12,185 research outputs found
Testing symmetries in effective models of higher derivative field theories
Higher derivative field theories with interactions raise serious doubts about
their validity due to severe energy instabilities. In many cases the
implementation of a direct perturbation treatment to excise the dangerous
negative-energies from a higher derivative field theory may lead to violations
of Lorentz and other symmetries. In this work we study a perturbative
formulation for higher derivative field theories that allows the construction
of a low-energy effective field theory being a genuine perturbations over the
ordinary-derivative theory and having a positive-defined Hamiltonian. We show
that some discrete symmetries are recovered in the low-energy effective theory
when the perturbative method to reduce the negative-energy degrees of freedom
from the higher derivative theory is applied. In particular, we focus on the
higher derivative Maxwell-Chern-Simons model which is a Lorentz invariant and
parity-odd theory in 2+1 dimensions. The parity violation arises in the
effective action of QED as a quantum correction from the massive fermionic
sector. We obtain the effective field theory which remains Lorentz invariant,
but parity invariant to the order considered in the perturbative expansion.Comment: 13 pages, Sec. III, additional references added, P symmetry revised,
accepted for publication in PR
Impurity in a granular gas under nonlinear Couette flow
We study in this work the transport properties of an impurity immersed in a
granular gas under stationary nonlinear Couette flow. The starting point is a
kinetic model for low-density granular mixtures recently proposed by the
authors [Vega Reyes F et al. 2007 Phys. Rev. E 75 061306]. Two routes have been
considered. First, a hydrodynamic or normal solution is found by exploiting a
formal mapping between the kinetic equations for the gas particles and for the
impurity. We show that the transport properties of the impurity are
characterized by the ratio between the temperatures of the impurity and gas
particles and by five generalized transport coefficients: three related to the
momentum flux (a nonlinear shear viscosity and two normal stress differences)
and two related to the heat flux (a nonlinear thermal conductivity and a cross
coefficient measuring a component of the heat flux orthogonal to the thermal
gradient). Second, by means of a Monte Carlo simulation method we numerically
solve the kinetic equations and show that our hydrodynamic solution is valid in
the bulk of the fluid when realistic boundary conditions are used. Furthermore,
the hydrodynamic solution applies to arbitrarily (inside the continuum regime)
large values of the shear rate, of the inelasticity, and of the rest of
parameters of the system. Preliminary simulation results of the true Boltzmann
description show the reliability of the nonlinear hydrodynamic solution of the
kinetic model. This shows again the validity of a hydrodynamic description for
granular flows, even under extreme conditions, beyond the Navier-Stokes domain.Comment: 23 pages, 11 figures; v2: Preliminary DSMC results from the Boltzmann
equation included, Fig. 11 is ne
Combined Relativistic and static analysis for all Delta B=2 operators
We analyse matrix elements of Delta B=2 operators by combining QCD results
with the ones obtained in the static limit of HQET. The matching of all the QCD
operators to HQET is made at NLO order. To do that we have to include the
anomalous dimension matrix up to two loops, both in QCD and HQET, and the one
loop matching for all the Delta B=2 operators. The matrix elements of these
operators are relevant for the prediction of the B-\bar B mixing, B_s meson
width difference and supersymmetric effects in Delta B=2 transitions.Comment: 3 pages, 1 figure. Lattice2001(heavyquark
Renormalization Constants of Quark Operators for the Non-Perturbatively Improved Wilson Action
We present the results of an extensive lattice calculation of the
renormalization constants of bilinear and four-quark operators for the
non-perturbatively O(a)-improved Wilson action. The results are obtained in the
quenched approximation at four values of the lattice coupling by using the
non-perturbative RI/MOM renormalization method. Several sources of systematic
uncertainties, including discretization errors and final volume effects, are
examined. The contribution of the Goldstone pole, which in some cases may
affect the extrapolation of the renormalization constants to the chiral limit,
is non-perturbatively subtracted. The scale independent renormalization
constants of bilinear quark operators have been also computed by using the
lattice chiral Ward identities approach and compared with those obtained with
the RI-MOM method. For those renormalization constants the non-perturbative
estimates of which have been already presented in the literature we find an
agreement which is typically at the level of 1%.Comment: 36 pages, 13 figures. Minor changes in the text and in one figure.
Accepted for publication on JHE
Fermion Condensate and Vacuum Current Density Induced by Homogeneous and Inhomogeneous Magnetic Fields in (2+1)-Dimensions
We calculate the condensate and the vacuum current density induced by
external static magnetic fields in (2+1)-dimensions. At the perturbative level,
we consider an exponentially decaying magnetic field along one cartesian
coordinate. Non-perturbatively, we obtain the fermion propagator in the
presence of a uniform magnetic field by solving the Schwinger-Dyson equation in
the rainbow-ladder approximation. In the large flux limit, we observe that both
these quantities, either perturbative (inhomogeneous) and non-perturbative
(homogeneous), are proportional to the external field, in agreement with early
expectations.Comment: 8 pages, 2 figures. Accepted for publication in Phys. Rev.
Bose-Einstein condensation in antiferromagnets close to the saturation field
At zero temperature and strong applied magnetic fields the ground sate of an
anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins.
We study the quantum phase transition as the field is reduced below an upper
critical and the system enters a XY-antiferromagnetic phase. Using a
bond operator representation we consider a model spin-1 Heisenberg
antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under
strong magnetic fields. We show that the transition at can be
interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical
results are used to analyze our magnetization versus field data in the organic
compound - (DTN) at very low temperatures. This is the
ideal BEC system to study this transition since is sufficiently low to
be reached with static magnetic fields (as opposed to pulsed fields). The
scaling of the magnetization as a function of field and temperature close to
shows excellent agreement with the theoretical predictions. It allows
to obtain the quantum critical exponents and confirm the BEC nature of the
transition at .Comment: 4 pages, 1 figure. Accepted for publication in PRB
An exact solution of the inelastic Boltzmann equation for the Couette flow with uniform heat flux
In the steady Couette flow of a granular gas the sign of the heat flux
gradient is governed by the competition between viscous heating and inelastic
cooling. We show from the Boltzmann equation for inelastic Maxwell particles
that a special class of states exists where the viscous heating and the
inelastic cooling exactly compensate each other at every point, resulting in a
uniform heat flux. In this state the (reduced) shear rate is enslaved to the
coefficient of restitution , so that the only free parameter is the
(reduced) thermal gradient . It turns out that the reduced moments of
order are polynomials of degree in , with coefficients that
are nonlinear functions of . In particular, the rheological properties
() are independent of and coincide exactly with those of the
simple shear flow. The heat flux () is linear in the thermal gradient
(generalized Fourier's law), but with an effective thermal conductivity
differing from the Navier--Stokes one. In addition, a heat flux component
parallel to the flow velocity and normal to the thermal gradient exists. The
theoretical predictions are validated by comparison with direct Monte Carlo
simulations for the same model.Comment: 16 pages, 4 figures,1 table; v2: minor change
Is This a Joke? Detecting Humor in Spanish Tweets
While humor has been historically studied from a psychological, cognitive and
linguistic standpoint, its study from a computational perspective is an area
yet to be explored in Computational Linguistics. There exist some previous
works, but a characterization of humor that allows its automatic recognition
and generation is far from being specified. In this work we build a
crowdsourced corpus of labeled tweets, annotated according to its humor value,
letting the annotators subjectively decide which are humorous. A humor
classifier for Spanish tweets is assembled based on supervised learning,
reaching a precision of 84% and a recall of 69%.Comment: Preprint version, without referra
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