95 research outputs found
Deviation from standard QED at large distances: influence of transverse dimensions of colliding beams on bremsstrahlung
The radiation at collision of high-energy particles is formed over a rather
long distances and therefore is sensitive to an environment. In particular the
smallness of the transverse dimensions of the colliding beams leads to
suppression of bremsstrahlung cross section for soft photons. This beam-size
effect was discovered and investigated at INP, Novosibirsk around 1980. At that
time an incomplete expression for the bremsstrahlung spectrum was calculated
and used because a subtraction associated with the extraction of pure
fluctuation process was not performed. Here this procedure is done. The
complete expression for the spectral-angular distribution of incoherent
bremsstrahlung probability is obtained. The case of Gaussian colliding beams is
investigated in details. In the case of flat beams the expressions for the
bremsstrahlung spectrum are simplified essentially. Comparison of theory with
VEPP4 and HERA data is performed. Possible application of the effect to linear
collider tuning is discussed.Comment: 23 pagers,5 figure
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
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
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
Polarization effects for pair creation by photon in oriented crystals at high energy
Pair creation by a photon in an oriented crystal is considered in a frame of
the quasiclassical operator method, which includes processes with polarized
particles. Under some quite generic assumptions the general expression is
derived for the probability of pair creation of longitudinally polarized
electron (positron) by circularly polarized photon in oriented crystal. In
particular cases and
( is the angle of incidence, angle between the momentum of initial
photon and axis (plane) of crystal, is the scale of a potential of axis
or a plane relative to which the angle is defined) one has
constant field approximation and the coherent pair production theory
correspondingly. Side by side with coherent process the probability of
incoherent pair creation is calculated, which differs essentially from
amorphous one. At high energy the pair creation in oriented crystals is
strongly enhanced comparing with amorphous medium. In appendixes the integral
polarization of positron is found in external field and for coherent and
incoherent mechanisms.Comment: 24 pages, 4 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
Electroproduction of electron-positron pair in oriented crystal at high energy
The process of electroproduction of the electron-positron pair by high energy
electron in an oriented single crystal is investigated. Two contributions are
considered: the direct (one-step) process via the virtual intermediate photon
and the cascade(two-step) process when the electron emits the real photon
moving in the field of axis and afterwards the photon converts into the pair.
The spectrum of created positron(electron) is found. It is shown that the
probability of the process is strongly enhanced comparing with the
corresponding amorphous medium.Comment: 15 pages, 7 figure
Transition radiation as a source of quasi-monochromatic X-rays
Transition radiation (TR) from ultrarelativistic particles is considered. It
is shown that performing collimation of the TR from the periodic N-foil stack
(parameters of which are selected in a appropriate manner) one obtains the
spectrum of the TR which has a form of a peak position of which
depends on the plasma frequency and the thickness of the radiator foils. The
height and width of the peak depend on the collimation angle . The
height of the peak for given is proportional to N. Selecting
parameters one can have the source of X-rays of desired frequency with rather
good monochromaticity.Comment: LaTeX 2.09, 12 pages, 3 eps figure
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