780 research outputs found
Ultraviolet-Renormalon Reexamined
We consider large-order perturbative expansions in QED and QCD. The
coefficients of the expansions are known to be dominated by the so called
ultraviolet (UV) renormalons which arise from inserting a chain of
vacuum-polarization graphs into photonic (gluonic) lines. In large orders the
contribution is associated with virtual momenta of order where
is external momentum, is the base of natural logs and is the order
of perturbation theory considered. To evaluate the UV renormalon we develop
formalism of operator product expansion (OPE) which utilizes the observation
that . When applied to the simplest graphs the formalism reproduces
the known results in a compact form. In more generality, the formalism reveals
the fact that the class of the renormalon-type graphs is not well defined. In
particular, graphs with extra vacuum-polarization chains are not suppressed.
The reason is that while inclusion of extra chains lowers the power of their contribution is enhanced by combinatorial factors.Comment: LaTex, 18 pages, 5 figures. Some numerical coefficients are corrected
once mor
Generalization of the Fierz-Pauli Action
We consider the Lagrangian of gravity covariantly amended by the mass and
polynomial interaction terms with arbitrary coefficients, and reinvestigate the
consistency of such a theory in the decoupling limit, up to the fifth order in
the nonlinearities. We calculate explicitly the self-interactions of the
helicity-0 mode, as well as the nonlinear mixing between the helicity-0 and -2
modes. We show that ghost-like pathologies in these interactions disappear for
special choices of the polynomial interactions, and argue that this result
remains true to all orders in the decoupling limit. Moreover, we show that the
linear, and some of the nonlinear mixing terms between the helicity-0 and -2
modes can be absorbed by a local change of variables, which then naturally
generates the cubic, quartic, and quintic Galileon interactions, introduced in
a different context. We also point out that the mixing between the helicity-0
and 2 modes can be at most quartic in the decoupling limit. Finally, we discuss
the implications of our findings for the consistency of the effective field
theory away from the decoupling limit, and for the Boulware-Deser problem.Comment: 18 pages, no figure
Masses and decay constants of bound states containing fourth family quarks from QCD sum rules
The heavy fourth generation of quarks that have sufficiently small mixing
with the three known SM families form hadrons. In the present work, we
calculate the masses and decay constants of mesons containing either both
quarks from the fourth generation or one from fourth family and the other from
known third family SM quarks in the framework of the QCD sum rules. In the
calculations, we take into account two gluon condensate diagrams as
nonperturbative contributions. The obtained results reduce to the known masses
and decay constants of the and quarkonia when the fourth
family quark is replaced by the bottom or charm quark.Comment: 15 Pages, 9 Figures and 6 Table
Tunneling-assisted impact ionization fronts in semiconductors
We propose a novel type of ionization front in layered semiconductor
structures. The propagation is due to the interplay of band-to-band tunneling
and impact ionization. Our numerical simulations show that the front can be
triggered when an extremely sharp voltage ramp () is
applied in reverse direction to a Si structure that is connected in
series with an external load. The triggering occurs after a delay of 0.7 to 0.8
ns. The maximal electrical field at the front edge exceeds .
