7,226 research outputs found
On the Consistency of a Fermion-Torsion Effective Theory
We discuss the possibility to construct an effective quantum field theory for
an axial vector coupled to a Dirac spinor field. A massive axial vector
describes antisymmetric torsion. The consistency conditions include unitarity
and renormalizability in the low-energy region. The investigation of the Ward
identities and the one- and two-loop divergences indicate serious problems
arising in the theory. The final conclusion is that torsion may exist as a
string excitation, but there are very severe restrictions for the existence of
a propagating torsion field, subject to the quantization procedure, at low
energies.Comment: LaTeX, 26 pages, 4 figure
On the cosmological effects of the Weyssenhoff spinning fluid in the Einstein-Cartan framework
The effects of non-Riemannian structures in Cosmology have been studied long
ago and are still a relevant subject of investigation. In the seventies, it was
discovered that singularity avoidance and early accelerated expansion can be
induced by torsion in the Einstein-Cartan theory. In this framework, torsion is
not dynamical and is completely expressed by means of the spin sources. Thus,
in order to study the effects of torsion in the Einstein-Cartan theory, one has
to introduce matter with spin. In principle, this can be done in several ways.
In this work we consider the cosmological evolution of the universe in the
presence of a constant isotropic and homogeneous axial current and the
Weyssenhoff spinning fluid. We analyse possible solutions of this model, with
and without the spinning fluid.Comment: Work presented at the 7th Alexander Friedmann International Seminar
on Gravitation and Cosmology, Joao Pessoa, Brazil, 29 Jun - 5 Jul 2008. No
figures, 5 pages. New version with dynamical equation corrected, new
reference and a brief comparison with its experimental bound
Eisenstein Series and String Thresholds
We investigate the relevance of Eisenstein series for representing certain
-invariant string theory amplitudes which receive corrections from BPS
states only. may stand for any of the mapping class, T-duality and
U-duality groups , or respectively.
Using -invariant mass formulae, we construct invariant modular functions
on the symmetric space of non-compact type, with the
maximal compact subgroup of , that generalize the standard
non-holomorphic Eisenstein series arising in harmonic analysis on the
fundamental domain of the Poincar\'e upper half-plane. Comparing the
asymptotics and eigenvalues of the Eisenstein series under second order
differential operators with quantities arising in one- and -loop string
amplitudes, we obtain a manifestly T-duality invariant representation of the
latter, conjecture their non-perturbative U-duality invariant extension, and
analyze the resulting non-perturbative effects. This includes the and
couplings in toroidal compactifications of M-theory to any
dimension and respectively.Comment: Latex2e, 60 pages; v2: Appendix A.4 extended, 2 refs added, thms
renumbered, plus minor corrections; v3: relation (1.7) to math Eis series
clarified, eq (3.3) and minor typos corrected, final version to appear in
Comm. Math. Phys; v4: misprints and Eq C.13,C.24 corrected, see note adde
Caracterização das Comunidades Subtidais de Macroinvertebrados Bentónicos de Substrato Rochoso da Ilha de São Vicente, Cabo Verde
The Cape Verde Archipelago location and its biogeographical features are of special
interest for Marine Ecology. However, there’s a lack of knowledge regarding the composition of
the coastal ecosystems in this region, especially about benthic macroinvertebrates subtidal
communities. Between August and October of 2007, eight locations around the island of São
Vicente were sampled. Within each of those spots, fragments of substratum were collected and
throughout the processing of the collected data, a total of 4032 individuals were counted, which
belong to 81 different species. Shannon’s Entropy and Gini-Simpson’s diversity index were
calculated, as the real number of species each one represented. By comparing the results,
differences between sampling stations and between indices within the same sampling station were
found. With the purpose of clustering the sampled locations according to the number of collected
organisms by species, a dendrogram was elaborated and a principal component analysis was carried
out. The considered sampling stations didn’t reveal significant differences according to the
composition of their benthic macroinvertebrates subtidal communities in terms of great taxonomic
groups or functional groups. It’s assumed that they differ only by minute traits
Quantum erasure in the presence of a thermal bath: the effects of system-environment microscopic correlations
We investigate the role of the environment in a quantum erasure setup in the
cavity quantum electrodynamics domain. Two slightly different schemes are
analyzed. We show that the effects of the environment vary when a scheme is
exchanged for another. This can be used to estimate the macroscopic parameters
related to the system-environment microscopic correlations.Comment: 10 pages, 2 figure
Control of state and state entanglement with a single auxiliary subsystem
We present a strategy to control the evolution of a quantum system. The novel
aspect of this protocol is the use of a \emph{single auxiliary subsystem}. Two
applications are given, one which allows for state preservation and another
which controls the degree of entanglement of a given initial state
- …