We report on the first measurement of spin-correlation parameters in quasifree electron scattering from vector-polarized deuterium. Polarized electrons were injected into an electron storage ring at a beam energy of 720 MeV. A Siberian snake was employed to preserve longitudinal polarization at the interaction point. Vector-polarized deuterium was produced by an atomic beam source and injected into an open-ended cylindrical cell, internal to the electron storage ring. The spin correlation parameter A V ed was measured for the reaction 2 H͑e, e 0 n͒ p at a four-momentum transfer squared of 0.21 ͑GeV͞c͒ 2 from which a value for the charge form factor of the neutron was extracted. [S0031-9007(99)09392-8] PACS numbers: 13.40. Gp, 14.20.Dh, 24.70. + s, 25.30.Fj Although the neutron has no net electric charge, it does have a charge distribution. Precise measurements [1] where thermal neutrons from a nuclear reactor are scattered from atomic electrons indicate that the neutron has a positive core surrounded by a region of negative charge. The actual distribution is described by the charge form factor G n E , which enters the cross section for elastic electron scattering. It is related to the Fourier transform of the charge distribution and is generally expressed as a function of Q 2 , the square of the four-momentum transfer. Data on G n E are important for our understanding of the nucleon and are essential for the interpretation of electromagnetic multipoles of nuclei, e.g., the deuteron.Since a practical target of free neutrons is not available, experimentalists mostly resorted to (quasi)elastic scattering of electrons from unpolarized deuterium [2,3] to determine this form factor. The shape of G n E as a function of Q 2 is relatively well known from high precision elastic electron-deuteron scattering [3]. However, in this case the cross section is dominated by scattering from the proton and, moreover, is sensitive to nuclear-structure uncertainties and reaction-mechanism effects. Consequently, the absolute scale of G n E still contains a systematic uncertainty of about 50%.Many of the aforementioned uncertainties can be significantly reduced through the measurement of electronuclear spin observables. The scattering cross section with both longitudinal polarized electrons and a polarized target for the 2 H͑e, e 0 N͒ reaction, can be written as [4]where S 0 is the unpolarized cross section, h the polarization of the electrons, and P d 1 (P d 2 ) the vector (tensor) polarization of the target. A e is the beam analyzing power, A V ͞T d the vector and tensor analyzing powers, and A V ͞T ed the vector and tensor spin-correlation parameters. The target analyzing powers and spin-correlation parameters depend on the orientation of the target spin. The polarization direction of the deuteron is defined by the angles Q d and F d in the frame where the z axis is along the direction of the three-momentum transfer (q) and the y axis is defined by the vector product of the incoming and outgoing electron momenta. A V ed ͑Q d 90 ±...

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We report new precise H(e,e(')p)pi(0) measurements at the Delta(1232) resonance at Q(2)=0.127 (GeV/c)(2) obtained at the MIT-Bates out-of-plane scattering facility which are particularly sensitive to the transverse electric amplitude (E2) of the gamma(*)N-->Delta transition. The new data have been analyzed together with those of earlier measurements to yield precise quadrupole to dipole amplitude ratios: Re(E(3/2)(1+)/M(3/2)(1+))=(-2.3+/-0.3(stat+syst)+/-0.6(model))% and Re(S(3/2)(1+)/M(3/2)(1+))=(-6.1+/-0.2(stat+syst)+/-0.5(model))% for M(3/2)(1+)=(41.4+/-0.3(stat+syst)+/-0.4(model))(10(-3)/m(pi(+))). The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q2, the dominance of mesonic effects.

The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electronproton elastic cross section ratio, R 2γ , a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of ≈20°to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved gas electron multiplier and multiwire proportional chamber detectors at 12°, as well as symmetric Møller or Bhabha calorimeters at 1.29°. A total integrated luminosity of 4.5 fb −1 was collected. In the extraction of R 2γ , radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of R 2γ , presented here for a wide range of virtual photon polarization 0.456 < ϵ < 0.978, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.