6,792 research outputs found
Weak Magnetism in Two Neutrino Double Beta Decay
We have extended the formalism for the two-neutrino double beta decay by
including the weak-magnetism term, as well as other second-forbidden
corrections. The weak magnetism diminishes the calculated half-lives in , independently of the nuclear structure. Numerical computations were
performed within the pn-QRPA, for , , ,
and nuclei. No one of the second-forbidden corrections modifies
significantly the spectrum shapes. The total reduction in the calculated half
lives varies from 6% up to 32%, and strongly depend on the nuclear interaction
in the particle-particle channel. We conclude that the higher order
effects in the weak Hamiltonian would hardly be observed in the two-neutrino
double beta experiments.Comment: 8 pages, latex, 1ps figure, to appear in Phys. Lett.
Solving the Constraints of General Relativity
I show in this letter that it is possible to solve some of the constraints of
the -ADM formalism for general relativity by using an approach similar
to the one introduced by Capovilla, Dell and Jacobson to solve the vector and
scalar constraints in the Ashtekar variables framework. I discuss the
advantages of using the ADM formalism and compare the result with similar
proposals for different Hamiltonian formulations of general relativity.Comment: 8 pages, LATEX, no figures, Preprint CGPG-94/11-
Competition between standard and exotic double beta decays
We discuss the contributions of higher order terms in weak Hamiltonian to the
standard two-neutrino double beta decay. The formalism for the unique first
forbidden transitions has been developed, and it is shown that they can alter
the two-electron energy spectrum. Yet, their effect is too small to screen the
detection of exotic neutrinoless double beta decays, which are candidates for
testing the physics beyond the standard model.Comment: 9 pages, latex, 1ps figures, minor changes, to appear in Phys. Lett.
meson exchange effect on nonmesonic hypernuclear weak decay observables
We analyze the influence of meson exchange on the main nonmesonic
hypernuclear weak decay observables: the total rate, , the
neutron-to-proton branching ratio, , and the proton asymmetry
parameter, . The meson exchange is added to the standard
strangeness-changing weak transition potential, which
includes the exchange of the complete pseudoscalar and vector mesons octet
(, , , , , ). Using a shell model formalism,
the meson weak coupling constants are adjusted to reproduce the recent
and experimental data for .
Numerical results for the remaining observables of and all the
observables of decays are presented. They clearly show that
the addition of the meson, in spite of improving some observables
values, is not enough to reproduce simultaneously all the measurements, and the
puzzle posed by the experimental data remains unexplained.Comment: 4 pages, 1 figure. Submitted for publication in Phys. Re
SO(4,C)-covariant Ashtekar-Barbero gravity and the Immirzi parameter
An so(4,C)-covariant hamiltonian formulation of a family of generalized
Hilbert-Palatini actions depending on a parameter (the so called Immirzi
parameter) is developed. It encompasses the Ashtekar-Barbero gravity which
serves as a basis of quantum loop gravity. Dirac quantization of this system is
constructed. Next we study dependence of the quantum system on the Immirzi
parameter. The path integral quantization shows no dependence on it. A way to
modify the loop approach in the accordance with the formalism developed here is
briefly outlined.Comment: 14 pages, LATEX; minor changes; misprints corrected; commutator of
two secondary second class constraints correcte
One pion production in neutrino-nucleon scattering and the different parametrizations of the weak vertex
The weak vertex provides an important contribution to the one
pion production in neutrino-nucleon and neutrino-nucleus scattering for
invariant masses below 1.4 GeV. Beyond its interest as a tool in neutrino
detection and their background analyses, one pion production in
neutrino-nucleon scattering is useful to test predictions based on the quark
model and other internal symmetries of strong interactions. Here we try to
establish a connection between two commonly used parametrizations of the weak
vertex and form factors (FF) and we study their effects on the
determination of the axial coupling , the common normalization of the
axial FF, which is predicted to hold 1.2 by using the PCAC hypothesis.
Predictions for the total cross sections within
the two approaches, which include the resonant and other
background contributions in a coherent way, are compared to experimental data.Comment: Submitted to Physics Letters
Real Ashtekar Variables for Lorentzian Signature Space-times
I suggest in this letter a new strategy to attack the problem of the reality
conditions in the Ashtekar approach to classical and quantum general
relativity. By writing a modified Hamiltonian constraint in the usual
Yang-Mills phase space I show that it is possible to describe space-times with
Lorentzian signature without the introduction of complex variables. All the
features of the Ashtekar formalism related to the geometrical nature of the new
variables are retained; in particular, it is still possible, in principle, to
use the loop variables approach in the passage to the quantum theory. The key
issue in the new formulation is how to deal with the more complicated
Hamiltonian constraint that must be used in order to avoid the introduction of
complex fields.Comment: 10 pages, LATEX, Preprint CGPG-94/10-
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