1,120 research outputs found
Chiral Bosons as solutions of the BV master equation 2D chiral gauge theories
We construct the chiral Wess-Zumino term as a solution for the
Batalin-Vilkovisky master equation for anomalous two-dimensional gauge
theories, working in an extended field-antifield space, where the gauge group
elements are introduced as additional degrees of freedom.
We analyze the Abelian and the non-Abelian cases, calculating in both cases
the BRST generator in order to show the physical equivalence between this
chiral solution for the master equation and the usual (non-chiral) one.Comment: 11 pages, TEX dialet, IF/UFRJ-94-
The role of the time gauge in the 2nd order formalism
We perform a canonical quantization of gravity in a second-order formulation,
taking as configuration variables those describing a 4-bein, not adapted to the
space-time splitting. We outline how, neither if we fix the Lorentz frame
before quantizing, nor if we perform no gauge fixing at all, is invariance
under boost transformations affected by the quantization.Comment: 4 pages, Proceedings of the II Stueckelberg Worksho
Mixmaster Chaoticity as Semiclassical Limit of the Canonical Quantum Dynamics
Within a cosmological framework, we provide a Hamiltonian analysis of the
Mixmaster Universe dynamics on the base of a standard Arnowitt-Deser-Misner
approach, showing how the chaotic behavior characterizing the evolution of the
system near the cosmological singularity can be obtained as the semiclassical
limit of the canonical quantization of the model in the same dynamical
representation. The relation between this intrinsic chaotic behavior and the
indeterministic quantum dynamics is inferred through the coincidence between
the microcanonical probability distribution and the semiclassical quantum one.Comment: 9 pages, 1 figur
Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient
We analyse the two-dimensional MHD configurations characterising the steady
state of the accretion disk on a highly magnetised neutron star. The model we
describe has a local character and represents the extension of the crystalline
structure outlined in Coppi (2005), dealing with a local model too, when a
specific accretion rate is taken into account. We limit our attention to the
linearised MHD formulation of the electromagnetic back-reaction characterising
the equilibrium, by fixing the structure of the radial, vertical and azimuthal
profiles. Since we deal with toroidal currents only, the consistency of the
model is ensured by the presence of a small collisional effect,
phenomenologically described by a non-zero constant Nernst coefficient (thermal
power of the plasma). Such an effect provides a proper balance of the electron
force equation via non zero temperature gradients, related directly to the
radial and vertical velocity components.
We show that the obtained profile has the typical oscillating feature of the
crystalline structure, reconciled with the presence of viscosity, associated to
the differential rotation of the disk, and with a net accretion rate. In fact,
we provide a direct relation between the electromagnetic reaction of the disk
and the (no longer zero) increasing of its mass per unit time. The radial
accretion component of the velocity results to be few orders of magnitude below
the equatorial sound velocity. Its oscillating-like character does not allow a
real matter in-fall to the central object (an effect to be searched into
non-linear MHD corrections), but it accounts for the out-coming of steady
fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics
Letters
Dark Energy as a Relic of the Vacuum-Energy Cancellation?
We analyze the dynamical implications of an exponential Lagrangian density
for the gravitational field, as referred to an isotropic FRW Universe. Then, we
discuss the features of the generalized deSitter phase, predicted by the new
Friedmann equation. The existence of a consistent deSitter solution arises only
if the ratio between the vacuum-energy density and that associated with the
fundamental length of the theory acquires a tantalizing negative character.
