Measurement of the electric form factor of the neutron at Q2=0.5 and 1.0 GeV2/c2

The electric form factor of the neutron was determined from measurements of the d-->(e-->,e'n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (15ND3) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G(n)(E)=0.0526+/-0.0033(stat)+/-0.0026(sys) and 0.0454+/-0.0054+/-0.0037 at Q(2)=0.5 and 1.0 (GeV/c)(2), respectively.     

Measurements of the electric form factor of the neutron up to Q2=3.4 GeV2 using the reaction 3He(e,e'n)pp

The electric form factor of the neutron was determined from studies of the reaction 3He(e,e'n)pp in quasielastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2 range over which it is known, we find G(E)(n)=0.0236±0.0017(stat)±0.0026(syst), 0.0208±0.0024±0.0019, and 0.0147±0.0020±0.0014 for Q(2)=1.72, 2.48, and 3.41 GeV2, respectively.     

Measurement of the electric form factor of the neutron through d-->(e-->,e(')n)p at Q2 = 0.5 (GeV/c)(2)

We report the first measurement using a solid polarized target of the neutron electric form factor G(n)(E) via d-->(e-->,e(')n)p. G(n)(E) was determined from the beam-target asymmetry in the scattering of longitudinally polarized electrons from polarized deuterated ammonia ( 15ND3). The measurement was performed in Hall C at Thomas Jefferson National Accelerator Facility in quasifree kinematics with the target polarization perpendicular to the momentum transfer. The electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle segmented detector. We find G(n)(E) = 0.04632+/-0.00616(stat)+/-0.00341(syst) at Q2 = 0.495 (GeV/c)(2).     

Nucleon form factors and the BLAST experiment

Measurements of the electric and magnetic form factors of the nucleon present a sensitive test of nucleon models and QCD-inspired theories. A precise knowledge of the neutron form factors at low Q² is also essential to reduce the systematic errors of parity violation experiments. At the MIT-Bates Linear Accelerator Center, the nucleon form factors have been measured by means of scattering of polarized electrons from vector-polarized hydrogen and deuterium. The experiment used the longitudinally polarized stored electron beam of the MIT-Bates South Hall Ring along with an isotopically pure, highly vector-polarized internal atomic hydrogen and deuterium target provided by an atomic beam source. The measurements have been carried out with the symmetric Bates Large Acceptance Spectrometer Toroid (BLAST) with enhanced neutron detection capability.     

Electron polarization in elastic ed scattering

The situation in which elastically scattered ultrarelativistic electrons that are partially polarized in the initial state are polarized at oriented deuterons with an axially symmetric mixed spin state is considered; an expression for the resulting polarization is obtained. The role played by the orientation of the target state’s symmetry axis and the structure of²H is studied. In the dependence of the scattered-electron polarization on the momentum transmitted, the conditions of existence of extrema are established; the extremal value may be 1 (when the scattered electrons are completely polarized). Analysis of the polarization of the scattered electrons permits not only the separation of the electromagnetic form factors of²H but also the establishment of the relative phases of all the reduced matrix elements, thereby providing new information on the²H structure. Taras Shevchencko Kiev University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 87–91, July, 1996.     

Q2 evolution of the generalized Gerasimov-Drell-Hearn integral for the neutron using a 3He target

We present data on the inclusive scattering of polarized electrons from a polarized 3He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5 degrees. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference sigma(1/2)-sigma(3/2). We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q2 of 0.1-0.9 GeV2.     

Field induced magnetic density in the paramagnetic compound LuCo2

The magnetization density induced in LuCo₂ by an applied magnetic field was measured by means of polarized neutron diffraction. The measurements were performed on a single crystal at 100 K in an applied field of 57.2 kOe. The observed density is localized on Co atoms with a form factor which is, within the experimental accuracy, similar to that of 3d electrons in Co metal. Weak additional magnetic amplitudes reveal a nonuniform polarization of the conduction band. Its mean value is opposite to the Co moment as in Co metal.     

