15,120 research outputs found
Coulomb corrections to bremsstrahlung in electric field of heavy atom at high energies
The differential and partially integrated cross sections are considered for
bremsstrahlung from high-energy electrons in atomic field with the exact
account of this field. The consideration exploits the quasiclassical electron
Green's function and wave functions in an external electric field. It is shown
that the Coulomb corrections to the differential cross section are very
susceptible to screening. Nevertheless, the Coulomb corrections to the cross
section summed up over the final-electron states are independent of screening
in the leading approximation over a small parameter ( is
a screening radius, is the electron mass, ). Bremsstrahlung from
an electron beam of the finite size on heavy nucleus is considered as well.
Again, the Coulomb corrections to the differential probability are very
susceptible to the beam shape, while those to the probability integrated over
momentum transfer are independent of it, apart from the trivial factor, which
is the electron-beam density at zero impact parameter. For the Coulomb
corrections to the bremsstrahlung spectrum, the next-to-leading terms with
respect to the parameters ( is the electron energy) and
are obtained.Comment: 13 pages, 4 figure
Electroproduction of electron-positron pair in a medium
The process of electron-positron pair creation by a high-energy electron in a
medium is analyzed. The spectral distribution over energies of created
particles is calculated for the direct and cascade mechanisms of the process.
The Coulomb corrections are included. The new formulation of the equivalent
photons method is developed which takes into account the influence of multiple
scattering. It is shown the effects of multiple scattering can be quite
effectively studied in the process under consideration.Comment: 10 pages, 4 figure
Polarized triplet production by circularly polarized photons
A process of the pair production by a circularly polarized photon in the
field of unpolarized atomic electron has been considered in the
Weizaecker-Williams approximation. The degree of longitudinal polarization of
positron and electron has been calculated. An exclusive cross-section as well
as a spectral distribution are obtained. We estimate the accuracy of our
calculations at the level of a few percent. We show the identity of the
positron polarization for considered process and for process of pair production
in the screened Coulomb field of nucleus.Comment: 9 pages, 3 picture
Production of relativistic positronium in collisions of photons and electrons with nuclei and atoms
We consider the production of ultrarelativistic positronium (Ps) in and processes where is an atom or a nucleus
with charge . For the photoproduction of para- and ortho-Ps and the
electroproduction of para-Ps we obtain the most complete description compared
with previous works. It includes high order corrections and
polarization effects. The accuracy of the obtained cross sections is determined
by omitted terms of the order of the inverse Ps Lorentz factor squared. The
studied high order multi-photon electroproduction of ortho-Ps dominates for the
collision of electrons with heavy atoms over the bremsstrahlung production from
the electron via a virtual photon proposed by Holvik and Olsen. Our results
complete and correct the studies of those authors.Comment: 19 pages, 9 figures, RevTex; v2: minor corrections for the accuracy
of the results, a discussion of the literature added in a footnote, one
additional reference; v3: diagram of Fig.2 correcte
A separability criterion for density operators
We give a necessary and sufficient condition for a mixed quantum mechanical
state to be separable. The criterion is formulated as a boundedness condition
in terms of the greatest cross norm on the tensor product of trace class
operators.Comment: REVTeX, 5 page
Deriving main field and secular variation models from synthetic Swarm satellite and observatory data
The CORALIE survey for southern extra-solar planets IX. A 1.3-day period brown dwarf disguised as a planet
In this article we present the case of HD 41004 AB, a system composed of a
K0V star and a 3.7-magnitude fainter M-dwarf companion separated by only 0.5
arcsec. An analysis of CORALIE radial-velocity measurements has revealed a
variation with an amplitude of about 50m/s and a periodicity of 1.3days. This
radial-velocity signal is consistent with the expected variation induced by the
presence a very low mass giant planetary companion to HD 41004 A, whose light
dominates the spectra. The radial-velocity measurements were then complemented
with a photometric campaign and with the analysis of the bisector of the
CORALIE Cross-Correlation Function (CCF). While the former revealed no
significant variations within the observational precision of 0.003-0.004 mag
(except for an observed flare event), the bisector analysis showed that the
line profiles are varying in phase with the radial-velocity. This latter
result, complemented with a series of simulations, has shown that we can
explain the observations by considering that HD 41004 B has a brown-dwarf
companion orbiting with the observed 1.3-day period. If confirmed, this
detection represents the first discovery of a brown dwarf in a very short
period (1.3-day) orbit around an M dwarf. Finally, this case should be taken as
a serious warning about the importance of analyzing the bisector when looking
for planets using radial-velocity techniques.Comment: 16 pages, 17 eps figures, A&A in press (Figure 11 not as in original
version due to size
Production of para-- and orthopositronium at relativistic heavy ion colliders
We consider the ortho-- and parapositronium production in the process Ps where A is a nucleus with the charge number Z. The inclusive cross
section and the energy distribution of the relativistic Ps are calculated which
are of primary interest from the experimental point of view. The accuracy of
the corresponding cross sections is given by omitting terms for the para--Ps and for the ortho--Ps production
where and 16 for the RHIC and the LHC. Within this
accuracy the multiphoton (Coulomb) corrections are taken into account. We show
that the RHIC and the LHC will be Ps factories with a productions rate of about
relativistic Ps per day. The fraction of the ortho--Ps is
expected to be of the same order as that of the para--Ps for Au--Au and Pb--Pb
collisions.Comment: 22 pages, 5 figures, RevTeX, misprint correcte
- …