This choice allows us to explain the present universe dark energy as a relic of
the vacuum-energy cancellation due to the cosmological constant intrinsically
contained in our scheme. The corresponding scalar-tensor description of the
model is addressed too, and the behavior of the scalar field is analyzed for
both negative and positive values of the cosmological term. In the first case,
the Friedmann equation is studied both in vacuum and in presence of external
matter, while, in the second case, the quantum regime is approached in the
framework of ''repulsive'' properties of the gravitational interaction, as
described in recent issues in Loop Quantum Cosmology. In particular, in the
vacuum case, we find a pure non-Einsteinian effect, according to which a
negative cosmological constant provides an accelerating deSitter dynamics, in
the region where the series expansion of the exponential term does not hold.Comment: 24 pages, 2 figures, to appear on IJMP
Contributions to the linear and nonlinear theory of the beam-plasma interaction
We focus our attention on some relevant aspects of the beam-plasma instability in order to refine some features of the linear and nonlinear dynamics. After a re-Analysis of the Poisson equation and of the assumption dealing with the background plasma in the form of a linear dielectric, we study the non-perturbative properties of the linear dispersion relation, showing the necessity for a better characterization of the mode growth rate in those flat regions of the distribution function where the Landau formula is no longer predictive. We then upgrade the original-body approach in O'Neil et al. (Phys. Fluids, vol. 14, 1971, pp. 1204-1212), in order to include a return current in the background plasma. This correction term is responsible for smaller saturation levels and growth rates of the Langmuir modes, as result of the energy density transferred to the plasma via the return current. Finally, we include friction effects, as those due to the collective influence of all the plasma charges on the motion of the beam particles. The resulting force induces a progressive resonance detuning, because particles are losing energy and decreasing their velocity. This friction phenomenon gives rise to a deformation of the distribution function, associated with a significant growth of the less energetic particle population. The merit of this work is to show how a fine analysis of the beam-plasma instability outlines a number of subtleties about the linear, intermediate and late dynamics which can be of relevance when such a system is addressed as a paradigm to describe relevant nonlinear wave-particle phenomena (Chen Zonca, Rev. Mod. Phys., vol. 88, 2016, 015008)
Oscillatory regime in the Multidimensional Homogeneous Cosmological Models Induced by a Vector Field
We show that in multidimensional gravity vector fields completely determine
the structure and properties of singularity. It turns out that in the presence
of a vector field the oscillatory regime exists in all spatial dimensions and
for all homogeneous models. By analyzing the Hamiltonian equations we derive
the Poincar\'e return map associated to the Kasner indexes and fix the rules
according to which the Kasner vectors rotate. In correspondence to a
4-dimensional space time, the oscillatory regime here constructed overlap the
usual Belinski-Khalatnikov-Liftshitz one.Comment: 9 pages, published on Classical and Quantum Gravit
Nonlinear velocity redistribution caused by energetic-particle-driven geodesic acoustic modes, mapped with the beam-plasma system
The nonlinear dynamics of energetic particle (EP) driven geodesic acoustic
modes (EGAM) in tokamaks is investigated, and compared with the beam-plasma
system (BPS). The EGAM is studied with the global gyrokinetic (GK)
particle-in-cell code ORB5, treating the thermal ions and EP (in this case,
fast ions) as GK and neglecting the kinetic effects of the electrons. The
wave-particle nonlinearity only is considered in the EGAM nonlinear dynamics.
The BPS is studied with a 1D code where the thermal plasma is treated as a
linear dielectric, and the EP (in this case, fast electrons) with an n-body
hamiltonian formulation. A one-to-one mapping between the EGAM and the BPS is
described. The focus is on understanding and predicting the EP redistribution
in phase space. We identify here two distint regimes for the mapping: in the
low-drive regime, the BPS mapping with the EGAM is found to be complete, and in
the high-drive regime, the EGAM dynamics and the BPS dynamics are found to
differ. The transition is described with the presence of a non-negligible
frequency chirping, which affects the EGAM but not the BPS, above the
identified drive threshold. The difference can be resolved by adding an ad-hoc
frequency modification to the BPS model. As a main result, the formula for the
prediction of the nonlinear width of the velocity redistribution around the
resonance velocity is provided
Integrable mixing of A_{n-1} type vertex models
Given a family of monodromy matrices {T_u; u=0,1,...,K-1} corresponding to
integrable anisotropic vertex models of A_{(n_u)-1}-type, we build up a related
mixed vertex model by means of glueing the lattices on which they are defined,
in such a way that integrability property is preserved. Algebraically, the
glueing process is implemented through one dimensional representations of
rectangular matrix algebras A(R_p,R_q), namely, the `glueing matrices' zeta_u.
Here R_n indicates the Yang-Baxter operator associated to the standard Hopf
algebra deformation of the simple Lie algebra A_{n-1}. We show there exists a
pseudovacuum subspace with respect to which algebraic Bethe ansatz can be
applied. For each pseudovacuum vector we have a set of nested Bethe ansatz
equations identical to the ones corresponding to an A_{m-1} quasi-periodic
model, with m equal to the minimal range of involved glueing matrices.Comment: REVTeX 28 pages. Here we complete the proof of integrability for
mixed vertex models as defined in the first versio
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