A polarized deuteron target in the d(e,e′)np reaction

Asymmetries for scattering of longitudinally polarized electrons from deuterons with a vector polarization as well as asymmetries for scattering of unpolarized electrons from deuterons with a tensor polarization have been considered for inclusive elastic ed scattering. The sensitivity of all asymmetries considered to the electric form factor of the neutron and to the spin structure of the deuteron wave function has been investigated.     

-Measurement of the proton's electric to magnetic form factor ratio from 1H(over -->)(e(over -->),e'p)

We report the first precision measurement of the proton electric to magnetic form factor ratio from spin-dependent elastic scattering of longitudinally polarized electrons from a polarized hydrogen internal gas target. The measurement was performed at the MIT-Bates South Hall Ring over a range of four-momentum transfer squared Q2 from 0.15 to 0.65 (GeV/c)(2). Significantly improved results on the proton electric and magnetic form factors are obtained in combination with existing cross-section data on elastic electron-proton scattering in the same Q2 region.     

Magnetic form factor of Fe0.66Ni0.34 invar alloy

The magnetic form factor of a single crystal Fe_0.66Ni_0.34 Invar alloy was measured at room temperature and at liquid N₂ temperature by means of the polarized neutron diffraction technique. The self-consistent form factor analysis was made possible with the use of a spin only Fe spherical form factor.     

Calibration and applications of polarized neutron thermometry at milli- and microkelvin temperatures

We have utilized the polarized neutron scattering technique for the determination of the nuclear temperature in copper from 20 mK down to 100 K. Using a cooling facility where two adiabatic demagnetization stages work in series we have calibrated the flipping ratio of neutrons scattered from the (200) Bragg-peak against the nuclear polarization over the whole polarization range. The observed calibration curve deviates considerably from the curve expected for an extinction-free sample. The polarized neutron thermometer is discussed and applied in measurements of the spin-lattice relaxation in copper.     

Relativistic neutron fireball from a supernova explosion as a possible source of chiral effect. The role of bremsstrahlung photons

We develop the idea, proposed earlier, of a possible role of neutrons, released in a supernova (SN) explosion, as a source of polarized electrons that cause chiral asymmetry of organic molecules in interstellar gas-dust clouds. The neutrons are carried away from the dense SN shell by a relativistic neutron fireball with Lorentz factor γ of order 100. At the early stage of this carrying away, the ejected polarized electrons generate circularly polarized photons as a result of bremsstrahlung in the fireball plasma. The photons of energy near 5 eV in the ultraviolet part of the spectrum show a high efficiency of the chiral effect. In the favorable case of low absorption in the interstellar medium this mechanism may appear to be more (by two orders of magnitude) efficient than the chiral effect of the polarized electrons from the same fireball.     

Model independent determination of GEn from the d(e′n)p reaction

Electrodisintegration of vector polarized deuterium by longitudinally polarized electrons offers a possibility for measuring the neutron electric form factor G_En. In principle, the electron asymmetry is essentially proportional to G_En when quasi-free kinematics are employed. We investigate the model dependence of this result by employing several NN force models both with and without meson-exchange and isobar corrections. We find the asymmetry to be remarkably model independent near the neutron quasi-elastic peak.     

Chiral effect of electrons from supernova outbursts in the neutron fireball model

The previously proposed idea about the possible role of neutrons released during the supernova (SN) outburst as a source of polarized electrons causing chiral asymmetry of organic molecules in interstellar gas—dust clouds is developed. A specific physical mechanism of removal of a large number of free neutrons beyond the dense SN shell via the relativistic neutron fireball with a Lorentz factor on the order of 100 is considered. Estimations show high efficiency of the chiral effect of electrons from the SN in the mechanism under consideration.     

