90 research outputs found
Abelian Landau-Pomeranchuk-Migdal effects
It is shown that the high-energy expansion of the scattering amplitude
calculated from Feynman diagrams factorizes in such a way that it can be
reduced to the eikonalized form up to the terms of inverse power in energy in
accordance with results obtained by solving the Klein-Gordon equation.
Therefore the two approaches when applied to the suppression of the emission of
soft photons by fast charged particles in dense matter should give rise to the
same results. A particular limit of thin targets is briefly discussed.Comment: 14 pages, LATEX, 1 Fig. ps, submitted to Mod. Phys. Lett.
Experimental Research of the Diffraction and Vavilov-Cherenkov Radiation Generation in a Teflon Target
Geometry of Vavilov-Cherekov (VChR) radiation when an electron moves close to
a dielectric target is in analogy to diffraction radiation (DR) geometry. In
this case we may expect DR generation from the upstream face of the target
besides that VChR. The joint observation of these booth types of radiation is
very interesting from the pseudo-photon viewpoint, which is applicable for
relativistic electrons. Unexpected results obtained in our experiment insist on
reflection about nature both DR and VChR. The experiment was performed on the
relativistic electron beam of the microtron of Tomsk Polytechnic University.Comment: This article will be published in Journal of Physic
Coherent Bremsstrahlung in Imperfect Periodic Atomic Structures
Coherent bremsstrahlung of high energy electrons moving in a
three-dimensional imperfect periodic lattice consisting of a complicated system
of atoms is considered. On the basis of the normalized probability density
function of the distribution of atomic centers in the fundamental cell the
relations describing coherent and incoherent contributions into cross sections
are obtained. In particular, the cross section of coherent bremsstrahlung in
complex polyatomic single crystals is found.
The peculiarities of formation and possibilities of utilization of coherent
processes are discussed.Comment: 23 pages, 8 figures, to be published in Phys. Rev.
Multi-photon effects in energy losses spectra
Effect of radiation of many photons by a single electron traversing a target
is discussed. When the summary energy of emitted photons (the energy losses
spectrum) is measured only, the photon spectrum is distorted comparing with the
photon spectrum in one interaction. Influence of this effect is discussed for
the cases (1) bremsstrahlung (described by Bethe-Heitler formula), (2) the
strong Landau-Pomeranchuk-Migdal effect and (3) transition radiation.
Qualitative picture of the phenomenon is discussed in detail. Comparison with
the recent SLAC experiment in relatively thick target (2.7% of the radiation
length), where the effect of emission of many photons by a projectile is very
essential, shows perfect agreement of the theory and data.Comment: LaTeX2.09, 19 pages, 5 PostScript figure
Effective dynamics of a nonabelian plasma out of equilibrium
Starting from kinetic theory, we obtain a nonlinear dissipative formalism
describing the nonequilibrium evolution of scalar colored particles coupled
selfconsistently to nonabelian classical gauge fields. The link between the
one-particle distribution function of the kinetic description and the variables
of the effective theory is determined by extremizing the entropy production.
This method does not rely on the usual gradient expansion in fluid dynamic
variables, and therefore the resulting effective theory can handle situations
where these gradients (and hence the momentum-space anisotropies) are expected
to be large. The formalism presented here, being computationally less demanding
than kinetic theory, may be useful as a simplified model of the dynamics of
color fields during the early stages of heavy ion collisions and in phenomena
related to parton energy loss.Comment: 20 two-column pages, 2 figures. v3: minor changes. Accepted for
publication in Phys. Rev.
Exotic radiation from a photonic crystal excited by an ultra-relativistic electron beam
We report the observation of an exotic radiation (unconventional
Smith-Purcell radiation) from a one-dimensional photonic crystal. The physical
origin of the exotic radiation is direct excitation of the photonic bands by an
ultra-relativistic electron beam. The spectrum of the exotic radiation follows
photonic bands of a certain parity, in striking contrast to the conventional
Smith-Purcell radiation, which shows solely a linear dispersion. Key
ingredients for the observation are the facts that the electron beam is in an
ultra-relativistic region and that the photonic crystal is finite. The origin
of the radiation was identified by comparison of experimental and theoretical
results.Comment: 4 pages, 5 figure
Tensor polarization of deuterons passing through matter
It is shown that the magnitude of tensor polarization of the deuteron beam,
which arises owing to the spin dichroism effect, depends appreciably on the
angular width of the detector that registers the deuterons transmitted through
the target. Even when the angular width of the detector is much smaller than
the mean square angle of multiple Coulomb scattering, the beam's tensor
polarization depends noticeably on rescattering. When the angular width of the
detector is much larger than the mean square angle of multiple Coulomb
scattering (as well as than the characteristic angle of elastic nuclear
scattering), tensor polarization is determined only by the total reaction cross
sections for deuteron-nucleus interaction, and elastic scattering processes
make no contribution to tensor polarization.Comment: 18 pages, 3 figures, to be published in IO
Density Matrix Kinetic Equation Describing a Passage of Fast Atomic Systems Through Matter
The quantum-mechanical consideration of a passage of fast dimesoatoms through
matter is given. A set of quantum-kinetic equations for the density matrix
elements describing their internal state evolution is derived. It is shown that
probabilistic description of internal dynamics of hydrogen-like atoms is
impossible even at sufficiently low energies because of the ``accidental''
degeneracy of their energy levels.Comment: 12 pages, LATEX, submitted to J. Phys.
The Landau-Pomeranchuk-Migdal effect and transition radiation in structured targets
The radiation from high-energy electrons is investigated for the case when a
target consists of several separated plates. The spectrum of radiation is
considered in the region in which the bremsstrahlung is under influence of the
multiple scattering of a projectile (the LPM effect), the polarization of a
medium and the hard part of the boundary radiation contribute. In this region
the general expression for the radiation spectrum is obtained for the -plate
target. A qualitative description of the arising interference pattern is given.Comment: LaTeX2.09, 22 pages, 3 PostScript figure
Observation of Resonant Diffusive Radiation in Random Multilayered Systems
Diffusive Radiation is a new type of radiation predicted to occur in randomly
inhomogeneous media due to the multiple scattering of pseudophotons. This
theoretical effect is now observed experimentally. The radiation is generated
by the passage of electrons of energy 200KeV-2.2MeV through a random stack of
films in the visible light region. The radiation intensity increases resonantly
provided the Cherenkov condition is satisfied for the average dielectric
constant of the medium. The observed angular dependence and electron resonance
energy are in agreement with the theoretical predictions. These observations
open a road to application of diffusive radiation in particle detection,
astrophysics, soft X-ray generation and etc.. `Comment: 4pages, 4figure
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