Polarized Electron Beam Research Articles

Characterisation and application of a SPLEED-based spin polarisation analyser

A commercial electron spin analyser, based on spin-polarised low-energy electron diffraction (SPLEED) from W(1 0 0), has been characterised with incident polarised electron beams from a standard GaAs polarised electron source. The dependence of the Sherman function on the scattering energy and elapse time after CO-flash of the tungsten crystal of the analyser have been measured. The influence of the stray magnetic field on the performance of the analyser has been investigated. The spin analyser has been applied in monitoring the reorientation transition of the easy magnetisation direction of Fe films on W(1 1 0) upon the exposure of CO adsorbent on the surface.

Transverse beam asymmetries measured from 4He and hydrogen targets

The HAPPEX Collaboration at Jefferson Lab has measured the transverse beam spin asymmetries (AT) for elastic electron scattering from proton and 4He targets. The experiment was conducted using a vertically polarized electron beam of energy ∼ 3 GeV, at a Q 2 ∼ 0.1 GeV^2 and a scattering angle θlab ∼ 6° . The preliminary results are reported here. The 4He measurement is the first measurement of AT from a nucleus. AT for 4He is non-negligible; therefore, it will be necessary to make measurements of AT for future parity-violating experiments using nuclear targets.

Laser Compton polarimetry at JLab and MAMI

For modern parity violation experiments it. is crucial to measure and monitor the electron beam polarization continuously. In the recent years different high-luminosity concepts, for precision Compton backscattering polarimetry, have been developed, to be used at modern CW electron beam accelerator facilities. As Compton backscattering polarimetry is free of intrinsic systematic uncertainties, it can be a superior alternative to other polarimetry techniques such as Moller and Mott scattering. State-of-the-art high-luminosity Compton backscattering designs currently in use and under development at JLab and Mainz are compared to each other. The latest results from the Mainz A4 Compton polarimeter are presented.

Low Q2measurements of the neutron and 3He spin structure

Thomas Jefferson National Accelerator Facility experiment E97-110 was performed to provide a precise measurement of the extended Gerasimov-Drell-Hearn integral and of moments of the neutron and of the 3He spin structure functions. The momentum transfer range 0.02 < Q2 < 0.3 (GeV/c)2 will allow us to test predictions of Chiral Perturbation Theory, and chek the GDH sum rule by extrapolating the integral to the real photon point. The data have been taken in Hall A using a highly polarized electron beam and a polarized 3He target. The status of the data analysis is discussed, and preliminary results are shown.

Proton Spin Structure in the Resonance Region

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085 GeV<W<1.910 GeV) at an average four momentum transfer of Q2=1.3 GeV2. Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A|| and A(perpendicular) to high precision, and extracted the asymmetries A1 and A2, and the spin structure functions g1 and g2. We found a notably nonzero A(perpendicular), significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

Development of a high average current polarized electron source with long cathode operational lifetime

Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over $2\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\mathrm{C}/{\mathrm{cm}}^{2}$ and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously for many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.

Physics with CLAS at Jefferson Lab

The CLAS collaboration at Jefferson Lab is engaged in a wide range of experiments, covering mostly meson and baryon spectroscopy, nucleon structure through elastic and deep inelastic scattering, nuclear transparency and nucleon correlations in nuclei. These experiments use the CEBAF highly polarized electron beam, or the secondary tagged photon beam, together with the CLAS detector (CEBAF Large Acceptance Spectrometer), to which specific experiments bring additional equipment. In this talk, examples of recent results on subjects mentioned hereabove will be given, with special emphasis on nucleon structure. A short description of the planned upgrade from CLAS to CLAS12 is presented.

Passive magnetic shielded spin polarized electron source with optical electron polarimeter

A new GaAs(100) spin polarized electron source with an optical polarimeter, which is employed in the field of polarized electron and gas atom collision, is presented in detail. The apparatus is passive-magnetic-shielded by a box and a cylinder made of nickel–iron–molybdenum soft magnetic alloy without Helmholtz coil arrangement. And a uniformly distributed residual magnetic field of less than 5×10−7 T is obtained near the collision area. The spin polarized electron beam is transmitted and focused onto collision point from photocathode by a set of electron optics with more than 25% transmission 95 cm distance through an 1 mm diameter aperture. Construction and operation of the apparatus, such as vacuum and magnetic shielding system, photocathode, laser optics, electron optics and polarimeter are discussed. The polarization of the spin polarized electron beam is determined to be 30.8±3.5% measured with a He optical polarimeter.

