3,957 research outputs found
On the leading OPE corrections to the ghost-gluon vertex and the Taylor theorem
This brief note is devoted to a study of genuine non-perturbative corrections
to the Landau gauge ghost-gluon vertex in terms of the non-vanishing
dimension-two gluon condensate. We pay special attention to the kinematical
limit which the bare vertex takes for its tree-level expression at any
perturbative order, according to the well-known Taylor theorem. Based on our
OPE analysis, we also present a simple model for the vertex, in acceptable
agreement with lattice data.Comment: Final version published in JHE
Anomalous resonance phenomena of solitary waves with internal modes
We investigate the non-parametric, pure ac driven dynamics of nonlinear
Klein-Gordon solitary waves having an internal mode of frequency . We
show that the strongest resonance arises when the driving frequency
, whereas when the resonance is weaker,
disappearing for nonzero damping. At resonance, the dynamics of the kink center
of mass becomes chaotic. As we identify the resonance mechanism as an {\em
indirect} coupling to the internal mode due to its symmetry, we expect similar
results for other systems.Comment: 4 pages, 4 figures, to appear in Phys Rev Let
On the existence of internal modes of sine-Gordon kinks
We study whether or not sine-Gordon kinks exhibit internal modes or
``quasimodes.'' By considering the response of the kinks to ac forces and
initial distortions, we show that neither intrinsic internal modes nor
``quasimodes'' exist in contrast to previous reports. However, we do identify a
different kind of internal mode bifurcating from the bottom edge of the phonon
band which arises from the discretization of the system in the numerical
simulations, thus confirming recent predictions.Comment: 4 pages, 2 figures, REVTeX, to appear as a Rapid Communication in
Phys Rev E (July 1st
Finite-size effects on multibody neutrino exchange
The effect of multibody massless neutrino exchanges between neutrons inside a
finite-size neutron star is studied. We use an effective Lagrangian, which
incorporates the effect of the neutrons on the neutrinos. Following Schwinger,
it is shown that the total interaction energy density is computed by comparing
the zero point energy of the neutrino sea with and without the star. It has
already been shown that in an infinite-size star the total energy due to
neutrino exchange vanishes exactly. The opposite claim that massless neutrino
exchange would produce a huge energy is due to an improper summation of an
infrared-divergent quantity. The same vanishing of the total energy has been
proved exactly in the case of a finite star in a one-dimensional toy model.
Here we study the three-dimensional case. We first consider the effect of a
sharp star border, assumed to be a plane. We find that there is a non-
vanishing of the zero point energy density difference between the inside and
the outside due to the refraction index at the border and the consequent
non-penetrating waves. An analytical and numerical calculation for the case of
a spherical star with a sharp border confirms that the preceding border effect
is the dominant one. The total result is shown to be infrared-safe, thus
confirming that there is no need to assume a neutrino mass. The ultraviolet
cut-offs, which correspond in some sense to the matching of the effective
theory with the exact one, are discussed. Finally the energy due to long
distance neutrino exchange is of the order of , i.e. negligible with respect to the neutron mass density.Comment: Latex file (Revtex), 34 pages, 8 postscripted figure
Electron-phonon renormalization of the absorption edge of the cuprous halides
Compared to most tetrahedral semiconductors, the temperature dependence of
the absorption edges of the cuprous halides (CuCl, CuBr, CuI) is very small.
