Simulation studies on the polarized positron generation from circularly polarized gamma-rays

Polarized positrons can be generated through the electron-positron pair creation from circularly polarized gamma-rays hitting a conversion target. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. Simulation studies on these two processes have been done with the Monte Carlo codes, CAIN and GEANT4. Using CAIN to simulate the Laser-Compton scattering process, the energy spectrum of the generated polarized photons could be obtained. GEANT4 was used to study the yield, energy spectrum and the mean polarization of the positrons emanating from the conversion target. To increase the yield of the generated positrons, an optimization study on the thickness of conversion target was also performed.     

Forward circularly polarized photon emission in longitudinally polarized e+e− collisions

We discuss the emission of circularly polarized forward photons in high energy e⁺e⁻ collisions with longitudinally polarized beams. We find substantial asymmetry under inversion of longitudinal electron polarization, thus supporting the proposal that measurement of circular polarization of forward emitted photons may allow for monitoring of longitudinal beam polarization.     

129Xe NMR – highly polarized thin films

We studied the macroscopic effects of nuclear magnetization. Highly polarized xenon is often used to increase the sensitivity in NMR investigations of porous media, diluted liquids or for imaging in the gas phase. In the condensed phase, however, highly nuclear spin polarized xenon also possesses a sizable magnetization due to the nuclear spin density. This results in an additional magnetic field, that is used to measure the polarization of the sample, when only the particle density is known. Here we find P_z≈0.8 corresponding to a spin temperature of 0.5 mK. We use isotopically enriched xenon with a ¹²⁹Xe abundance of 0.71. At high abundance of ¹²⁹Xe and high nuclear polarization the dipolar linewidth is considerably reduced. We find for small angle excitation a reduction from 650 Hz to 400 Hz. We investigate this using a thin film geometry. The susceptibility effects of the substrate and the Xe film are treated. The macroscopic angle between the normal of the film and the external field strongly changes the polarization induced line shift and line width. The first follows an expected cos²θ dependence with an understood amplitude the latter however is not understood up to now. Relaxation of ¹²⁹Xe in the condensed film is observed to be T₁=15±1.8 min, much faster than expected. To cite this article: P. Gerhard et al., C. R. Physique 5 (2004).     

Depolarization of backscattered linearly polarized light from ZnO thin film

The depolarization behavior of backscattered linearly polarized light from ZnO thin film was investigated experimentally.The results show that the characteristics are related to both the polarization orientation and wavelength of linearly polarized incident light.When the incident light is s-polarized,the depo- larization behaviors are different for different wavelengths.When the incident light is p-polarized,the depolarization behaviors,on the contrary,are similar for different wavelengths.In addition,there is an optimal incident angle for depolarization of linearly polarized light with different wavelengths,which is equal to their effective Brewster angles,respectively.     

Can a linearly polarized light beam polarize atomic spin?

Lax et al. [Phys. Rev. 11 (1975) 1365] discovered that a light beam in vacuum is not a transverse wave but does have a longitudinal field component. We investigate atomic and molecular electric dipole transitions induced by such a light beam, in particular, linearly polarized in a transverse plane. We derive the selection rules and the transition rates for various quantization axes using the paraxial approximation up to the first order of 1/kw, where k is the wave number and w is the transverse size of the light beam. The light beam is able to yield atomic spin polarization in the direction perpendicular to both the optical axis and the transverse electric field, and its magnitude is approximately 1/kw times that generated by a circularly polarized light wave with the similar intensity.     

