2,905 research outputs found
The Post-Newtonian Limit of f(R)-gravity in the Harmonic Gauge
A general analytic procedure is developed for the post-Newtonian limit of
-gravity with metric approach in the Jordan frame by using the harmonic
gauge condition. In a pure perturbative framework and by using the Green
function method a general scheme of solutions up to order is shown.
Considering the Taylor expansion of a generic function it is possible to
parameterize the solutions by derivatives of . At Newtonian order,
, all more important topics about the Gauss and Birkhoff theorem are
discussed. The corrections to "standard" gravitational potential
(-component of metric tensor) generated by an extended uniform mass
ball-like source are calculated up to order. The corrections, Yukawa
and oscillating-like, are found inside and outside the mass distribution. At
last when the limit is considered the -gravity converges
in General Relativity at level of Lagrangian, field equations and their
solutions.Comment: 16 pages, 10 figure
Tube Model for Light-Front QCD
We propose the tube model as a first step in solving the bound state problem
in light-front QCD. In this approach we neglect transverse variations of the
fields, producing a model with 1+1 dimensional dynamics. We then solve the two,
three, and four particle sectors of the model for the case of pure glue SU(3).
We study convergence to the continuum limit and various properties of the
spectrum.Comment: 29 page
Melting of hexagonal skyrmion states in chiral magnets
Skyrmions are spiral structures observed in thin films of certain magnetic materials (Uchida et al 2006 Science 311 359–61). Of the phases allowed by the crystalline symmetries of these materials (Yi et al 2009 Phys. Rev. B 80 054416), only the hexagonally packed phases (SCh) have been observed. Here the melting of the SCh phase is investigated using Monte Carlo simulations. In addition to the usual measure of skyrmion density, chiral charge, a morphological measure is considered. In doing so it is shown that the low-temperature reduction in chiral charge is associated with a change in skyrmion profiles rather than skyrmion destruction. At higher temperatures, the loss of six-fold symmetry is associated with the appearance of elongated skyrmions that disrupt the hexagonal packing
Masses of the physical mesons from an effective QCD--Hamiltonian
The front form Hamiltonian for quantum chromodynamics, reduced to an
effective Hamiltonian acting only in the space, is solved
approximately. After coordinate transformation to usual momentum space and
Fourier transformation to configuration space a second order differential
equation is derived. This retarded Schr\"odinger equation is solved by
variational methods and semi-analytical expressions for the masses of all 30
pseudoscalar and vector mesons are derived. In view of the direct relation to
quantum chromdynamics without free parameter, the agreement with experiment is
remarkable, but the approximation scheme is not adequate for the mesons with
one up or down quark. The crucial point is the use of a running coupling
constant , in a manner similar but not equal to the one of
Richardson in the equal usual-time quantization. Its value is fixed at the Z
mass and the 5 flavor quark masses are determined by a fit to the vector meson
quarkonia.Comment: 18 pages, 4 Postscript figure
Klein-Gordon Equation in Hydrodynamical Form
We follow and modify the Feshbach-Villars formalism by separating the
Klein-Gordon equation into two coupled time-dependent Schroedinger equations
for particle and antiparticle wave function components with positive
probability densities. We find that the equation of motion for the probability
densities is in the form of relativistic hydrodynamics where various forces
have their classical counterparts, with the additional element of the quantum
stress tensor that depends on the derivatives of the amplitude of the wave
function. We derive the equation of motion for the Wigner function and we find
that its approximate classical weak-field limit coincides with the equation of
motion for the distribution function in the collisionless kinetic theory.Comment: 13 page
Massive Dirac particles on the background of charged de-Sitter black hole manifolds
We consider the behavior of massive Dirac fields on the background of a
charged de-Sitter black hole. All black hole geometries are taken into account,
including the Reissner-Nordstr\"{o}m-de-Sitter one, the Nariai case and the
ultracold case. Our focus is at first on the existence of bound quantum
mechanical states for the Dirac Hamiltonian on the given backgrounds. In this
respect, we show that in all cases no bound state is allowed, which amounts
also to the non-existence of normalizable time-periodic solutions of the Dirac
equation. This quantum result is in contrast to classical physics, and it is
shown to hold true even for extremal cases. Furthermore, we shift our attention
on the very interesting problem of the quantum discharge of the black holes.
Following Damour-Deruelle-Ruffini approach, we show that the existence of
level-crossing between positive and negative continuous energy states is a
signal of the quantum instability leading to the discharge of the black hole,
and in the cases of the Nariai geometry and of the ultracold geometries we also
calculate in WKB approximation the transmission coefficient related to the
discharge process.Comment: 19 pages, 11 figures. Macro package: Revtex4. Changes concern mainly
the introduction and the final discussion in section VI; moreover, Appendix D
on the evaluation of the Nariai transmission integral has been added.
References adde
Generalized geometric quantum speed limits
The attempt to gain a theoretical understanding of the concept of time in quantum mechanics has triggered significant progress towards the search for faster and more efficient quantum technologies. One of such advances consists in the interpretation of the time-energy uncertainty relations as lower bounds for the minimal evolution time between two distinguishable states of a quantum system, also known as quantum speed limits. We investigate how the nonuniqueness of a bona fide measure of distinguishability defined on the quantum-state space affects the quantum speed limits and can be exploited in order to derive improved bounds. Specifically, we establish an infinite family of quantum speed limits valid for unitary and nonunitary evolutions, based on an elegant information geometric formalism. Our work unifies and generalizes existing results on quantum speed limits and provides instances of novel bounds that are tighter than any established one based on the conventional quantum Fisher information. We illustrate our findings with relevant examples, demonstrating the importance of choosing different information metrics for open system dynamics, as well as clarifying the roles of classical populations versus quantum coherences, in the determination and saturation of the speed limits. Our results can find applications in the optimization and control of quantum technologies such as quantum computation and metrology, and might provide new insights in fundamental investigations of quantum thermodynamics
Strong Orientation Effects in Ionization of H by Short, Intense, High-Frequency Light Sources
We present three dimensional time-dependent calculations of ionization of
arbitrarily spatially oriented H by attosecond, intense, high-frequency
laser fields. The ionization probability shows a strong dependence on both the
internuclear distance and the relative orientation between the laser field and
the internuclear axis.Comment: 4 pages, 4 figure
Quantum Master Equation of Particle in Gas Environment
The evolution of the reduced density operator of Brownian particle is
discussed in single collision approach valid typically in low density gas
environments. This is the first succesful derivation of quantum friction caused
by {\it local} environmental interactions. We derive a Lindblad master equation
for , whose generators are calculated from differential cross section of
a single collision between Brownian and gas particles, respectively. The
existence of thermal equilibrium for is proved. Master equations
proposed earlier are shown to be particular cases of our one.Comment: 6 pages PlainTeX, 23-March-199
Tetramixing of vector and pseudoscalar mesons: A source of intrinsic quarks
The tetramixing of pseudoscalar mesons --- and
vector mesons --- are studied in the light-cone
constituent quark model, and such mixing of four mesons provides a natural
source for the intrinsic charm components of light mesons. By mixing
with the light mesons, the charmonium states and could decay
into light mesons more naturally, without introducing gluons or a virtual
photon as intermediate states. Thus, the introduction of light quark components
into is helpful to reproduce the new experimental data of
decays. The mixing matrices and the behaviors of the transition form
factors are also calculated and compared with experimental data.Comment: 20 pages, 10 figures. Version for publication in PR
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