in an electron storage ring has been performed at the VEPP-3 facility. A clam-shell storage cell, which is opened while injecting electrons into the storage ring and closed during operation, was used to increase the target thickness viewed by the detector system. The average tensor polarization of the target was found to be p zz = 0.59 ± 0.14 and the figure of merit was increased by more than a factor of thirty in comparison with a direct jet thickness. The measured target thickness and polarization are in a good agreement with the expected values.