Spin dependent 2D electron scattering by nanomagnets

The 2D scattering problem of an electron by a magnetized nanoparticle is solved in the Born approximation with account of the dipole-dipole interaction of the magnetic moments of electron and nanomagnet. The scattering amplitudes in this problem are the two-component spinors. They are obtained as functions of the electron spin orientation, the electron energy and show anisotropy in scattering angle. The initially polarized beam of electrons scattered by the nanomagnet consists of electrons with no spin flipped and spin flipped. The majority of electrons with no spin flipped are scattered by small angles. The majority electrons with spin flipped are scattered in the vicinity of the scattering angles π/2 and 3π/2. This can be used as one more method of controlling the spin currents.     

Differential cross sections for ionization of laser-aligned atoms by electron impact

The first experimental data are given for (e,2e) ionization from laser-aligned atoms. A linearly polarized laser excited Mg atoms to the 31P? state prior to ionization by low energy electrons. The scattered and ejected electrons were detected in coincidence and the differential cross section determined for a range of alignment angles. An asymmetric coplanar geometry was used, with one electron fixed and the other detected at different angles. The data are compared to that from the spherically symmetric 31S? state. Significant differences are found, in both magnitude and angular distribution.     

Polarized electron-proton scattering at highQ 2

We show that polarized electron-proton scattering is an excellent tool to measure helicity changing amplitudes. The asymmetry of polarized protons measured with longitudinally polarized electrons determines the ratio of Pauli (F ²) to Dirac (F ¹) form factors. For the leading Fock state the Pauli form factor originates only from helicity changing quarkgluon interactions which are zero for zero quark masses. Therefore at high momentum transfer the ratioF ²/F ¹ depends essentially on the up and down quark masses.     

Parity-violating electron scattering from 4He and the strange electric form factor of the nucleon

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from 4He at an average scattering angle = 5.7 degrees and a four-momentum transfer Q2 = 0.091 GeV2 . From these data, for the first time, the strange electric form factor of the nucleon G(E)s can be isolated. The measured asymmetry of A(PV) = (6.72 +/- 0.84(stat) +/- 0.21(syst) x 10(-6) yields a value of G(E)s = -0.038 +/- 0.042(stat) +/- 0.010(syst), consistent with zero.     

Dynamic mean field theory of the SK-spin glass

The ground state of the holmium ion in ferromagnetic HoAl₂ is discussed in the light of available experimental evidence, including hitherto unpublished NMR measurements on¹⁶⁵Ho. The measured quadrupole splitting is not consistent with the ground state derived from neutron form factor experiments, but is compatible with exchange and crystal field parameters derived from magnetization measurements and neutron spectroscopy in the context of the conventional three-parameter mean-field model. A more detailed analysis of the NMR data indicates that the exchange interaction in HoAl₂ is over 20% stronger than that derived, using mean-field theory, from the Curie temperature. Using the revised exchange constant and a weighted average of published crystal field parameters, we obtain = (9.39 ± 0.05) _B for the moment on the Ho³⁺ ion atT =0. The contribution of orbitally polarized conduction electrons to the hyperfine field at the holmium nucleus is estimated to be (–1.4±2.0) T.     

Parity-violating electron scattering – an experimental overview

We discuss the current status and future plans of world-wide efforts of parity-violating asymmetry measurements in the scattering of longitudinally polarized electrons off unpolarized fixed targets. One thrust is the measurements of nucleon neutral weak form factors at intermediate four-momentum transfer (0.1 < Q ² < 1) (GeV/c)² which provides information about the role of virtual strange quarks on the charge and current distributions inside nucleons. A new topic is the elastic neutral weak amplitude at very low Q ² from scattering off a heavy spinless nucleus, which is sensitive to the presence of a neutron skin. Finally, we discuss the neutral current elastic amplitude at very low Q ² off protons and electrons and in the DIS regime off deuterium, which allows precision measurements of the weak mixing angle at low energy and is thus sensitive to new physics at the TeV scale. The physics implications of recent results, potential measurements from experiments under construction as well as new ideas at future facilities are discussed.