2,622 research outputs found
Regge description of spin-spin asymmetry in photon diffractive dissociation
We explore the possibility whether the gluon helicity distribution can be extracted from a comparison of experimental data on the
longitudinal spin-spin asymmetry in diffractive deep
inelastic scattering with calculations performed within the framework of
perturbative QCD (pQCD). The data could be obtained at the future HERA collider
in scattering of polarized electrons/positrons off polarized protons. In this
paper we look for such kinematical regions where contributions to from
soft processes (reggeon exchanges) are suppressed to guarantee an applicability
of pQCD. It is shown that for the square of the center-of-mass energy
, the hadronic diffractive mass , the momentum transferred to the proton , and
the longitudinal spin-spin asymmetry due to reggeon
exchanges is less than 10^{-4}. This value is presumably lower than the
asymmetry which can be measured with modern experimental technique. This means
that the pQCD prediction can be reliably compared with data in this kinematical
region.Comment: 44 pages, LaTeX, 12 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.
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
Synchrotron radiation from a charge moving along a helical orbit inside a dielectric cylinder
The radiation emitted by a charged particle moving along a helical orbit
inside a dielectric cylinder immersed into a homogeneous medium is
investigated. Expressions are derived for the electromagnetic potentials,
electric and magnetic fields, and for the spectral-angular distribution of
radiation in the exterior medium. It is shown that under the Cherenkov
condition for dielectric permittivity of the cylinder and the velocity of the
particle image on the cylinder surface, strong narrow peaks are present in the
angular distribution for the number of radiated quanta. At these peaks the
radiated energy exceeds the corresponding quantity for a homogeneous medium by
some orders of magnitude. The results of numerical calculations for the angular
distribution of radiated quanta are presented and they are compared with the
corresponding quantities for radiation in a homogeneous medium. The special
case of relativistic charged particle motion along the direction of the
cylinder axis with non-relativistic transverse velocity (helical undulator) is
considered in detail. Various regimes for the undulator parameter are
discussed. It is shown that the presence of the cylinder can increase
essentially the radiation intensity.Comment: 18 pages, 8 EPS figure
Whightman function and scalar Casimir densities for a wedge with a cylindrical boundary
Whightman function, vacuum expectation values of the field square, and the
energy-momentum tensor are investigated for a scalar field inside a wedge with
and without a coaxial cylindrical boundary. Dirichlet boundary conditions are
assumed on the bounding surfaces. The vacuum energy-momentum tensor is
evaluated in the general case of the curvature coupling parameter. Making use
of a variant of the generalized Abel-Plana formula, expectation values are
presented as the sum of two terms. The first one corresponds to the geometry
without a cylindrical boundary and the second one is induced by the presence of
this boundary. The asymptotic behaviour of the field square, vacuum energy
density and stresses near the boundaries are investigated. The additional
vacuum forces acting on the wedge sides due the presence of the cylindrical
boundary are evaluated and it is shown that these forces are attractive. As a
limiting case, the geometry of two parallel plates perpendicularly intersected
by a third one is analyzed.Comment: 19 pages, 6 figures, new section is added on the VEVs for the region
outside the cylidrical shell, discussion and references added, accepted for
publication in J. Phys.
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