101 research outputs found
Generation of circularly polarized photons for a linear collider polarized positron source
Various methods of obtaining longitudinally polarized positrons for future
linear colliders are reviewed. Special attention is paid to the schemes using
circularly polarized high-energy photons for positron production. Most
effectively such photons are obtained from electrons passing through a helical
undulator or colliding with a circularly polarized laser wave. Spectrum and
polarization of radiation emitted during helical motion of electrons are
considered in detail. A new simple presentation of known formulas is used to
account for the influence of the wave intensity, of the electron-beam angular
divergence, of the collimation of radiation, and of the lateral and temporal
profiles of the laser bunch on the radiation properties.Comment: 16 pages, 6 figure
Emission of polarized photons from unpolarized electrons moving in crystals
Radiation emitted by unpolarized high-energy electrons penetrating crystals
may be linearly polarized. This occurs when the particle velocity makes an
angle, with respect to some major crystal axis, being sufficiently larger than
the axial-channelling angle. For such orientation, a complete description of
spectral and polarization characteristics of the radiation is derived. At
planar channelling, a non-perturbative contribution to the probability of the
process appears caused by the plane field, and we must solve exactly a
one~-~dimensional mechanical problem. For that, the approximate form of the
actual plane potential is suggested which provides a precise fit for any
crystal plane and an analytical solution to the motion problem. In a practical
case, we must consider electron-photon showers developing in sufficiently thick
crystals. For the first time, this development is described taking into account
the polarization of photons. We discuss qualitative features of the phenomenon,
present results of numerical calculations for thin and thick crystals, and
evaluate the possibility of the use of differently oriented crystals in a
polarized hard photon source.Comment: 16 pages, 7 PostScript figure
Coulomb effects in the spin-dependent contribution to the intra-beam scattering rate
Coulomb effects in the intra-beam scattering are taken into account in a way
providing correct description of the spin-dependent contribution to the beam
loss rate. It allows one to calculate this rate for polarized beams
at arbitrarily small values of the ratio ,
characterizing relative change of the electron energy in the laboratory system
during scattering event.Comment: 8 pages, 2 figure
Photon splitting in atomic fields
Photon splitting due to vacuum polarization in the electric field of an atom
is considered. We survey different theoretical approaches to the description of
this nonlinear QED process and several attempts of its experimental
observation. We present the results of the lowest-order perturbation theory as
well as those obtained within the quasiclassical approximation being exact in
the external field strength. The experiment where photon splitting was really
observed for the first time is discussed in details. The results of this
experiment are compared with recent theoretical estimations.Comment: 45 pages, 24 figure
Comparison of theory with experiment for positron production from high-energy electrons moving along crystal axes
Various positron distributions are obtained using an approach developed
earlier for the description of electron-photon showers in axially aligned
single crystals. Based on these distributions, characteristics of the positron
yield measured in recent experiments are calculated. Theoretical estimations
display a rather good agreement with experimental results obtained using 3 to
10 GeV electrons aligned to the - axis of the tungsten crystals.Comment: 10 pages, 6 Postscript figure
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