Circularly polarized localized near-field radiation at the nanoscale

Diffraction-limited circularly polarized electromagnetic radiation has been widely used in the literature for various applications at both optical and microwave frequencies. With advances in nanotechnology, emerging plasmonic nano-optical applications, such as all-optical magnetic recording, require circularly polarized electromagnetic radiation beyond the diffraction limit. In this study, a plasmonic cross-dipole nano-antenna is investigated to obtain a circularly polarized near-field optical spot with a size smaller than the diffraction limit of light. A cross-dipole nano-antenna is composed of four metallic nano-rods placed at a perpendicular orientation with respect to each other. The performance of the nano-antenna is investigated through numerical simulations. In the first part of this study, the nano-antenna is illuminated with a diffraction-limited circularly polarized radiation. An optimal antenna geometry is specified to obtain an intense optical spot that satisfies two necessary conditions for circular polarization: a phase difference of 90° and a unit amplitude ratio between the electric field components in the vicinity of the antenna gap. In the second part of this study, the nano-antenna is illuminated with diffraction-limited linearly polarized radiation. It is shown that the phase difference between the electric field components can be adjusted by selecting either different antenna lengths or different gap distances in the vertical and horizontal directions. Due to the relatively short wavelength of surface plasma waves on the antenna, it is demonstrated that the phase difference can be sufficient to obtain circularly polarized light. An optimal physical configuration for the nano-antenna and the polarization angle of the incident light is identified to obtain a circularly polarized optical spot beyond the diffraction limit from diffraction-limited linearly polarized radiation.     

Tight focusing properties of radially polarized Lorentz–Gaussian beam

Focusing properties of radially polarized Lorentz–Gauss beam with one on-axis optical vortex was investigated by vector diffraction theory. Results show that intensity distribution in the focal region can be altered considerably by charge number of the optical vortex and the beam parameters. Many novel focal patterns may occur, Such as Peak-centered, donut focal shapes which is potentially useful in optical tweezers, material processing and laser printing.     

Two-loop electroweak vertex corrections for polarized Møller scattering

The contributions to the electron-electron-photon vertex of two-loop electroweak corrections are calculated. The relative correction to the parity-violating asymmetry of Møller scattering for the case of 11 GeV electron scattered off the electron at rest is found to be about–0.0034 and should be taken into account at future experiment MOLLER at JLab.     

Azimuthal distributions in radiative decay of a polarized τ lepton

We investigate various distributions over emitted photon angles, especially over the azimuthal angle, in the one-meson radiative decay of the polarized τ lepton, τ^– → π^–γν_τ. In connection with this, the photon phase space is discussed in more detail because it is nontrivial in the case of a polarized τ lepton. The decay matrix element contains both the inner bremsstrahlung and the resonance (structural) contributions. The azimuthal dependence of some observables are calculated. They are the asymmetry of the differential decay width caused by the τ-lepton polarization, the Stokes parameters of the emitted photon itself, and the correlation parameters describing the influence of τ-lepton polarization on the photon Stokes parameters. A numerical estimation is done in the τ-lepton rest frame for an arbitrary direction of the τ-lepton polarization 3-vector. The vector and axial-vector form factors describing the structure-dependent part of the decay amplitude are determined using the chiral effective theory with resonances (RχT). It is found that the features of the azimuthal distributions allows separating various terms in the spin-dependent contribution. The so-called up–down and right–left asymmetries are also calculated.     

Tensor polarized γ-deuteron Compton scattering in effective field theory

The differential cross section for γ-deuteron Compton scattering from a tensor polarized deuteron is computed in an effective field theory. The first non-vanishing contributions to this differential cross section are the interference terms between the leading electric coupling diagrams and the subleading single potential pion exchange diagrams or the subleading magnetic moment coupling diagrams. At 90° photon scattering angle, only the pion term contributes at this order to the tensor polarized differential cross section. This provides a clean way to study the photon pion dynamics in the two nucleon sector. The effect is measurable for photon energies between 40 and 80 MeV provided the uncertainty in the measured cross sections are ≲ 7%.     

Aspects of χ and ψ production in polarized proton-proton collisions

Several topics of relevance to low transverse momentum and _1,2(c ) production in polarized proton-proton collisions are discussed. The leadingO( _S ³ ) contributions to the lowp _{T 1} production cross-sections viagg, qg, andq initial states are calculated as well as the corresponding spin-spin asymmetries. We find that ₁ production increases relative to direct and ₂ production, providing up to 25% of the observablee ^* e ^– pairs arising from decays inpp collisions at s=500 GeV. The spin-dependence of ₁ production, however, is much smaller than for either direct or ₂ production and so will likely be far less useful than either process in probing the polarized gluon structure function of the proton. A subset of theO( _S ³ ) radiative corrections to ₂ production involving initial state quarks are also performed and compared to leading ordergg₂ predictions.     

