637 research outputs found
Solutions of Podolsky's Electrodynamics Equation in the First-Order Formalism
The Podolsky generalized electrodynamics with higher derivatives is
formulated in the first-order formalism. The first-order relativistic wave
equation in the 20-dimensional matrix form is derived. We prove that the
matrices of the equation obey the Petiau-Duffin-Kemmer algebra. The
Hermitianizing matrix and Lagrangian in the first-order formalism are given.
The projection operators extracting solutions of field equations for states
with definite energy-momentum and spin projections are obtained, and we find
the density matrix for the massive state. The -matrix Schrodinger
form of the equation is derived, and the Hamiltonian is obtained. Projection
operators extracting the physical eigenvalues of the Hamiltonian are found.Comment: 17 pages, minor corrections, published versio
Bipolaron-SO(5) Non-Fermi Liquid in a Two-channel Anderson Model with Phonon-assisted Hybridizations
We analyze non-Fermi liquid (NFL) properties along a line of critical points
in a two-channel Anderson model with phonon-assisted hybridizations. We succeed
in identifying hidden nonmagnetic SO(5) degrees of freedom for
valence-fluctuation regime and analyze the model on the basis of boundary
conformal field theory. We find that the NFL spectra along the critical line,
which is the same as those in the two-channel Kondo model, can be alternatively
derived by a fusion in the nonmagnetic SO(5) sector. The leading irrelevant
operators near the NFL fixed points vary as a function of Coulomb repulsion U;
operators in the spin sector dominate for large U, while those in the SO(5)
sector do for small U, and we confirm this variation in our numerical
renormalization group calculations. As a result, the thermodynamic singularity
for small U differs from that of the conventional two-channel Kondo problem.
Especially, the impurity contribution to specific heat is proportional to
temperature and bipolaron fluctuations, which are coupled electron-phonon
fluctuations, diverge logarithmically at low temperatures for small U.Comment: 16 pages, 4 figures, 3 table
Gauge invariance and non-constant gauge couplings
It is shown that space-time dependent gauge couplings do not completely break
gauge invariance. We demonstrate this in various gauge theories.Comment: 18 page
Reduction formalism for Dirac fermions on de Sitter spacetime
The reduction formulas for Dirac fermions are derived, using the exact
solutions of free Dirac equation on de Sitter spacetime. In the framework of
the perturbation theory one studies the Green functions and derive the
scatering amplitude in the first orders of perturbation theory.Comment: 12 pages, no figure
A Maximally Symmetric Vector Propagator
We derive the propagator for a massive vector field on a de Sitter background
of arbitrary dimension. This propagator is de Sitter invariant and possesses
the proper flat spacetime and massless limits. Moreover, the retarded Green's
function inferred from it produces the correct classical response to a test
source. Our result is expressed in a tensor basis which is convenient for
performing quantum field theory computations using dimensional regularization.Comment: 21 pages, no figures, uses LaTeX 2 epsilon, version 2 has an error in
eqn (86) corrected and an updated reference lis
The Schwinger mechanism and graphene
The Schwinger mechanism, the production of charged particle-antiparticle
pairs in a macroscopic external electric field, is derived for 2+1 dimensional
theories. The rate of pair production per unit area for four species of
massless fermions, with charge , in a constant electric field is given
by where is
the speed of light for the two-dimensional system. To the extent undoped
graphene behaves like the quantum field-theoretic vacuum for massless fermions
in 2+1 dimensions, the Schwinger mechanism should be testable experimentally. A
possible experimental configuration for this is proposed. Effects due to
deviations from this idealized picture of graphene are briefly considered. It
is argued that with present day samples of graphene, tests of the Schwinger
formula may be possible.Comment: Extensive revisions. The distinction between the vacuum decay rate
and the pair production rate in the Schwinger mechanism is now stressed. The
discussion of quality of sample needed for a viable experimental test has
been significantly expanded. References adde
DRA method: Powerful tool for the calculation of the loop integrals
We review the method of the calculation of multiloop integrals suggested in
Ref.\cite{Lee2010}.Comment: 6 pages, contribution to ACAT2011 proceedings, Uxbridge, London,
September 5-9, 2011, typos are correcte
Asymptotic Expansions of Feynman Amplitudes in a Generic Covariant Gauge
We show in this paper how to construct Symanzik polynomials and the Schwinger
parametric representation of Feynman amplitudes for gauge theories in an
unspecified covariant gauge. The complete Mellin representation of such
amplitudes is then established in terms of invariants (squared sums of external
momenta and squared masses). From the scaling of the invariants by a parameter
we extend for the present situation a theorem on asymptotic expansions,
previously proven for the case of scalar field theories, valid for both
ultraviolet and infrared behaviors of Feynman amplitudes.Comment: 10 pages, revtex, no figure
Some exact properties of the gluon propagator
Recent numerical studies of the gluon propagator in the minimal Landau and
Coulomb gauges in space-time dimension 2, 3, and 4 pose a challenge to the
Gribov confinement scenario.
We prove, without approximation, that for these gauges, the continuum gluon
propagator in SU(N) gauge theory satisfies the bound . This holds for Landau
gauge, in which case is the dimension of space-time, and for Coulomb gauge,
in which case is the dimension of ordinary space and is the
instantaneous spatial gluon propagator. This bound implies that , where is the gluon propagator at momentum , and
consequently in Landau gauge in space-time , and in Coulomb
gauge in space dimension , but D(0) may be finite in higher dimension.
These results are compatible with numerical studies of the Landau-and
Coulomb-gauge propagator.
In 4-dimensional space-time a regularization is required, and we also prove
an analogous bound on the lattice gluon propagator, . Here we have taken the
infinite-volume limit of lattice gauge theory at fixed lattice spacing, and the
lattice momentum componant is a continuous angle . Unexpectedly, this implies a bound on the {\it high-momentum} behavior of
the continuum propagator in minimum Landau and Coulomb gauge in 4 space-time
dimensions which, moreover, is compatible with the perturbative renormalization
group when the theory is asymptotically free.Comment: 13 page
Potentially Large One-loop Corrections to WIMP Annihilation
We compute one-loop corrections to the annihilation of non--relativistic
particles due to the exchange of a (gauge or Higgs) boson with
mass in the initial state. In the limit this leads to
the "Sommerfeld enhancement" of the annihilation cross section. However, here
we are interested in the case \mu \lsim m_\chi, where the one--loop
corrections are well--behaved, but can still be sizable. We find simple and
accurate expressions for annihilation from both and wave initial
states; they differ from each other if . In order to apply our
results to the calculation of the relic density of Weakly Interacting Massive
Particles (WIMPs), we describe how to compute the thermal average of the
corrected cross sections. We apply this formalism to scalar and Dirac fermion
singlet WIMPs, and show that the corrections are always very small in the
former case, but can be very large in the latter. Moreover, in the context of
the Minimal Supersymmetric Standard Model, these corrections can decrease the
relic density of neutralinos by more than 1%, if the lightest neutralino is a
strongly mixed state.Comment: 25 pages, 8 figures. Added an appendix showing that the approximation
works well in a scalar toy model. To be published in PRD
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