3,556 research outputs found
The elastic electron-deuteron scattering beyond one-photon exchange
We discuss the elastic ed scattering beyond Born approximation. It is shown
that the reaction amplitude contains six generalized form factors, but only
three linearly independent combinations of them (we call them generalized
charge, quadrupole and magnetic form factors) contribute to the reaction cross
section in the second order perturbation theory. We examine two-photon exchange
and find that it includes two types of diagrams, when two virtual photons
interact with the same nucleon and when the photons interact with different
nucleons. Estimations based on nonrelativistic calculations with the deuteron
wave function for realistic NN potential show that the main contribution to the
generalized charge, quadrupole and magnetic form factors comes from diagrams of
the first type.Comment: v2, published version in PR
Analysis of noise-induced transitions from regular to chaotic oscillations in the Chen system
The stochastically perturbed Chen system is studied within the parameter region which permits both regular and chaotic oscillations. As noise intensity increases and passes some threshold value, noise-induced hopping between close portions of the stochastic cycle can be observed. Through these transitions, the stochastic cycle is deformed to be a stochastic attractor that looks like chaotic. In this paper for investigation of these transitions, a constructive method based on the stochastic sensitivity function technique with confidence ellipses is suggested and discussed in detail. Analyzing a mutual arrangement of these ellipses, we estimate the threshold noise intensity corresponding to chaotization of the stochastic attractor. Capabilities of this geometric method for detailed analysis of the noise-induced hopping which generates chaos are demonstrated on the stochastic Chen system. © 2012 American Institute of Physics
Search for evidence of two photon contribution in elastic electron proton data
We reanalyze the most recent data on elastic electron proton scattering. We
look for a deviation from linearity of the Rosenbluth fit to the differential
cross section, which would be the signature of the presence of two photon
exchange. The two photon contribution is parametrized by a one parameter
formula, based on symmetry arguments. The present data do not show evidence for
such deviation.Comment: 15 pages 3 figures More details on the fitting procedure, more
explicit explanation
Could Only Fermions Be Elementary?
In standard Poincare and anti de Sitter SO(2,3) invariant theories,
antiparticles are related to negative energy solutions of covariant equations
while independent positive energy unitary irreducible representations (UIRs) of
the symmetry group are used for describing both a particle and its
antiparticle. Such an approach cannot be applied in de Sitter SO(1,4) invariant
theory. We argue that it would be more natural to require that (*) one UIR
should describe a particle and its antiparticle simultaneously. This would
automatically explain the existence of antiparticles and show that a particle
and its antiparticle are different states of the same object. If (*) is adopted
then among the above groups only the SO(1,4) one can be a candidate for
constructing elementary particle theory. It is shown that UIRs of the SO(1,4)
group can be interpreted in the framework of (*) and cannot be interpreted in
the standard way. By quantizing such UIRs and requiring that the energy should
be positive in the Poincare approximation, we conclude that i) elementary
particles can be only fermions. It is also shown that ii) C invariance is not
exact even in the free massive theory and iii) elementary particles cannot be
neutral. This gives a natural explanation of the fact that all observed neutral
states are bosons.Comment: The paper is considerably revised and the following results are
added: in the SO(1,4) invariant theory i) the C invariance is not exact even
for free massive particles; ii) neutral particles cannot be elementar
Exact identification of the radion and its coupling to the observable sector
Braneworld models in extra dimensions can be tested in laboratory by the
coupling of the radion to the Standard Model fields. The identification of the
radion as a canonically normalized field involves a careful General Relativity
treatment: if a bulk scalar is responsible for the stabilization of the system,
its fluctuations are entangled with the perturbations of the metric and they
also have to be taken into account (similarly to the well-developed theory of
scalar metric perturbations in 4D cosmology with a scalar field). Extracting a
proper dynamical variable in a warped geometry/scalar setting is a nontrivial
task, performed so far only in the limit of negligible backreaction of the
scalar field on the background geometry. We perform the general calculation,
diagonalizing the action up to second order in the perturbations and
identifying the physical eigenmodes of the system for any amplitude of the bulk
scalar. This computation allows us to derive a very simple expression for the
exact coupling of the eigenmodes to the Standard Model fields on the brane,
valid for an arbitrary background configuration. As an application, we discuss
the Goldberger-Wise mechanism for the stabilization of the radion in the
Randall-Sundrum type models. The existing studies, limited to small amplitude
of the bulk scalar field, are characterized by a radion mass which is
significantly below the physical scale at the observable brane. We extend them
beyond the small backreaction regime. For intermediate amplitudes, the radion
mass approaches the electroweak scale, while its coupling to the observable
brane remains nearly constant. At very high amplitudes, the radion mass instead
decreases, while the coupling sharply increases. Severe experimental
constraints are expected in this regime.Comment: 20 pages, 6 figure
Cosmological Fluctuations from Infra-Red Cascading During Inflation
We propose a qualitatively new mechanism for generating cosmological
fluctuations from inflation. The non-equilibrium excitation of interacting
scalar fields often evolves into infra-red (IR) and ultra-violet (UV)
cascading, resulting in an intermediate scaling regime. We observe elements of
this phenomenon in a simple model with inflaton \phi and iso-inflaton \chi
fields interacting during inflation via the coupling g^2 (\phi-\phi_0)^2
\chi^2. Iso-inflaton particles are created during inflation when they become
instantaneously massless at \phi=\phi_0, with occupation numbers not exceeding
unity. We point out that very quickly the produced \chi particles become heavy
and their multiple re-scatterings off the homogeneous condensate \phi(t)
generates bremschtrahlung radiation of light inflaton IR fluctuations with high
occupation numbers. The subsequent evolution of these IR fluctuations is
qualitatively similar to that of the usual inflationary fluctuations, but their
initial amplitude is different. The IR cascading generates a bump-shaped
contribution to the cosmological curvature fluctuations, which can even
dominate over the usual fluctuations for g^2>0.06. The IR cascading curvature
fluctuations are significantly non-gaussian and the strength and location of
the bump are model-dependent, through g^2 and \phi_0. The effect from IR
cascading fluctuations is significantly larger than that from the momentary
slowing-down of \phi(t). With a sequence of such bursts of particle production,
the superposition of the bumps can lead to a new broad band non-gaussian
component of cosmological fluctuations added to the usual fluctuations. Such a
sequence of particle creation events can, but need not, lead to trapped
inflation.Comment: 13 pages, 9 figures. Accepted for publication in Phys. Rev.
Polarization effects in the reaction and determination of the meson form factors in the time--like region
The electron positron annihilation reaction into four pion production has
been studied, through the channel . The
differential (and total) cross sections and various polarization observables
for this reaction have been calculated in terms of the electromagnetic form
factors of the corresponding current. The elements of the
spin--density matrix of the meson were also calculated. Numerical
estimations have been done, with the help of phenomenological form factors
obtained in the space--like region of the momentum transfer squared and
analytically extended to the time-like region.Comment: 19 pages, 2 figures, to appear in Phys Rev
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