2,994 research outputs found
Nuclear attenuation of high energy multi-hadron systems in the string model
Nuclear attenuation of the multi-hadron systems in the string model is
considered. The improved two-scale model with set of parameters obtained
recently for the single hadron attenuation is used for calculation of the
multiplicity ratios of the one-, two- and three-hadron systems electroproduced
on nuclear and deuterium targets. The comparison of the features of the one-,
two- and three-hadron systems is performed. The predictions of the model for
multiplicity ratios of multi-hadron systems as functions of different
convenient variables are presented.Comment: 7 pages, 6 figure
Simple parameterization of nuclear attenuation data
Based on the nuclear attenuation data obtained by the HERMES experiment on
nitrogen and krypton nuclei, it is shown that the nuclear attenuation
can be parametrised in a form of a linear polynomial + , where is the formation time, which depends on the energy of the
virtual photon and fraction of that energy carried by the final
hadron. Three widely known parameterizations for were used for the
performed fit. The fit parameters and do not depend on
and
Pseudoclassical theories of Majorana, Weyl and Majorana--Weyl particles
A pseudoclassical theories of Majorana, Weyl and Majorana--Weyl particles in
the space--time dimensions are constructed. The canonical quantization
of these theories is carried out and as a result we obtain the quantum
mechanical description of neutral particle in , Weyl particle in
and neutral Weyl particle in . In dimensional
space--time the description of the neutral particle coincides with the field
theoretical description of the Majorana particle in the Foldy--Wouthuysen
representation. In dimensions the neutral Weyl particle coincides with
the Majorana--Weyl particle in the Foldy--Wouthuysen representation.Comment: LATEX, 17 pages, no figure
On the theory of coherent pair production in crystals in presence of acoustic waves
The influence of hypersonic waves excited in a single crystal is investigated
on the process of electron-positron pair creation by high-energy photons. The
coherent part of the corresponding differential cross-section is derived as a
function of the amplitude and wave number of the hypersound. The values of the
parameters are specified for which the latter affects remarkably on the pair
creation cross-section. It is shown that under certain conditions the presence
of hypersonic waves can result in enhancement of the process cross-section.Comment: 10 pages, 3 EPS figure
Casimir densities for a spherical boundary in de Sitter spacetime
Two-point functions, mean-squared fluctuations, and the vacuum expectation
value of the energy-momentum tensor operator are investigated for a massive
scalar field with an arbitrary curvature coupling parameter, subject to a
spherical boundary in the background of de Sitter spacetime. The field is
prepared in the Bunch-Davies vacuum state and is constrained to satisfy Robin
boundary conditions on the sphere. Both the interior and exterior regions are
considered. For the calculation in the interior region, a mode-summation method
is employed, supplemented with a variant of the generalized Abel-Plana formula.
This allows us to explicitly extract the contributions to the expectation
values which come from de Sitter spacetime without boundaries. We show that the
vacuum energy-momentum tensor is non-diagonal with the off-diagonal component
corresponding to the energy flux along the radial direction. With dependence on
the boundary condition and the mass of the field, this flux can be either
positive or negative. Several limiting cases of interest are then studied. In
terms of the curvature coupling parameter and the mass of the field, two very
different regimes are realized, which exhibit monotonic and oscillatory
behavior of the vacuum expectation values, respectively, far from the sphere.
The decay of the boundary induced expectation values at large distances from
the sphere is shown to be power-law (monotonic or oscillating), independent of
the value of the field mass.Comment: 32 pages, 4 figures, new paragraph about generalizations, discussion
and references added, accepted for publication in Phys. Rev.
Structural and energetic properties of nickel clusters:
The four most stable structures of Ni clusters with from 2 to 150
have been determined using a combination of the embedded-atom method in the
version of Daw, Baskes and Foiles, the {\it variable metric/quasi-Newton}
method, and our own {\it Aufbau/Abbau} method. A systematic study of
energetics, structure, growth, and stability of also larger clusters has been
carried through without more or less severe assumptions on the initial
geometries in the structure optimization, on the symmetry, or on bond lengths.
It is shown that cluster growth is predominantly icosahedral with of
{\it fcc}, {\it tetrahedral} and {\it decahedral} growth. For the first time in
unbiased computations it is found that Ni is the multilayer (third
Mackay) icosahedron. Further, we point to an enhanced ability of {\it fcc}
clusters to compete with the icosahedral and decahedral structures in the
vicinity of N=79. In addition, it is shown that conversion from the {\it
hcp}/anti-Mackay kind of icosahedral growth to the {\it fcc}/Mackay one occurs
within a transition layer including several cluster sizes. Moreover, we present
and apply different analytical tools in studying structural and energetic
properties of such a large class of clusters. These include means for
identifying the overall shape, the occurrence of atomic shells, the similarity
of the clusters with, e.g., fragments of the {\it fcc} crystal or of a large
icosahedral cluster, and a way of analysing whether the -atom cluster can be
considered constructed from the -atom one by adding an extra atom. In
addition, we compare in detail with results from chemical-probe experiment.
Maybe the most central result is that first for clusters with above 80
general trends can be identified.Comment: 37 pages, 11 figure
Electromagnetic field and radiation for a charge moving along a helical trajectory inside a waveguide with dielectric filling
We investigate the electromagnetic field generated by a point charge moving
along a helical trajectory inside a circular waveguide with conducting walls
filled by homogeneous dielectric. The parts corresponding to the radiation
field are separated and the formulae for the radiation intensity are derived
for both TE and TM waves. It is shown that the main part of the radiated quanta
is emitted in the form of the TE waves. Various limiting cases are considered.
The results of the numerical calculations show that the insertion of the
waveguide provides an additional mechanism for tuning the characteristics of
the emitted radiation by choosing the parameters of the waveguide and filling
medium.Comment: 17 pages, 9 figures, discussion, graphs, and references adde
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