5,953 research outputs found
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
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
Spectra of radiation and created particles at intermediate energy in oriented crystal taking into account energy loss
The spectral distribution of positron created by photon and the spectral
distribution of photons radiated from electron in an oriented single crystal of
intermediate thickness is calculated at intermediate energies. The energy loss
of charged particles as well as photon absorption are taken into account. The
used basic probabilities of processes include the action of field of axis as
well as the multiple scattering of radiating electron or particles of the
created pair (the Landau-Pomeranchuk-Migdal (LPM) effect)Comment: 11 pages, 5 figures, report at "Channeling 2008" Workshop, Erice,
Italy, October 200
Jet quenching in thin plasmas
We investigate the energy loss of quarks and gluons produced in hard
processes resulting from final state rescatterings in a finite quark-gluon
plasma. The angular distribution of the soft gluon bremsstrahlung induced by
n_s=1 rescatterings in the plasma is computed in the Gyulassy-Wang model.
Special focus is on how the interference between the initial hard radiation
amplitude, the multiple induced Gunion-Bertsch radiation amplitudes, and gluon
rescattering amplitudes modifies the classical parton cascade results.Comment: 4 pages in Latex and 3 EPS figures, Proceedings of the Quark
Matter'99 Conference, 10-15 May, 1999, Torin
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
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
Deciphering the properties of the medium produced in heavy ion collisions at RHIC by a pQCD analysis of quenched large spectra
We discuss the question of the relevance of perturbative QCD calculations for
analyzing the properties of the dense medium produced in heavy ion collisions.
Up to now leading order perturbative estimates have been worked out and
confronted with data for quenched large hadron spectra. Some of
them are giving paradoxical results, contradicting the perturbative framework
and leading to speculations such as the formation of a strongly interacting
quark-gluon plasma. Trying to bypass some drawbacks of these leading order
analysis and without performing detailed numerical investigations, we collect
evidence in favour of a consistent description of quenching and of the
characteristics of the produced medium within the pQCD framework.Comment: 10 pages, 3 figure
Beam-size effect in Bremsstrahlung
The smallness of the transverse dimensions of the colliding beams leads to
suppression of bremsstrahlung cross section. This beam-size effect was
discovered and investigated at INP, Novosibirsk. Different mechanisms of
radiation are discussed. Separation of coherent and incoherent radiation is
analyzed in detail. For linear collider this suppression affects the whole
radiation spectrum. It is shown that objections to the subtraction procedure in
\cite{BK2} are groundless.Comment: Plenary talk at the Workshop "Quantum Aspects of Beam Physics",
Hiroshima, Japan, January 7-11, 2003. To be pulished in the Proceedings
(World Scientific, Singapore
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