1,422 research outputs found
A relativistic coupled-channel formalism for the pion form factor
The electromagnetic form factor of a confined quark-antiquark pair is
calculated within the framework of point-form relativistic quantum mechanics.
The dynamics of theexchanged photon is explicitly taken into account by
treating theelectromagnetic scattering of an electron by a meson as a
relativistic two-channel problem for a Bakamjian-Thomas type mass operator.
This approach guarantees Poincare invariance. Using a Feshbach reduction the
coupled-channel problem can be converted into a one-channel problem for the
elastic electron-meson channel. By comparing the one-photon-exchange optical
potential at the constituent and hadronic levels, we are able to unambiguously
identify the electromagnetic meson form factor. Violations of
cluster-separability properties, which are inherent in the Bakamjian-Thomas
approach, become negligible for sufficiently large invariant mass of the
electron-meson system. In the limit of an infinitely large invariant mass, an
equivalence with form-factor calculations done in front-form relativistic
quantum mechanics is established analytically.Comment: 3 pages, 1 figure, submitted to EPJ Web of Conference
Point-form quantum field theory and meson form factors
We shortly review point-form quantum field theory, i.e. the canonical
quantization of a relativistic field theory on a Lorentz-invariant surface of
the form . As an example of how point-form quantum field
theory may enter the framework of relativistic quantum mechanics we discuss the
calculation of the electromagnetic form factor of a confined quark-antiquark
pair (e.g. the pion).Comment: 3 pages, 2 figures. Based on a talk presented by W. Schweiger at the
20th European Conference on Few-Body Problems in Physics, September 10-14
2007, Pisa, Ital
Reggeon and pion contributions in semi-exclusive diffractive processes at HERA
A detailed analysis of semi-exclusive diffractive processes in e-p DIS at
HERA, with the diffractive final states in the forward direction is presented.
The contributions of the subleading f_2, \omega, a_2, \rho reggeons and the
pion exchanges to the diffractive structure function with the forward proton or
neutron are estimated. It is found that the (a_2,\rho) reggeons are entirely
responsible for the forward neutron production at x_P < 10^{-3}. The \pi N
production in the forward region is estimated using the Deck mechanism. The
significance of this reaction for the processes measured at HERA, especially
with the leading neutron, is discussed.Comment: Strongly revised version accepted for publication in Phys.Rev.D.
Latex, 14 pages with 5 eps figures include
The colour dipole approach to small-x processes
We explain why it is possible to formulate a wide variety of high energy
(small-x) photon-proton processes in terms of a universal dipole cross section
and compare and contrast various parameterizations of this function that exist
in the literature.Comment: 6 pages, latex, 2 figures. Contribution to Durham Collider Workshop
(Sept 99) proceeding
Two-dimensional MHD model of the reconnection diffusion region
International audienceMagnetic reconnection is an important process providing a fast conversion of magnetic energy into thermal and kinetic plasma energy. In this concern, a key problem is that of the resistive diffusion region where the reconnection process is initiated. In this paper, the diffusion region is associated with a nonuniform conductivity localized to a small region. The nonsteady resistive incompressible MHD equations are solved numerically for the case of symmetric reconnection of antiparallel magnetic fields. A Petschek type steady-state solution is obtained as a result of time relaxation of the reconnection layer structure from an arbitrary initial stage. The structure of the diffusion region is studied for various ratios of maximum and minimum values of the plasma resistivity. The effective length of the diffusion region and the reconnection rate are determined as functions of the length scale and the maximum of the resistivity. For sufficiently small length scale of the resistivity, the reconnection rate is shown to be consistent with Petschek's formula. By increasing the resistivity length scale and decreasing the resistivity maximum, the reconnection layer tends to be wider, and correspondingly, the reconnection rate tends to be more consistent with that of the Parker-Sweet regime
Interchange instability of a curved current layerconvecting in the magnetosheath from the bow shock towards themagnetopause
International audienceThis paper deals with nonsteady perturbations of the magnetosheath parameters which are related to variations of the interplanetary magnetic field from north to south under a constant solar wind dynamic pressure. The magnetic field changes its direction within a thin layer which is convected with the plasma from the bow shock to the ionopause. In the course of time, this current layer is amplified during its motion towards the magnetopause. The intensity of the current is increasing, the layer thickness is decreasing, and the gradients of parameters are becoming much sharper while the layer is approaching the magnetopause. The curvature radius of this layer is decreasing while it is draping around the magnetopause. This curved layer structure with reversed magnetic field in the magnetosheath is found to be unstable with respect to the interchange instability. The growth rate of the instability is obtained for different positions of the layer. Key words. Magnetospheric physics (magnetosheath
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