CuCl and CuBr show a small increase of the gap with increasing
temperature, with a change in the slope of vs. at around 150 K: above
this temperature, the variation of with becomes even smaller. This
unusual behavior has been clarified for CuCl by measurements of the low
temperature gap vs. the isotopic masses of both constituents, yielding an
anomalous negative shift with increasing copper mass. Here we report the
isotope effects of Cu and Br on the gap of CuBr, and that of Cu on the gap of
CuI. The measured isotope effects allow us to understand the corresponding
temperature dependences, which we also report, to our knowledge for the first
time, in the case of CuI. These results enable us to develop a more
quantitative understanding of the phenomena mentioned for the three halides,
and to interpret other anomalies reported for the temperature dependence of the
absorption gap in copper and silver chalcogenides; similarities to the behavior
observed for the copper chalcopyrites are also pointed out.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
Thermal diffusion of supersonic solitons in an anharmonic chain of atoms
We study the non-equilibrium diffusion dynamics of supersonic lattice
solitons in a classical chain of atoms with nearest-neighbor interactions
coupled to a heat bath. As a specific example we choose an interaction with
cubic anharmonicity. The coupling between the system and a thermal bath with a
given temperature is made by adding noise, delta-correlated in time and space,
and damping to the set of discrete equations of motion. Working in the
continuum limit and changing to the sound velocity frame we derive a
Korteweg-de Vries equation with noise and damping. We apply a collective
coordinate approach which yields two stochastic ODEs which are solved
approximately by a perturbation analysis. This finally yields analytical
expressions for the variances of the soliton position and velocity. We perform
Langevin dynamics simulations for the original discrete system which fully
confirm the predictions of our analytical calculations, namely noise-induced
superdiffusive behavior which scales with the temperature and depends strongly
on the initial soliton velocity. A normal diffusion behavior is observed for
very low-energy solitons where the noise-induced phonons also make a
significant contribution to the soliton diffusion.Comment: Submitted to PRE. Changes made: New simulations with a different
method of soliton detection. The results and conclusions are not different
from previous version. New appendixes containing information about the system
energy and soliton profile
First lattice QCD estimate of the g_{D^* D pi} coupling
We present the results of the first lattice QCD study of the strong coupling
g_{D^* D pi}. From our simulations in the quenched approximation, we obtain
g_{D^* D pi} = 18.8 +/- 2.3^{+1.1}_{-2.0} and hat(g)_c = 0.67 +/-
0.08^{+0.04}_{-0.06}. Whereas previous theoretical studies gave different
predictions, our result favours a large value for hat(g)_c. It agrees very well
with the recent experimental value by CLEO. hat(g) varies very little with the
heavy mass and we find in the infinite mass limit hat(g)_infinity = 0.69(18).Comment: 24 pages, 7 figures; references added, corrected typos, Comments
added about the continuum limi
Preliminary Calculation of from Green Functions with Dynamical Quarks
We present preliminary results on the computation of the QCD running coupling
constant in the scheme and Landau gauge with two flavours of
dynamical Wilson quarks. Gluon momenta range up to about 7 GeV ( 5.6,
5.8 and 6.0) with a constant dynamical-quark mass. This range already allows to
exhibit some evidence for a sizable correction to the asymptotic
behaviour, as in the quenched approximation, although a fit without power
corrections is still possible with a reasonable . Following the
conclusions of our quenched study, we take into account correction to
the asymptotic behaviour. We find , which leads to
. The latter result has to be taken as a
preliminary indication rather than a real prediction in view of the systematic
errors still to be controlled. Still, being two sigmas below the experimental
result makes it very encouraging.Comment: 14 pages, 3 figs., 2 tabs., revte
On the massive gluon propagator, the PT-BFM scheme and the low-momentum behaviour of decoupling and scaling DSE solutions
We study the low-momentum behaviour of Yang-Mills propagators obtained from
Landau-gauge Dyson-Schwinger equations (DSE) in the PT-BFM scheme. We compare
the ghost propagator numerical results with the analytical ones obtained by
analyzing the low-momentum behaviour of the ghost propagator DSE in Landau
gauge, assuming for the truncation a constant ghost-gluon vertex and a simple
model for a massive gluon propagator. The asymptotic expression obtained for
the regular or decoupling ghost dressing function up to the order is proven to fit pretty well the numerical PT-BFM results.
Furthermore, when the size of the coupling renormalized at some scale
approaches some critical value, the numerical PT-BFM propagators tend to behave
as the scaling ones. We also show that the scaling solution, implying a
diverging ghost dressing function, cannot be a DSE solution in the PT-BFM
scheme but an unattainable limiting case.Comment: 16 pages, 2 figs., 2 tabs (updated version to be published in JHEP
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