CP violation and electric-dipole-moment at low energy τ production with polarized electrons

The new proposals for high luminosity B/Flavor factories, near and on top of the ϒ resonances, allow for a detailed investigation of CP-violation in the τ-pair production. In particular, bounds on the tau electric dipole moment can be obtained of genuine CP-odd observables related to the τ-pair production. We perform an independent analysis from low energy (10 GeV) data by means of linear spin observables. We show that for a longitudinally polarized electron beam a CP-odd asymmetry, associated to the normal polarization term, can be measured at these low energy facilities both at resonant and non-resonant energies. In this way stringent and independent bounds to the tau electric dipole moment, which are orders of magnitude below other high or low energy previous bounds, can be obtained.

Measurement of the x- and Q2-dependence of the asymmetry A1 on the nucleon

We report results for the virtual photon asymmetry $A_1$ on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ($^{15}$NH$_3$) and deuteron ($^{15}$ND$_3$) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present paper, we concentrate on our results for $A_1(x,Q^2)$ and the related ratio $g_1/F_1(x,Q^2)$ in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer $Q^2$ from 0.05 to 5.0 GeV$^2$ and in final-state invariant mass $W$ up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong $Q^2$--dependence of $A_1(x,Q^2)$ for $W$ below 2 GeV. At higher $W$, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but $A_1(x,Q^2)$ is not strictly $Q^2$--independent. We add significantly to the world data set at high $x$, up to $x = 0.6$. Our data exceed the SU(6)-symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative $d$-quark polarization up to our highest $x$. This data setshould improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.

Polarization effects ine++e−→d¯+dand determination of time-like deuteron form factors

Polarization effects in the reaction $e^++e^-\to \bar d+d$ have been investigated for the case of longitudinally polarized electron beam and arbitrary polarization of the produced deuteron, with the aim of a determination of the time-like complex deuteron electromagnetic form factors. General expressions of polarization observables are derived and numerical estimations have been carried out by means of various models of deuteron electromagnetic form factors, for kinematical conditions near threshold.

Space–time structure of new physics with polarized beams at the linear collider

We approach the issue of the discovery of new physics at high energies associated with the proposed International Linear Collider in the presence of longitudinal as well as transverse electron and positron beam polarization. We determine the beam polarization dependence and the angular distribution of a particle of arbitrary spin in a one-particle inclusive final state produced in e+e- collisions through the interference of γ or Z amplitude with the amplitude from new interactions having arbitrary space–time structure. We thus extend the results of Dass and Ross, proposed at the time of the discovery of neutral currents, to beyond the standard model currents. We also extend the case of e+e- annihilation in the s-channel to the production of bosons due to t- and u-channel processes. Our work provides an approach to model-independent determination of the space–time structure of beyond the standard model interactions. We briefly discuss applications of the framework to popular extensions of the standard model, and demonstrate that our framework is general enough to account for certain results in the minimal supersymmetric standard model.

Experimental study on helium optical electron polarimetry

Optical electron polarimetry is suitable for calibration of a spin-polarized electron source, especially for measurement of polarization of spin-polarized electron beam. In this paper, a new optical electron polarimeter is described, which is based on the circularly polarized He radiation induced by the bombarding of He atoms with spin-polarized electrons. The theoretical basis of the optical electron polarimetry and the structure of the optical electron polarimeter are discussed. The measurement of polarization of spin-polarized electrons produced from a new GaAs (100) spin-polarized electron source is carried out. The result of polarization of 30.8% for our spin-polarized electron source is obtained using the He optical electron polarimeter.

Single production of the gauge bosonsW±via polarizede−γcollisions in the littlest Higgs model

In the framework of the littlest Higgs($LH$) model, we study single production of the standard model($SM$) gauge boson $W_{s}^{-}$ and the heavy gauge boson $W_{H}^{-}$ via polarized $e^{-}\gamma$ collisions. We find that the corrections of the $LH$ model to the cross section $\sigma(W_{s}^{-})$ might be observed only for the scale parameter $f\leq 1.5TeV$ and the mixing parameter $c'\geq 0.4$ in future high energy linear $e^{+}e^{-}$ collider($LC$) experiment with the center-of-mass($CM$) energy $\sqrt{S}=500GeV$ and a yearly integrated luminosity $\pounds=100fb^{-1}$. However, with a suitably chosen polarization of the initial electron and position beams, the possible signals of the heavy gauge boson $W_{H}^{-}$ can be easily detected via $e^{-}\gamma$ collisions in future $LC$ experiment with $\sqrt{S}=3TeV$ and $\pounds=500fb^{-1}$.