The front velocity is 40 times faster than the saturated drift velocity
. The front passes through the base with a thickness of
within approximately 30 ps, filling it with dense electron-hole plasma. This
passage is accompanied by a voltage drop from 8 kV to dozens of volts. In this
way a voltage pulse with a ramp up to can be applied to the
load. The possibility to form a kilovolt pulse with such a voltage rise rate
sets new frontiers in pulse power electronics.Comment: 12 pages, 6 figure
A Note on the Cosmological Dynamics in Finite-Range Gravity
In this note we consider the homogeneous and isotropic cosmology in the
finite-range gravity theory recently proposed by Babak and Grishchuk. In this
scenario the universe undergoes late time accelerated expansion if both the
massive gravitons present in the model are tachyons. We carry out the phase
space analysis of the system and show that the late-time acceleration is an
attractor of the model.Comment: RevTex, 4 pages, two figures, New references added, To appear in
IJMP
Scalar Quarkonia at Finite Temperature
Masses and decay constants of the scalar quarkonia, with
quantum numbers are calculated in the framework of
the QCD sum rules approach both in vacuum and finite temperature. The masses
and decay constants remain unchanged up to but they start to
diminish with increasing the temperature after this point. At near the critic
or deconfinement temperature, the decay constants reach approximately to 25% of
their values in vacuum, while the masses are decreased about 6% and 23% for
bottom and charm cases, respectively. The results at zero temperature are in a
good consistency with the existing experimental values and predictions of the
other nonperturbative approaches. Our predictions on the decay constants in
vacuum as well as the behavior of the masses and decay constants with respect
to the temperature can be checked in the future experiments.Comment: 12 Pages, 9 Figures and 2 Table
Hadronic Light-by-Light Scattering in the Muonium Hyperfine Splitting
We consider an impact of hadronic light-by-light scattering on the muonium
hyperfine structure. A shift of the hyperfine interval is calculated with the light-by-light scattering approximated
by exchange of pseudoscalar and pseudovector mesons. Constraints from the
operator product expansion in QCD are used to fix parameters of the model
similar to the one used earlier for the hadronic light-by-light scattering in
calculations of the muon anomalous magnetic moment. The pseudovector exchange
is dominant in the resulting shift, . Although the effect is tiny it is useful in understanding
the level of hadronic uncertainties.Comment: 16 pages, 7 figures, a reference adde
Compressible hydromagnetic nonlinearities in the predecoupling plasma
The adiabatic inhomogeneities of the scalar curvature lead to a compressible
flow affecting the dynamics of the hydromagnetic nonlinearities. The influence
of the plasma on the evolution of a putative magnetic field is explored with
the aim of obtaining an effective description valid for sufficiently large
scales. The bulk velocity of the plasma, computed in the framework of the
LambdaCDM scenario, feeds back into the evolution of the magnetic power spectra
leading to a (nonlocal) master equation valid in Fourier space and similar to
the ones discussed in the context of wave turbulence. Conversely, in physical
space, the magnetic power spectra obey a Schroedinger-like equation whose
effective potential depends on the large-scale curvature perturbations.
Explicit solutions are presented both in physical space and in Fourier space.
It is argued that curvature inhomogeneities, compatible with the WMAP 7yr data,
shift to lower wavenumbers the magnetic diffusivity scale.Comment: 29 page
Numerical evidence of chiral magnetic effect in lattice gauge theory
The chiral magnetic effect is the generation of electric current of quarks
along external magnetic field in the background of topologically nontrivial
gluon fields. There is a recent evidence that this effect is observed by the
STAR Collaboration in heavy ion collisions at RHIC. In our paper we study
qualitative signatures of the chiral magnetic effect using quenched lattice
simulations. We find indications that the electric current is indeed enhanced
in the direction of the magnetic field both in equilibrium configurations of
the quantum gluon fields and in a smooth gluon background with nonzero
topological charge. In the confinement phase the magnetic field enhances the
local fluctuations of both the electric charge and chiral charge densities. In
the deconfinement phase the effects of the magnetic field become smaller,
possibly due to thermal screening. Using a simple model of a fireball we obtain
a good agreement between our data and experimental results of the STAR
Collaboration.Comment: 14 pages, 14 figures, uses RevTeX 4.0; revision: references and
comments added, figures corrected, published versio
Rapid dissipation of magnetic fields due to Hall current
We propose a mechanism for the fast dissipation of magnetic fields which is
effective in a stratified medium where ion motions can be neglected. In such a
medium, the field is frozen into the electrons and Hall currents prevail.
Although Hall currents conserve magnetic energy, in the presence of density
gradients, they are able to create current sheets which can be the sites for
efficient dissipation of magnetic fields. We recover the frequency,
, for Hall oscillations modified by the presence of density
gradients. We show that these oscillations can lead to the exchange of energy
between different components of the field. We calculate the time evolution and
show that magnetic fields can dissipate on a timescale of order
. This mechanism can play an important role for magnetic
dissipation in systems with very steep density gradients where the ions are
static such as those found in the solid crust of neutron stars.Comment: 9 pages, changed fig.
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