42 research outputs found
Theoretical study of the Compton effect with correlated three-photon emission: From the differential cross section to high-energy triple-photon entanglement
The three-photon Compton effect is studied. An incoming photon undergoes
triple scattering off a free electron, which leads to the emission of three
entangled photons. We investigate the properties of both the total cross
section, assuming a low-energy cutoff for the detected photons, and the
differential cross section. Particular emphasis is laid on evaluating
polarization-resolved cross sections. The entanglement of the final
three-photon state is analyzed.Comment: 14 pages; RevTe
Effect of a strong laser field on photoproduction by relativistic nuclei
We study the influence of a strong laser field on the Bethe-Heitler
photoproduction process by a relativistic nucleus. The laser field propagates
in the same direction as the incoming high-energy photon and it is taken into
account exactly in the calculations. Two cases are considered in detail. In the
first case, the energy of the incoming photon in the nucleus rest frame is much
larger than the electron's rest energy. The presence of the laser field may
significantly suppress the photoproduction rate at soon available values of
laser parameters. In the second case, the energy of the incoming photon in the
rest frame of the nucleus is less than and close to the electron-positron pair
production threshold. The presence of the laser field allows for the pair
production process and the obtained electron-positron rate is much larger than
in the presence of only the laser and the nuclear field. In both cases we have
observed a strong dependence of the rate on the mutual polarization of the
laser field and of the high-energy photon and the most favorable configuration
is with laser field and high-energy photon linearly polarized in the same
direction. The effects discussed are in principle measurable with presently
available proton accelerators and laser systems.Comment: 21 pages, 4 figure
Barrier control in tunneling e^+ e^- photoproduction
Tunneling electron-positron pair production is studied in a new setup in
which a strong low-frequency and a weak high-frequency laser field propagate in
the same direction and collide head-on with a relativistic nucleus. The
electron-positron pair production rate is calculated analytically in the limit
in which in the nucleus rest frame the strong field is undercritical and the
frequency of the weak field is below and close to the pair production
threshold. By changing the frequency of the weak field one can reduce the
tunneling barrier substantially. As a result tunneling pair production is shown
to be observable with presently available technology.Comment: 4 pages, 3 figure
Numerical calculation of Bessel, Hankel and Airy functions
The numerical evaluation of an individual Bessel or Hankel function of large
order and large argument is a notoriously problematic issue in physics.
Recurrence relations are inefficient when an individual function of high order
and argument is to be evaluated. The coefficients in the well-known uniform
asymptotic expansions have a complex mathematical structure which involves Airy
functions. For Bessel and Hankel functions, we present an adapted algorithm
which relies on a combination of three methods: (i) numerical evaluation of
Debye polynomials, (ii) calculation of Airy functions with special emphasis on
their Stokes lines, and (iii) resummation of the entire uniform asymptotic
expansion of the Bessel and Hankel functions by nonlinear sequence
transformations.
In general, for an evaluation of a special function, we advocate the use of
nonlinear sequence transformations in order to bridge the gap between the
asymptotic expansion for large argument and the Taylor expansion for small
argument ("principle of asymptotic overlap"). This general principle needs to
be strongly adapted to the current case, taking into account the complex phase
of the argument. Combining the indicated techniques, we observe that it
possible to extend the range of applicability of existing algorithms. Numerical
examples and reference values are given.Comment: 18 pages; 7 figures; RevTe
Stochastic modelling of reaction-diffusion processes: algorithms for bimolecular reactions
Several stochastic simulation algorithms (SSAs) have been recently proposed
for modelling reaction-diffusion processes in cellular and molecular biology.
In this paper, two commonly used SSAs are studied. The first SSA is an
on-lattice model described by the reaction-diffusion master equation. The
second SSA is an off-lattice model based on the simulation of Brownian motion
of individual molecules and their reactive collisions. In both cases, it is
shown that the commonly used implementation of bimolecular reactions (i.e. the
reactions of the form A + B -> C, or A + A -> C) might lead to incorrect
results. Improvements of both SSAs are suggested which overcome the
difficulties highlighted. In particular, a formula is presented for the
smallest possible compartment size (lattice spacing) which can be correctly
implemented in the first model. This implementation uses a new formula for the
rate of bimolecular reactions per compartment (lattice site).Comment: 33 pages, submitted to Physical Biolog
An adaptive hierarchical domain decomposition method for parallel contact dynamics simulations of granular materials
A fully parallel version of the contact dynamics (CD) method is presented in
this paper. For large enough systems, 100% efficiency has been demonstrated for
up to 256 processors using a hierarchical domain decomposition with dynamic
load balancing. The iterative scheme to calculate the contact forces is left
domain-wise sequential, with data exchange after each iteration step, which
ensures its stability. The number of additional iterations required for
convergence by the partially parallel updates at the domain boundaries becomes
negligible with increasing number of particles, which allows for an effective
parallelization. Compared to the sequential implementation, we found no
influence of the parallelization on simulation results.Comment: 19 pages, 15 figures, published in Journal of Computational Physics
(2011
Enhanced ionization of acetylene in intense laser pulses is due to energy upshift and field coupling of multiple orbitals
Synopsis We describe a new enhanced ionization mechanism for polyatomic molecules. It works via a significant energy up-shift of valence orbitals for stretched bonds and a strong concomitant increase in the coupling between multiple molecular orbitals
Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3)
Ice flow forced by gravity is governed by the full Stokes (FS) equations,
which are computationally expensive to solve due to the nonlinearity
introduced by the rheology. Therefore, approximations to the FS equations are
commonly used, especially when modeling a marine ice sheet (ice sheet, ice
shelf, and/or ice stream) for 103 years or longer. The shallow ice
approximation (SIA) and shallow shelf approximation (SSA) are commonly used
but are accurate only for certain parts of an ice sheet. Here, we report a
novel way of iteratively coupling FS and SSA that has been implemented in
Elmer/Ice and applied to conceptual marine ice sheets. The FS–SSA coupling
appears to be very accurate; the relative error in velocity compared to FS is
below 0.5 % for diagnostic runs and below 5 % for prognostic runs.
Results for grounding line dynamics obtained with the FS–SSA coupling are
similar to those obtained from an FS model in an experiment with a periodical
temperature forcing over 3000 years that induces grounding line advance and
retreat. The rapid convergence of the FS–SSA coupling shows a large
potential for reducing computation time, such that modeling a marine ice
sheet for thousands of years should become feasible in the near future.
Despite inefficient matrix assembly in the current implementation,
computation time is reduced by 32 %, when the coupling is applied to a
3-D ice shelf.</p