37 research outputs found
Charge asymmetry in the differential cross section of high-energy e+e- photoproduction in the field of a heavy atom
First quasiclassical correction to the differential cross section of
high-energy electron-positron photoproduction in the electric field of a heavy
atom is obtained with the exact account of the field. This correction is
responsible for the charge asymmetry in this process. When the
transverse momentum of at least one of the produced particles is much larger
than the electron mass , the charge asymmetry can be as large as tens
percent. We also estimate the contribution to the charge
asymmetry coming from the Compton-type diagram. For heavy nuclei, this
contribution is negligible. For light nuclei, is noticeable
only when the angle between the momenta of electron and positron is of order of
( is the photon energy) while the transverse momenta of both
particles are much larger than .Comment: 19 pages, 7 figure
Angular distribution of high-energy photoproduction close to the end of spectrum
We consider the differential cross section of electron-positron pair
production by a high-energy photon in a strong Coulomb field close to the end
of the electron or positron spectrum. When the momentum transfer largely
exceeds the electron mass, the cross section is obtained analytically in a
compact form. Coulomb corrections essentially modify the cross section even for
moderate values of the nuclear charge number . In the same kinematical
region, the angular distribution for bound-free pair production,
bremsstrahlung, and photorecombination is also obtained.Comment: 12 pages, 4 figure
Spatial distributions in static heavy-light mesons: a comparison of quark models with lattice QCD
Lattice measurements of spatial distributions of the light quark bilinear
densities in static mesons allow to test directly and in detail the wave
functions of quark models. These distributions are gauge invariant quantities
directly related to the spatial distribution of wave functions. We make a
detailed comparison of the recent lattice QCD results with our own quark
models, formulated previously for quite different purposes. We find a striking
agreement not only between our two quark models, but also with the lattice QCD
data for the ground state in an important range of distances up to about 4/GeV.
Moreover the agreement extends to the L=1 states [j^P=(1/2)^+]. An explanation
of several particular features completely at odds with the non-relativistic
approximation is provided. A rather direct, somewhat unexpected and of course
approximate relation between wave functions of certain quark models and QCD has
been established.Comment: 40 pages, 5 figures (version published in PRD
Perturbation Theory for Metastable States of the Dirac Equation with Quadratic Vector Interaction
The spectral problem of the Dirac equation in an external quadratic vector
potential is considered using the methods of the perturbation theory. The
problem is singular and the perturbation series is asymptotic, so that the
methods for dealing with divergent series must be used. Among these, the
Distributional Borel Sum appears to be the most well suited tool to give
answers and to describe the spectral properties of the system. A detailed
investigation is made in one and in three space dimensions with a central
potential. We present numerical results for the Dirac equation in one space
dimension: these are obtained by determining the perturbation expansion and
using the Pad\'e approximants for calculating the distributional Borel
transform. A complete agreement is found with previous non-perturbative results
obtained by the numerical solution of the singular boundary value problem and
the determination of the density of the states from the continuous spectrum.Comment: 10 pages, 1 figur
Variational Two Fermion Wave Equations in QED: Muonium Like Systems
We consider a reformulation of QED in which covariant Green functions are
used to solve for the electromagnetic field in terms of the fermion fields. The
resulting modified Hamiltonian contains the photon propagator directly. A
simple Fock-state variational trial function is used to derive relativistic
two-fermion equations variationally from the expectation value of the
Hamiltonian of the field theory. The interaction kernel of the equation is
shown to be, in essence, the invariant M-matrix in lowest order. Solutions of
the two-body equations are presented for muonium like system for small coupling
strengths. The results compare well with the observed muonium spectrum, as well
as that for hydrogen and muonic hydrogen. Anomalous magnetic moment effects are
discussed
Polarization of the electron and positron produced in combined Coulomb and strong laser fields
The process of production in the superposition of a Coulomb and a
strong laser field is considered. The pair production rate integrated over the
momentum and summed over the spin projections of one of the particles is
derived exactly in the parameters of the laser field and in the Born
approximation with respect to the Coulomb field. The case of a monochromatic
circularly polarized laser field is considered in detail. A very compact
analytical expression of the pair production rate and its dependence on the
polarization of one of the created particles is obtained in the quasiclassical
approximation for the experimentally relevant case of an undercritical laser
field. As a result, the polarization of the created electron (positron) is
derived.Comment: 16 pages, no figure
Casimir-Polder interaction between an excited atom and a gas dielectric medium
The Casimir-Polder potential for interaction between an excited atom and a
ground-state one in the retarded case obtained with the help of perturbation
technique drops as R^-2 with the distance between the atoms [E.A. Power,
T.Thirunamachandran, Phys. Rev. A, 47, 2539 (1993)]. It results in diverdent
integrals for interaction between an excited atom and a dilute gas medium. We
investigate interaction between two atoms embedded in a dielectric medium with
the help of non-perturbative approach. We take into account absorption of
photons in the medium. This approach solves the problem of divergence. We
consider interaction between an excited atom and a planar dielectric gas medium
of ground-state atoms. We show that the retarded interaction between an excited
atom and a gas of ground-state atoms is not oscillating but follows a simple
power law. We show that to obtain coventional non-retarded expression for the
van der Waals force between an excited atom and a dilute gas the distance
between the atom and the interface should be much smaller than the free mean
pass of a photon in the medium. Interaction between an excited atom and a
hemisphere of ground-state atoms is considered.Comment: 23 pages, 6 figure
High-energy photoproduction cross section close to the end of spectrum
We consider the cross section of electron-positron pair production by a
high-energy photon in a strong Coulomb field close to the end of electron or
positron spectrum. We show that the cross section essentially differs from the
result obtained in the Born approximation as well as form the result which
takes into account the Coulomb corrections under assumption that both electron
and positron are ultrarelativistic. The cross section of bremsstrahlung in a
strong Coulomb field by a high-energy electron is also obtained in the region
where the final electron is not ultrarelativistic.Comment: 20 pages, 4 figure
Interacting spinor and scalar fields in Bianchi type-I Universe filled with viscous fluid: exact and numerical solutions
We consider a self-consistent system of spinor and scalar fields within the
framework of a Bianchi type I gravitational field filled with viscous fluid in
presence of a term. Exact self-consistent solutions to the
corresponding spinor, scalar and BI gravitational field equations are obtained
in terms of , where is the volume scale of BI universe. System of
equations for and \ve, where \ve is the energy of the viscous fluid,
is deduced. Some special cases allowing exact solutions are thoroughly studied.Comment: 18 pages, 6 figure
Foldy-Wouthuysen Transformation for a Spinning Particle with Anomalous Magnetic Moment
We study the Foldy-Wouthuysen transformation for a pseudoclassical particle
with anomalous magnetic moment in an external, stationary electromagnetic
field. We show that the transformation can be expressed in a closed form for
neutral particles in purely electrostatic fields and for neutral and charged
particles in external magnetostatic fields. The explicit expressions of the
diagonalized Hamiltonians are calculated.Comment: 10 page