4,115 research outputs found
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
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
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
Delbr\"uck scattering in combined Coulomb and laser fields
We study Delbr\"uck scattering in a Coulomb field in the presence of a laser
field. The amplitudes are calculated in the Born approximation with respect to
the Coulomb field and exactly in the parameters of the laser field having
arbitrary strength, spectral content and polarization. The case of high energy
initial photon energy is investigated in detail for a monochromatic circularly
polarized laser field. It is shown that the angular distribution of the process
substantially differs from that for Delbr\"uck scattering in a pure Coulomb
field. The value of the cross section under discussion may exceed the latter at
realistic laser parameters that essentially simplify the possibility of the
experimental observation of the phenomenon. The effect of high order terms in
the quantum intensity parameter of the laser field is found to be very
important already at relatively small .Comment: 21 pages, 4 figure
Partonic Energy Loss and the Drell-Yan Process
We examine the current status of the extraction of the rate of partonic
energy loss in nuclei from A dependent data. The advantages and difficulties of
using the Drell-Yan process to measure the energy loss of a parton traversing a
cold nuclear medium are discussed. The prospects of using relatively low energy
proton beams for a definitive measurement of partonic energy loss are
presented.Comment: 12 pages, 2 figure
Laser-dressed vacuum polarization in a Coulomb field
We investigate quantum electrodynamic effects under the influence of an
external, time-dependent electromagnetic field, which mediates dynamic
modifications of the radiative corrections. Specifically, we consider the
quantum electrodynamic vacuum-polarization tensor under the influence of two
external background fields: a strong laser field and a nuclear Coulomb field.
We calculate the charge and current densities induced by a nuclear Coulomb
field in the presence of a laser field. We find the corresponding induced
scalar and vector potentials. The induced potential, in first-order
perturbation theory, leads to a correction to atomic energy levels. The
external laser field breaks the rotational symmetry of the system.
Consequently, the induced charge density is not spherically symmetric, and the
energy correction therefore leads to a "polarized Lamb shift." In particular,
the laser generates an additional potential with a quadrupole moment. The
corresponding laser-dressed vacuum-polarization potential behaves like 1/r**3
at large distances, unlike the Uehling potential that vanishes exponentially
for large r. Our investigation might be useful for other situations where
quantum field theoretic phenomena are subjected to external fields of a rather
involved structure.Comment: 13 pages, RevTe
Sample dispersion in isotachophoresis with Poiseuille counterflow
A particular mode of isotachophoresis (ITP) employs a pressure-driven flow
opposite to the sample electromigration direction in order to anchor a sample
zone at a specific position along a channel or capillary. We investigate this
situation using a two-dimensional finite-volume model based on the
Nernst-Planck equation. The imposed Poiseuille flow profile leads to a
significant dispersion of the sample zone. This effect is detrimental for the
resolution in analytical applications of ITP. We investigate the impact of
convective dispersion, characterized by the area-averaged width of a sample
zone, for various values of the sample P\'{e}clet-number, as well as the
relative mobilities of the sample and the adjacent electrolytes. A
one-dimensional model for the area-averaged concentrations based on a
Taylor-Aris-type effective axial diffusivity is shown to yield good agreement
with the finite-volume calculations. This justifies the use of such simple
models and opens the door for the rapid simulation of ITP protocols with
Poiseuille counterflow
Parton energy loss due to synchrotron-like gluon emission
We develop a quasiclassical theory of the synchrotron-like gluon radiation.
Our calculations show that the parton energy loss due to the synchrotron gluon
emission may be important in the jet quenching phenomenon if the plasma
instabilities generate a sufficiently strong chromomagnetic field. Our gluon
spectrum disagrees with that obtained by Shuryak and Zahed within the
Schwinger's proper time method.Comment: 11 pages, 3 eps figure
Medium-modified fragmentation of b-jets tagged by a leading muon in ultrarelativistic heavy ion collisions
The possibility to observe the medium-modified fragmentation of hard b-quarks
tagged by a leading muon in ultrarelativistic heavy ion collisions is analyzed.
We have found that reasonable statistics, ~20000 events per 1 month of LHC run
with lead beams, can be expected for the realistic geometrical acceptance and
kinematic cuts. The numerical estimates on the effect of the medium-induced
softening b-jet fragmentation function are given.Comment: 12 pages in LaTeX (including 3 figures in EPS-format
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