Photon polarization and spin asymmetry in the elementary process of bremsstrahlung

Experimental and theoretical results are reviewed concerning the photon polarization and spin asymmetry in the elementary process of bremsstrahlung. In electron–photon coincidence experiments using an unpolarized primary beam (300 keV) the electron–nucleus bremsstrahlung was found to be almost completely linearly polarized. The same behavior was found in electron–electron bremsstrahlung. By using a transversely polarized electron beam the photon emission asymmetry was measured for fixed direction of the outgoing electrons.

Accurate measurement of the electron beam polarization in JLab Hall A using Compton polarimetry

A major advance in accurate electron beam polarization measurement has been achieved at Jlab Hall A with a Compton polarimeter based on a Fabry–Perot cavity photon beam amplifier. At an electron energy of 4.6 GeV and a beam current of 40 μ A, a total relative uncertainty of 1.5% is typically achieved within 40 min of data taking. Under the same conditions monitoring of the polarization is accurate at a level of 1%. These unprecedented results make Compton polarimetry an essential tool for modern parity-violation experiments, which require very accurate electron beam polarization measurements.

Single-spin asymmetries in the Bethe-Heitler process e − + p → e − + γ + p induced by loop corrections

The single-spin beam and target asymmetries in the hard electroproduction process e − + p → e − + γ + p induced by the loop radiative corrections to the vertex part of lepton interaction are considered. The physical reason for the appearance of such asymmetries is the nonzero imaginary part of the amplitude (on the level of radiative corrections) caused by diagrams with photon radiation from the outgoing electron. We calculate the single-spin beam and target asymmetries at a longitudinally polarized electron beam or at arbitrary polarizations of the target proton for the CLAS and HERMES experimental conditions.

Recent Results from BLAST

The Bates Large Acceptance Spectrometer Toroid experiment, BLAST, at the MIT-Bates Linear Accelerator Laboratory is designed to study in a systematic manner the spindependent electromagnetic interaction in few-nucleon systems at momentum transfers below 1 GeV/c. Utilizing a polarized electron beam, highly polarized internal gas targets of H and D, and a symmetric detector configuration, BLAST is able to make simultaneous measurements of several reaction channels for different combinations of beam helicity and target polarization (vector for H, both vector and tensor for D). BLAST will provide new data on the nucleon and deuteron form factors as well as study few body physics and pion production. Preliminary results are presented.

Relativistically invariant analysis of polarization effects in exclusive deuteron electrodisintegration process

A general formalism for the calculation of the differential cross-section and polarization observables, for the process of deuteron electrodisintegration, is developed in the framework of relativistic impulse approximation. A detailed analysis of the general structure of the differential cross-section and polarization observables for the e − + d → e − + n + p reaction is derived, using the formalism of the structure functions. The obtained expressions have a general nature and they hold in the one-photon-exchange mechanism, assuming P-invariance of the hadron electromagnetic interaction. The model of relativistic impulse approximation is introduced and the final state interaction is taken into account by means of the unitarization of the helicity amplitudes. A detailed description of the unitarization procedure is also presented. Using different parametrizations of the deuteron wave functions, the following polarization observables are calculated in the kinematical region of quasi-elastic deuteron electrodisintegration: the asymmetry for the scattering of longitudinally polarized electrons on a polarized deuteron target, the proton and neutron polarizations (for longitudinally polarized electron beam or vector-polarized deuteron target). The sensitivity to the neutron electric form factor is also thoroughly investigated. The predictions of the model are compared with the results of recent polarization measurements and a good agreement with the existing experimental data has been obtained.

Mapping the spin-dependent electron reflectivity of Fe and Co ferromagnetic thin films

Spin Polarized Low Energy Electron Microscopy is used as a spin dependent spectroscopic probe to study the spin dependent specular reflection of a polarized electron beam from two different magnetic thin film systems: Fe/W(110) and Co/W(110). The reflectivity and spin-dependent exchange-scattering asymmetry are studied as a function of electron kinetic energy and film thickness, as well as the time dependence. The largest value of the figure of merit for spin polarimetry is observed for a 5 monolayer thick film of Co/W(110) at an electron kinetic energy of 2eV. This value is 2 orders of magnitude higher than previously obtained with state of the art Mini-Mott polarimeter. We discuss implications of our results for the development of an electron-spin-polarimeter using the exchange-interaction at low energy.