9,325 research outputs found
Connection between the elastic GEp/GMp and P to Delta form factors
It is suggested that the falloff in Qsq of the P to Delta magnetic form
factor GM* is related to the recently observed falloff of the elastic electric
form factor GEp/GMp. Calculation is carried out in the framework of a GPD
mechanism
Boundary critical behaviour at -axial Lifshitz points: the special transition for the case of a surface plane parallel to the modulation axes
The critical behaviour of -dimensional semi-infinite systems with
-component order parameter is studied at an -axial bulk
Lifshitz point whose wave-vector instability is isotropic in an -dimensional
subspace of . Field-theoretic renormalization group methods are
utilised to examine the special surface transition in the case where the
potential modulation axes, with , are parallel to the surface.
The resulting scaling laws for the surface critical indices are given. The
surface critical exponent , the surface crossover exponent
and related ones are determined to first order in
\epsilon=4+\case{m}{2}-d. Unlike the bulk critical exponents and the surface
critical exponents of the ordinary transition, is -dependent already
at first order in . The \Or(\epsilon) term of is
found to vanish, which implies that the difference of and
the bulk exponent is of order .Comment: 21 pages, one figure included as eps file, uses IOP style file
Compton telescope with coded aperture mask: Imaging with the INTEGRAL/IBIS Compton mode
Compton telescopes provide a good sensitivity over a wide field of view in
the difficult energy range running from a few hundred keV to several MeV. Their
angular resolution is, however, poor and strongly energy dependent. We present
a novel experimental design associating a coded mask and a Compton detection
unit to overcome these pitfalls. It maintains the Compton performance while
improving the angular resolution by at least an order of magnitude in the field
of view subtended by the mask. This improvement is obtained only at the expense
of the efficiency that is reduced by a factor of two. In addition, the
background corrections benefit from the coded mask technique, i.e. a
simultaneous measurement of the source and background. This design is
implemented and tested using the IBIS telescope on board the INTEGRAL satellite
to construct images with a 12' resolution over a 29 degrees x 29 degrees field
of view in the energy range from 200 keV to a few MeV. The details of the
analysis method and the resulting telescope performance, particularly in terms
of sensitivity, are presented
Generalized parton distributions in the deuteron
We introduce generalized quark and gluon distributions in the deuteron, which
can be measured in exclusive processes like deeply virtual Compton scattering
and meson electroproduction. We discuss the basic properties of these
distributions, and point out how they probe the interplay of nucleon and parton
degrees of freedom in the deuteron wave function
Logarithmic corrections in the two-dimensional Ising model in a random surface field
In the two-dimensional Ising model weak random surface field is predicted to
be a marginally irrelevant perturbation at the critical point. We study this
question by extensive Monte Carlo simulations for various strength of disorder.
The calculated effective (temperature or size dependent) critical exponents fit
with the field-theoretical results and can be interpreted in terms of the
predicted logarithmic corrections to the pure system's critical behaviour.Comment: 10 pages, 4 figures, extended version with one new sectio
Effects of surfaces on resistor percolation
We study the effects of surfaces on resistor percolation at the instance of a
semi-infinite geometry. Particularly we are interested in the average
resistance between two connected ports located on the surface. Based on general
grounds as symmetries and relevance we introduce a field theoretic Hamiltonian
for semi-infinite random resistor networks. We show that the surface
contributes to the average resistance only in terms of corrections to scaling.
These corrections are governed by surface resistance exponents. We carry out
renormalization group improved perturbation calculations for the special and
the ordinary transition. We calculate the surface resistance exponents
\phi_{\mathcal S \mathnormal} and \phi_{\mathcal S \mathnormal}^\infty for
the special and the ordinary transition, respectively, to one-loop order.Comment: 19 pages, 3 figure
Radiation Induced Membrane Changes and Programmed Cell Death: Possible Interrelationships
A short review of the evidence that lymphocyte membranes are a target for the initiation of irradiation induced programmed cell death (PCD) is given. It is assumed that for lymphocytes PCD represents an essential physiological mechanism in order to prevent degeneration of the biological system involved. Initiation of PCD can be obtained by a pharmacological activation as well as with irradiation. In both cases, protein kinase-C (PKC) is involved in the signal transduction from the cellular membrane to the nucleus where, by means of a metabolically active process, DNA fragmentation is induced. It is hypothesized that processes connected to lipid peroxidation in the cell membrane constitute a primary effect of irradiation induced PCD, where membrane fluidization or a compensatory process aimed to the maintenance of membrane fluidity (membrane homeoviscosity hypothesis) are likely to be involved
Transverse Deformation of Parton Distributions and Transversity Decomposition of Angular Momentum
Impact parameter dependent parton distributions are transversely distorted
when one considers transversely polarized nucleons and/or quarks. This provides
a physical mechanism for the T-odd Sivers effect in semi-inclusive
deep-inelastic scattering. The transverse distortion can also be related to
Ji's sum rule for the angular momentum carried by the quarks. The distortion of
chirally odd impact parameter dependent parton distributions is related to
chirally odd GPDs. This result is used to provide a decomposition of the quark
angular momentum w.r.t. quarks of definite transversity. Chirally odd GPDs can
thus be used to determine the correlation between quark spin and quark angular
momentum in unpolarized nucleons. Based on the transverse distortion, we also
suggest a qualitative connection between chirally odd GPDs and the Boer-Mulders
effect.Comment: 12 pages, 1 figure, version to appear in PR
The target asymmetry in hard vector-meson electroproduction and parton angular momenta
The target asymmetry for electroproduction of vector mesons is investigated
within the handbag approach. While the generalized parton distribution (GPD) H
is taken from a previous analysis of the elctroproduction cross section, we
here construct the GPD E from double distributions and constrain it by the
Pauli form factors of the nucleon, positivity bounds and sum rules. Predictions
for the target asymmetry are given for various vector mesons and discussed how
experimental data on the asymmetry will further constrain E and what we may
learn about the angular momenta the partons carry.Comment: 24 pages, 11 figures, late
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