5,516 research outputs found
Nonmesonic weak decay spectra of He
To comprehend the recent Brookhaven National Laboratory experiment E788 on
He, we have outlined a simple theoretical framework, based on the
independent-particle shell model, for the one-nucleon-induced nonmesonic weak
decay spectra. Basically, the shapes of all the spectra are tailored by the
kinematics of the corresponding phase space, depending very weakly on the
dynamics, which is gauged here by the one-meson-exchange-potential. In spite of
the straightforwardness of the approach a good agreement with data is acheived.
This might be an indication that the final-state-interactions and the
two-nucleon induced processes are not very important in the decay of this
hypernucleus. We have also found that the exchange potential with soft
vertex-form-factor cutoffs GeV, GeV), is able to account simultaneously for the available experimental
data related to and for H, He,
and He.Comment: 12 pages, 4 figures, 1 table, submitted for publication; v2: major
revision, 18 pages, one author added, table, figures and bibliography change
On the propagation speed of evanescent modes
The group-velocity of evanescent waves (in undersized waveguides, for
instance) was theoretically predicted, and has been experimentally verified, to
be Superluminal (v_g > c). By contrast, it is known that the precursor speed in
vacuum cannot be larger than c. In this paper, by computer simulations based on
Maxwell equations only, we show the existence of both phenomena. In other
words, we verify the actual possibility of Superluminal group velocities,
without violating the so-called (naive) Einstein causality. (Subject classes:
General physics, Classical physics, Optics, Special Relativity; PACS nos.:
73.40Gk, 03.80+z, 03.65Bz; Keywords: evanescent waves; tunnelling photons;
Hartman effect; group velocity; Superluminal waves; precursors; transient
waves; front velocity; Maxwell equations; electromagnetic waves; computer
simulations; Special Relativity; Extended Relativity).Comment: plain LaTeX file (14 pages), plus 15 figures in .jp
BF Actions for the Husain-Kuchar Model
We show that the Husain-Kuchar model can be described in the framework of BF
theories. This is a first step towards its quantization by standard
perturbative QFT techniques or the spin-foam formalism introduced in the
space-time description of General Relativity and other diff-invariant theories.
The actions that we will consider are similar to the ones describing the
BF-Yang-Mills model and some mass generating mechanisms for gauge fields. We
will also discuss the role of diffeomorphisms in the new formulations that we
propose.Comment: 21 pages (in DIN A4 format), minor typos corrected; to appear in
Phys. Rev.
The Flavours of Coffee Ground. The coffee waste as accelerator for new local businesses
Annual generation of Spent Coffee Grounds (SCG) is estimated around six million tonnes per year. They currently do not have a commercial value and are disposed of in landfills or as compost.
Systemic Design approach developed by Politecnico di Torino (DAD) wants to provide an holistic vision in which these production are linked together through relationships, output and input, flows of energy and materials, in order to make the SCG recovery activity complex, with almost no waste.
This research studies how to make real and profitable a system that values this waste considering the local condition
The structure of the distortion free-energy density in nematics: second-order elasticity and surface terms
Nuclear masses, deformations and shell effects
We show that the Liquid Drop Model is best suited to describe the masses of
prolate deformed nuclei than of spherical nuclei. To this end three Liquid Drop
Mass formulas are employed to describe nuclear masses of eight sets of nuclei
with similar quadrupole deformations. It is shown that they are able to fit the
measured masses of prolate deformed nuclei with an RMS smaller than 750 keV,
while for the spherical nuclei the RMS is, in the three cases, larger than 2000
keV. The RMS of the best fit of the masses of semi-magic nuclei is also larger
than 2000 keV. The parameters of the three models are studied, showing that the
surface symmetry term is the one which varies the most from one group of nuclei
to another. In one model, isospin dependent terms are also found to exhibit
strong changes. The inclusion of shell effects allows for better fits, which
continue to be better in the prolate deformed nuclei regionComment: 10 pages, 8 tables, Proc. of the XXXIV Nuclear Physics Symposium,
January 4-7 2011, Cocoyoc, Morelos, Mexico. IOP Journal of Physics:
Conference Series (in press
Numerical study of surface-induced reorientation and smectic layering in a nematic liquid crystal
Surface-induced profiles of both nematic and smectic order parameters in a
nematic liquid crystal, ranging from an orienting substrate to "infinity", were
evaluated numerically on base of an extended Landau theory. In order to obtain
a smooth behavior of the solutions at "infinity" a boundary energy functional
was derived by linearizing the Landau energy around its equilibrium solutions.
We find that the intrinsic wave number of the smectic structure, which plays
the role of a coupling between nematic and smectic order, strongly influences
the director reorientation. Whereas the smectic order is rapidly decaying when
moving away from the surface, the uniaxial nematic order parameter shows an
oscillatory behavior close to the substrate, accompanied by a non-zero local
biaxiality.Comment: LaTeX, 17 pages, with 4 postscript figure
Unitary evolution of free massless fields in de Sitter space-time
We consider the quantum dynamics of a massless scalar field in de Sitter
space-time. The classical evolution is represented by a canonical
transformation on the phase space for the field theory. By studying the
corresponding Bogoliubov transformations, we show that the symplectic map that
encodes the evolution between two instants of time cannot be unitarily
implemented on any Fock space built from a SO(4)-symmetric complex structure.
We will show also that, in contrast with some effectively lower dimensional
examples arising from Quantum General Relativity such as Gowdy models, it is
impossible to find a time dependent conformal redefinition of the massless
scalar field leading to a quantum unitary dynamics.Comment: 20 pages. Comments and references adde
Quantum Cylindrical Waves and Sigma Models
We analyze cylindrical gravitational waves in vacuo with general polarization
and develop a viewpoint complementary to that presented recently by Niedermaier
showing that the auxiliary sigma model associated with this family of waves is
not renormalizable in the standard perturbative sense.Comment: 11 pages (DIN A4), accepted in International Journal of Modern
Physics
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