The electric form factor of the neutron G E,n has been measured in the quasifree 2 H͑ e, e 0 n͒p reaction using the 855 MeV polarized cw electron beam of the Mainz Microtron MAMI. The polarization of the scattered neutrons was analyzed in a polarimeter consisting of two walls of plastic scintillators. The precession of the neutron spin in a magnetic field was used for the first time to circumvent the measurement of the effective analyzing power of the polarimeter and the beam polarization. In this way G E,n could be determined with little model dependence and experimental uncertainties. The result G E,n ͑0.34 GeV 2 ͞c 2 ͒ 0.0611 6 0.0069 stat ͑ 10.0069 20.0055 ͒ syst is larger than previously assumed.The internal structure of protons and neutrons at low energies is still an unsolved problem. At momentum transfers of the order Q 2 ϳ L 2 QCD , where L QCD ഠ 250 MeV 2 ͞c 2 is the scale parameter of quantum chromodynamics (QCD), perturbative solutions are not possible. Rather, the strictly nonlinear QCD has to be solved in the framework of lattice gauge theory or approximated by effective field theories. Sensitive tests of these solutions require the detailed comparison with low energy observables, such as the excitation spectrum of the nucleon or the electric and magnetic form factors. In particular, the form factors, which are measured by elastic electronnucleon scattering, contain the information on the spatial distribution of charge and magnetization in the nucleon. The electric form factor of the neutron G E,n is of special significance since the charge of the neutron vanishes and a nonzero form factor must come from a nonuniform spatial distribution of valence quarks or the sea of correlated quark-antiquark pairs, e.g., pions.The experimental information on G E,n is still unsatisfactory for two reasons. First, the lack of a free neutron target requires the use of nuclear targets thus introducing binding effects. Second, the standard Rosenbluth separation of the longitudinal cross section arising from the electric form factor G E,n ͑Q 2 ͒ and the transverse cross section due to the magnetic form factor G M,n ͑Q 2 ͒ inare limited because G 2 E,n ͑Q 2 ͒ ø tG 2 M,n ͑Q 2 ͒. Here s Mott denotes the Mott cross section, t Q 2 ͞4M 2 , and e ͓1 1 2͑1 1 t͒ tan 2 q e ͞2͔ 21 . Recently, such a separation of G E,n in the range Q 2 Ӎ 1.75 4 GeV 2 ͞c 2 yielded data compatible with G E,n ϵ 0 [1].
Parametric x-ray or quasi-Cherenkov radiation is produced by the passage of an electron through a crystal. A critical absorber technique has been employed to investigate its linewidth. Experiments have been performed with the 855 MeV electron beam from the Mainz Microtron MAMI. Thin absorber foils were mounted in front of a CCD camera serving as a position sensitive photon detector. Upper limits of the linewidth of 1.2 and 3.5 eV were determined for the (111) and (022) reflections of silicon at photon energies of 4966 and 8332 eV. These limits originate from geometrical line broadening effects that can be optimized to reach the ultimate limit given by the finite length of the wave train.[S0031-9007(97)04050-7] PACS numbers: 41.50. + h, 78.70.Dm Parametric x-ray radiation (PXR) is produced when charged particles traverse a crystal. This radiation can be understood as the coherent superposition of the elementary waves emitted from the atoms which are induced by the electromagnetic field of the passing particle. In an equivalent picture they can be described as the diffraction of the virtual photons associated with the moving particles or as a quasi-Cherenkov radiation in a medium with a periodic dielectricity. The most intriguing feature of PXR is the appearance of a sharp quasimonochromatic x-ray beam close to the Bragg angle. The narrow angular distribution consists of one peak above and one below the symmetry plane of the crystal. Their widths are characterized by the angle u ph ͓1͞g 2 1 ͑v p ͞v͒ 2 ͔ 1͞2 , with g the Lorentz factor of the moving particle, v the angular frequency of the emitted photon, and v p the plasma frequency of the crystal, 31 eV for Si. The theoretical description of the intensity distribution of PXR [1-5], suitably modified for self-absorption and multiple scattering effects, have been tested in a broad range of electron energies extending from 3.5 MeV [6,7] to about 1 GeV [8]. It was found to be accurate within 12%. On the other hand, very little is known about the energy width of PXR. The various theoretical descriptions of PXR predict a very narrow linewidth of less than a few meV. Actually, if it is assumed that a charged particle passes a very thick and perfect single crystal on a straight line and if self-absorption of the photons in the crystal can be neglected, simply a d function results for the line shape. Measurements of the linewidth have been performed up to now at the low beam energy of 6.8 MeV only. A variance of the linewidth of 48 eV has been determined for a 55 mm thick diamond crystal at a photon energy of 8.98 keV [9]. This rather large linewidth originates at low electron energy from the multiple scattering of electrons in the crystal.In this Letter a measurement of PXR at the Mainz Microtron MAMI with a silicon crystal at photon energies of about 5.0 and 8.3 keV is presented. For the high beam energy of MAMI of 855 MeV a line broadening by multiple scattering of the electrons in the crystal can be neglected. This fact originates from the rather surprising result ...
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