563 research outputs found
Solutions of evolution equations for medium-induced QCD cascades
In this paper we present solutions of evolution equations for inclusive
distribution of gluons as produced by jet traversing quark-gluon plasma. We
reformulate the original equations in such a form that the virtual and
unresolved-real emissions as well as unresolved collisions with medium are
resummed in a Sudakov-type form factor. The resulting integral equations are
then solved most efficiently with use of newly developed Markov Chain Monte
Carlo algorithms implemented in a dedicated program called MINCAS. Their
results for a gluon energy density are compared with an analytical solution and
a differential numerical method. Some results for gluon transverse-momentum
distributions are also presented. They exhibit interesting patterns not
discussed so far in the literature, in particular a departure from the Gaussian
behaviour - which does not happen in approximate analytical solutions.Comment: 21 pages. Final versio
Fuzzy cellular model for on-line traffic simulation
This paper introduces a fuzzy cellular model of road traffic that was
intended for on-line applications in traffic control. The presented model uses
fuzzy sets theory to deal with uncertainty of both input data and simulation
results. Vehicles are modelled individually, thus various classes of them can
be taken into consideration. In the proposed approach, all parameters of
vehicles are described by means of fuzzy numbers. The model was implemented in
a simulation of vehicles queue discharge process. Changes of the queue length
were analysed in this experiment and compared to the results of NaSch cellular
automata model.Comment: The original publication is available at http://www.springerlink.co
Medium induced QCD cascades: broadening, entropy and rescattering during branching
We study evolution equations describing jet propagation through quark--gluon
plasma (QGP). In particular we investigate the contribution of momentum
transfer during branching and find that such a contribution is sizeable.
Furthermore, we study various approximations, such as the Gaussian
approximation and the diffusive approximation to the jet-broadening term. We
notice that in order to reproduce the BDIM equation (without the momentum
transfer in the branching) the diffusive approximation requires a very large
value of the jet-quenching parameter . We also quantify the solutions
by calculating time dependence of entropy associated with each of the
distributions.Comment: 21 page
On Theoretical Uncertainties of the W Angular Distribution in W-Pair Production at LEP2 Energies
We discuss theoretical uncertainties of the distribution in the cosine of the
W polar angle projected into a measurement of the anomalous triple gauge-boson
coupling \lambda=\lambda_{\gamma}=\lambda_Z at LEP2 energies for the tandem of
the Monte Carlo event generators KoralW and YFSWW3 and for the Monte Carlo
event generator RacoonWW. Exploiting numerical results of these programs and
cross-checks with experimental fitting procedures, we estimate that the
theoretical uncertainty of the value of \lambda due to electroweak corrections,
as obtained at LEP2 with the help of these programs, is ~0.005, about half of
the expected experimental error for the combined LEP2 experiments (~0.010). We
use certain idealized event selections; however, we argue that these results
are valid for realistic LEP2 measurements.Comment: 14 pages, 3 Postscript figure
Multiple scattering of photons by atomic hyperfine multiplets
Mesoscopic interference effects in multiple scattering of photons depend
crucially on the internal structure of the scatterers. In the present article,
we develop the analytical theory of multiple photon scattering by cold atoms
with arbitrary internal hyperfine multiplets. For a specific application, we
calculate the enhancement factor of elastic coherent backscattering as a
function of detuning from an entire hyperfine multiplet of neighboring
resonances that cannot be considered isolated. Our theory permits to understand
why atoms behave differently from classical Rayleigh point-dipole scatterers,
and how the classical description is recovered for larger but still microscopic
objects like molecules or clusters.Comment: minor changes, published versio
One-loop corrections to the Drell-Yan process in SANC (I). The charged current case
Radiative corrections to the charged current Drell-Yan processes are
revisited. Complete one-loop electroweak corrections are calculated within the
automatic SANC system. Electroweak scheme dependence and the choice of the
factorization scale are discussed. Comparisons with earlier calculations are
presented.Comment: extended version submitted to EPJ
First-order Raman spectra of double perovskites AB''{1/2}O3
First principles computations of Raman intensities were performed for
perovskite-family compound CaAlNbO (CAN). This compound
features 1:1 (NaCl-type) ordering of Al and Nb superimposed onto the
octahedral tilting. Raman tensor for CAN was computed using the package for
first-principles computations ABINIT (URL \underline {http://www.abinit.org}).
Computations performed for both untilted cubic () and tilted
monoclinic () CAN structures showed that the strongest Raman lines are
associated with the ordering of Al and Nb. The computed spectrum agreed
qualitatively with the experimental data measured on powder (CAN is available
in polycrystalline form only). The effect of cation disorder on the Raman
intensities was considered using phenomenological theory of light scattering in
the vicinity of a phase transition. We suggest that, for certain modes, the
corresponding Raman intensities depend primarily on the average long range
order while, for other modes, the intensities are determined by fluctuations of
the order parameter.Comment: 4 figures, submitte
Critical and Ictal Phases in Simulated EEG Signals on a Small-World Network
Healthy brain function is marked by neuronal network dynamics at or near the critical phase, which separates regimes of instability and stasis. A failure to remain at this critical point can lead to neurological disorders such as epilepsy, which is associated with pathological synchronization of neuronal oscillations. Using full Hodgkin-Huxley (HH) simulations on a Small-World Network, we are able to generate synthetic electroencephalogram (EEG) signals with intervals corresponding to seizure (ictal) or non-seizure (interictal) states that can occur based on the hyperexcitability of the artificial neurons and the strength and topology of the synaptic connections between them. These interictal simulations can be further classified into scale-free critical phases and disjoint subcritical exponential phases. By changing the HH parameters, we can model seizures due to a variety of causes, including traumatic brain injury (TBI), congenital channelopathies, and idiopathic etiologies, as well as the effects of anticonvulsant drugs. The results of this work may be used to help identify parameters from actual patient EEG or electrocorticographic (ECoG) data associated with ictogenesis, as well as generating simulated data for training machine-learning seizure prediction algorithms
Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals
The effect of the powder average on the peak shapes and positions in neutron
pair distribution functions of polycrystalline materials is examined. It is
shown that for highly anisotropic crystals, the powder average leads to shifts
in peak positions and to non-Gaussian peak shapes. The peak shifts can be as
large as several percent of the lattice spacing
Calculation of the Two-Loop Heavy-Flavor Contribution to Bhabha Scattering
We describe in detail the calculation of the two-loop corrections to the QED
Bhabha scattering cross section due to the vacuum polarization by heavy
fermions. Our approach eliminates one mass scale from the most challenging part
of the calculation and allows us to obtain the corrections in a closed
analytical form. The result is valid for arbitrary values of the heavy fermion
mass and the Mandelstam invariants, as long as s,t,u >> m_e^2.Comment: 43 pages, 8 figures; added reference
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