Exchange effects during δ-isobar photoproduction on a polarized deuteron

Within the framework of the relativistic impulse approximation and the nucleon spectator model we have investigated the influence of exchange effects in the photoproduction of δ isobars on a polarized deuteron with subsequent decay into a proton and a π⁻-meson. We used two forms of propagator for the spinor-vector field of an δ isobar in the calculations. Nuclear Physics Research Institute, Tomsk Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 2, pp. 3–8, February, 1999.     

Studies of basic limitations on production, transport and acceleration of the high intensity polarized H− beam in the RHIC polarized ion source

Basic limitations on the high-intensity polarized H^? ion beam production and transport were experimentally studied in charge-exchange collisions of the neutral atomic hydrogen beam in the Na-vaporjet ionizer cell. The energy dependence of space-charge effects on the beam instabilities and losses were studied and described in the model of synthetic H⁺ — H^? beam transport. A xenon gas admixture to the H^? ion production cell and beam transport line greatly improves the space-charge compensation, which is also successfully described in the simulations. These studies are the part of the polarized source upgrade project for RHIC.     

Cross-relaxation processes of polarizedβ-active nuclei in various crystalline solids

Cross-processes of neutron activated, polarized nucleiI with surrounding stable host nucleiS are observed in diverse solids, using an in-beam NMR technique where the polarizedI nuclei are produced by capture of polarized thermal neutrons. The polarization is determined by measuring the 0°–180° asymmetry of the-decay radiation of these nuclei. The following cross-processes are studied:⁸Li–⁶Li in^natLi metal;⁸Li–⁷Li in⁷Li metal;⁸Li–⁶Li in^natLiF (single crystal);⁸Li–⁹³Nb in⁷LiNbO₃ (single crystal);²⁰F–¹⁹F in MgF₂ (single crystal). Two applications are presented: a) Determination of the temperature dependence of the electrical quadrupole coupling constant of⁹³Nb in LiNbO₃; b) Observation of radiation induced point defects in MgF₂ and their thermal annealing. All measurements are performed without any radiofrequency irradiation.     

The polarized spectrum of benzene-d 6 single crystal at 2000 Å

The Roothaan open-shell LCAO-MO-SCF method was coupled with the Simons model potential in calculations of internuclear distances and ionization potentials for Li₃, Na₃, K₃, Rb₃, and Cs₃ taking core polarization into account.     

Dual-band asymmetric transmission of linearly polarized wave using Π-shaped metamaterial

A bilayered chiral metamaterial with Π-shaped structure is proposed, which demonstrates to exhibit dual-band asymmetric transmission (AT) of linearly polarized electromagnetic (EM) waves in two opposite directions. Incident x-polarized wave is almost converted to y-polarized wave while incident y-polarized wave is locked through the Π-shaped slab at 10.82 GHz; incident y-polarized wave is almost converted to x-polarized wave while incident x-polarized wave cannot pass through the Π-shaped slab at 14.1 GHz. The property of the AT is similar to the diode-switching characteristics, and the EM wave can be switched on/off by changing the polarization state of the incident wave. The surface current distributions of the Π-shaped structure are discussed to look into the physical mechanism.     

Physics opportunities with polarized e − and e + beams at TESLA

Beam polarization at e⁺-e^– linear colliders will be a powerful tool for high precision analyses. Often it is assumed that the full information from polarization effects is provided by polarization of the electron beam and no further information can be obtained by the simultaneous polarization of the positrons. In this paper we point out the advantages of polarizing both beams, and summarize the polarization-related results of the Higgs, Electroweak, QCD, SUSY and Alternative Theories working groups of the ECFA/DESY workshop for a planned linear collider operating in the energy range s = 500-800 GeV.     

Circularly polarized sub-1.5 cycle laser pulses at 1.8 μm

The generation of intense carrier-envelope-phase-stabilized sub-1.5 cycle circularly polarized pulses at 1.8 μm is reported. After changing the polarization of the pulses produced by an optical parameter amplifier to circular, selected nonlinear medium parameters are found to be able to expand the spectrum to supercontinuum (1,300–2,100 nm) with a extremely high transmittance (>65 %). Using such laser pulses, polarization control of terahertz emission is demonstrated.