960 research outputs found
Polarization operator approach to electron-positron pair production in combined laser and Coulomb fields
The optical theorem is applied to the process of electron-positron pair
creation in the superposition of a nuclear Coulomb and a strong laser field. We
derive new representations for the total production rate as two-fold integrals,
both for circular laser polarization and for the general case of elliptic
polarization, which has not been treated before. Our approach allows us to
obtain by analytical means the asymptotic behaviour of the pair creation rate
for various limits of interest. In particular, we consider pair production by
two-photon absorption and show that, close to the energetic threshold of this
process, the rate obeys a power law in the laser frequency with different
exponents for linear and circular laser polarization. With the help of the
upcoming x-ray laser sources our results could be tested experimentally.Comment: 10 pages, 3 figure
Coulomb corrections and multiple e+e- pair production in ultra-relativistic nuclear collisions
We consider the problem of Coulomb corrections to the inclusive cross
section. We show that these corrections in the limiting case of small charge
number of one of the nuclei coincide with those to the exclusive cross section.
Within our approach we also obtain the Coulomb corrections for the case of
large charge numbers of both nuclei.Comment: 7 pages, REVTeX
Bremsstrahlung in alpha-Decay Reexamined
A high-statistics measurement of bremsstrahlung emitted in the alpha decay of
210Po has been performed, which allows to follow the photon spectra up to
energies of ~ 500 keV. The measured differential emission probability is in
good agreement with our theoretical results obtained within the quasi classical
approximation as well as with the exact quantum mechanical calculation. It is
shown that due to the small effective electric dipole charge of the radiating
system a significant interference between the electric dipole and quadrupole
contributions occurs, which is altering substantially the angular correlation
between the alpha particle and the emitted photon.Comment: 10 pages, 5 figures, v2: fix of small typo
Finite nuclear size and Lamb shift of p-wave atomic states
We consider corrections to the Lamb shift of p-wave atomic states due to the
finite nuclear size (FNS). In other words, these are radiative corrections to
the atomic isotop shift related to FNS. It is shown that the structure of the
corrections is qualitatively different from that for s-wave states. The
perturbation theory expansion for the relative correction for a -state
starts from -term, while for -states it starts
from term. Here is the fine structure constant and is
the nuclear charge. In the present work we calculate the -terms for
-states, the result for -state reads
. Even more interesting are
-states. In this case the ``correction'' is by several orders of
magnitude larger than the ``leading'' FNS shift.Comment: 4 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
On the nature of Coulomb corrections to the e^+e^- pair production in ultrarelativistic heavy-ion collisions
We manifest the origin of the wrong conclusion made by several groups of
authors on the absence of Coulomb corrections to the cross section of the
e^+e^- pair production in ultrarelativistic heavy-ion collisions. The source of
the mistake is connected with an incorrect passage to the limit in the
expression for the cross section. When this error is eliminated, the Coulomb
corrections do not vanish and agree with the results obtained within the
Weizs\"acker-Williams approximation.Comment: 7 pages, LaTe
Structure of the Coulomb and unitarity corrections to the cross section of pair production in ultra-relativistic nuclear collisions
We analyze the structure of the Coulomb and unitarity corrections to the
single pair production as well as the cross section for the multiple pair
production. In the external field approximation we consider the probability of
pair production at fixed impact parameter between colliding
ultra-relativistic heavy nuclei. We obtain the analytical result for this
probability at large as compared to the electron Compton wavelength. We
estimate also the unitary corrections to the total cross section of the
process.Comment: 10 pages, 2 figures, RevTeX, references correcte
Bound-free pair production in ultra-relativistic ion collisions at the LHC collider: Analytic approach to the total and differential cross sections
A theoretical investigation of the bound-free electron-positron pair
production in relativistic heavy ion collisions is presented. Special attention
is paid to the positrons emitted under large angles with respect to the beam
direction. The measurement of these positrons in coincidence with the
down--charged ions is in principle feasible by LHC experiments. In order to
provide reliable estimates for such measurements, we employ the equivalent
photon approximation together with the Sauter approach and derive simple
analytic expressions for the differential pair--production cross section, which
compare favorably to the results of available numerical calculations. Based on
the analytic expressions, detailed calculations are performed for collisions of
bare Pb ions, taking typical experimental conditions of the LHC
experiments into account. We find that the expected count rate strongly depends
on the experimental parameters and may be significantly enhanced by increasing
the positron-detector acceptance cone.Comment: 10 pages, 4 figure
Combined effect of coherent Z exchange and the hyperfine interaction in atomic PNC
The nuclear spin-dependent parity nonconserving (PNC) interaction arising
from a combination of the hyperfine interaction and the coherent,
spin-independent, PNC interaction from Z exchange is evaluated using many-body
perturbation theory. For the 6s-7s transition in 133Cs, we obtain a result that
is about 40% smaller than that found previously by Bouchiat and Piketty [Phys.
Lett. B 269, 195 (1991)]. Applying this result to 133Cs, leads to an increase
in the experimental value of nuclear anapole moment and exacerbates differences
between constraints on PNC meson coupling constants obtained from the Cs
anapole moment and those obtained from other nuclear parity violating
experiments. Nuclear spin-dependent PNC dipole matrix elements, including
contributions from the combined weak-hyperfine interaction, are also given for
the 7s-8s transition in 211Fr and for transitions between ground-state
hyperfine levels in K, Rb, Cs, Ba+, Au, Tl, Fr, and Ra+.Comment: Revtex4 preprint 19 pages 4 table
Strong suppression of Coulomb corrections to the cross section of e+e- pair production in ultrarelativistic nuclear collisions
The Coulomb corrections to the cross section of pair production in
ultrarelativistic nuclear collisions are calculated in the next-to-leading
approximation with respect to the parameter
( are the Lorentz factors of colliding nuclei). We found
considerable reduction of the Coulomb corrections even for large
due to the suppression of the production of pair
with the total energy of the order of a few electron masses in the rest frame
of one of the nuclei. Our result explains why the deviation from the Born
result were not observed in the experiment at SPS.Comment: 4 pages, RevTe
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