14,576 research outputs found
On propagation of high-energy photon in a medium in presence of an external field
The polarization tensor is calculated which originates from interaction of a
photon with the electron-positron field. The effects of multiple scattering of
electrons and positrons in a medium side by side with an external field are
included. The corresponding general representation of polarization tensor is
found in the form of two-dimentional integral. The both effects may be
essential for propagation of high-energy photons in oriented single crystals.Comment: 12 pages, 1 figur
Spectrum and polarization of coherent and incoherent radiation and the LPM effect in oriented single crystal
The spectrum and the circular polarization of radiation from longitudinally
polarized high-energy electrons in oriented single crystal are considered using
the method which permits inseparable consideration of both the coherent and the
incoherent mechanisms of photon emission. The spectral and polarization
properties of radiation are obtained and analyzed. It is found that in some
part of spectral distribution the influence of multiple scattering (the
Landau-Pomeranchuk-Migdal (LPM) effect) attains the order of 7 percent. The
same is true for the influence of multiple scattering on the polarization part
of the radiation intensity. The degree of circular polarization of total
intensity of radiation is found. It is shown that the influence of multiple
scattering on the photon polarization is similar to the influence of the LPM
effect on the total intensity of radiation: it appears only for relatively low
energies of radiating electron and has the order of 1 percent, while at higher
energies the crystal field action excludes the LPM effect.Comment: 13 pages, 7 figure
Coherent and incoherent pair creation by a photon in oriented single crystal
The new approach is developed for study of electron-positron pair production
by a photon in oriented single crystal. It permits indivisible consideration of
both coherent and incoherent mechanisms of pair creation and includes the
action of field of axis (or plane) as well as the multiple scattering of
particles of the created pair (the Landau-Pomeranchuk-Migdal (LPM) effect).
From obtained integral probability of pair creation, it follows that multiple
scattering appears only for relatively low energy of photon, while at higher
photon energy the field action excludes the LPM effect. The found results agree
quite satisfactory with recent CERN experiment.Comment: 10 pages, 3 figure
Pair creation by a photon in an electric field
The process of pair creation by a photon in a constant and homogeneous
electric field is investigated basing on the polarization operator in the
field. The total probability of the process is found in a relatively simple
form. At high energy the quasiclassical approximation is valid. The corrections
to the standard quasiclassical approximation (SQA) are calculated. In the
region relatively low photon energies, where SQA is unapplicable, the new
approximation is used. It is shown that in this energy interval the probability
of pair creation by a photon in electric field exceeds essentially the
corresponding probability in a magnetic field. This approach is valid at the
photon energy much larger than "vacuum" energy in electric field: . For smaller photon energies the low energy approximation is developed.
At the found probability describes the absorption of soft
photon by the particles created by an electric field.Comment: 15 pages, 6 figure
Quantum theory of transition radiation and transition pair creation
Theory of the transition radiation and the transition pair creation is
developed in the frame of QED. The spectral-angular distributions of
probability of the transition radiation and of the transition pair creation are
found. The total energy losses of and the total probability of pair creation
are calculated and analyzed. Features of radiation and pair creation processes
in a superdence medium (typical for white dwarfs) are discussed.Comment: LaTeX, 12 pages, 3 eps figure
Pair creation by a photon in a strong magnetic field
The process of pair creation by a photon in a strong magnetic field is
investigated basing on the polarization operator in the field. The total
probability of the process is found in a relatively simple form. The
probability exhibits a "saw-tooth" pattern because of divergences arising when
the electron and positron are created at threshold of the Landau energy levels.
The pattern will be washed out at averaging over any smooth photon energy
distribution. The new results are obtained in the scope of the quasiclassical
approach: 1) in the case when the magnetic field is the
critical field) the new formulation extends the photon energy interval to the
case when the created particles are not ultrarelativistic; 2) the correction to
the standard quasiclassical approximation is found showing the range of
applicability of the approach at high photon energy as well. The very important
conclusion is that for both cases and the results of
the quasiclassical calculation are very close to averaged probabilities of
exact theory in a very wide range of photon energies. The quasiclassical
approximation is valid also for the energy distribution if the electron and
positron are created on enough high levels.Comment: 21 pages, 6 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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