122 research outputs found
On the connection between the pinch technique and the background field method
The connection between the pinch technique and the background field method is
further explored. We show by explicit calculations that the application of the
pinch technique in the framework of the background field method gives rise to
exactly the same results as in the linear renormalizable gauges. The general
method for extending the pinch technique to the case of Green's functions with
off-shell fermions as incoming particles is presented. As an example, the
one-loop gauge independent quark self-energy is constructed. We briefly discuss
the possibility that the gluonic Green's functions, obtained by either method,
correspond to physical quantities.Comment: 13 pages and 3 figures, all included in a uuencoded file, to appear
in Physical Review
Gluon mass generation without seagull divergences
Dynamical gluon mass generation has been traditionally plagued with seagull
divergences, and all regularization procedures proposed over the years yield
finite but scheme-dependent gluon masses. In this work we show how such
divergences can be eliminated completely by virtue of a characteristic
identity, valid in dimensional regularization. The ability to trigger the
aforementioned identity hinges crucially on the particular Ansatz employed for
the three-gluon vertex entering into the Schwinger-Dyson equation governing the
gluon propagator. The use of the appropriate three-gluon vertex brings about an
additional advantage: one obtains two separate (but coupled) integral
equations, one for the effective charge and one for the gluon mass. This system
of integral equations has a unique solution, which unambiguously determines
these two quantities. Most notably, the effective charge freezes in the
infrared, and the gluon mass displays power-law running in the ultraviolet, in
agreement with earlier considerations.Comment: 37 pages, 9 figures; minor typos corrected and a few brief
explanatory remarks adde
Displacement Operator Formalism for Renormalization and Gauge Dependence to All Orders
We present a new method for determining the renormalization of Green
functions to all orders in perturbation theory, which we call the displacement
operator formalism, or the D-formalism, in short. This formalism exploits the
fact that the renormalized Green functions may be calculated by displacing by
an infinite amount the renormalized fields and parameters of the theory with
respect to the unrenormalized ones. With the help of this formalism, we are
able to obtain the precise form of the deformations induced to the Nielsen
identities after renormalization, and thus derive the exact dependence of the
renormalized Green functions on the renormalized gauge-fixing parameter to all
orders. As a particular non-trivial example, we calculate the gauge-dependence
of at two loops in the framework of an Abelian Higgs model, using a
gauge-fixing scheme that preserves the Higgs-boson low-energy theorem for
off-shell Green functions. Various possible applications and future directions
are briefly discussed.Comment: 41 pages, 8 figure
The heavy quark decomposition of the S-matrix and its relation to the pinch technique
We propose a decomposition of the S-matrix into individually gauge invariant
sub-amplitudes, which are kinematically akin to propagators, vertices, boxes,
etc. This decompsition is obtained by considering limits of the S-matrix when
some or all of the external particles have masses larger than any other
physical scale. We show at the one-loop level that the effective gluon
self-energy so defined is physically equivalent to the corresponding gauge
independent self-energy obtained in the framework of the pinch technique. The
generalization of this procedure to arbitrary gluonic -point functions is
briefly discussed.Comment: 11 uuencoded pages, NYU-TH-94/10/0
Evolutionary Dynamics of Multiple Sublineages of H5N1 Influenza Viruses in Nigeria, 2006-2008.
CP violation through particle mixing and the H-A lineshape
We consider the possibility of looking for CP-mixing effects in two-Higgs
doublet models (and particularly in the MSSM) by studying the lineshape of the
CP-even (H) and CP-odd (A) neutral scalars. In most cases H and A come quite
degenerate in mass, and their s-channel production would lead to nearly
overlapping resonances. CP-violating effects may connect these two Higgs
bosons, giving origin to one-loop particle mixing, which, due to their mass
proximity, can be resonantly enhanced. The corresponding transition amplitude
contains then CP-even and CP-odd components; besides the signal of
intereference between both amplitudes, leading to a CP-odd asymmetry, we
propose to look for the mixing probability itself, a quantity which, although
CP-even, can originate only from a CP-odd amplitude. We show that, in general,
the effect of such a mixing probability cannot be mimicked by (or be
re-absorbed into) a simple redefinition of the H and A masses in the context of
a CP-conserving model. Specifically, the effects of the CP-mixing are such
that, either the mass-splitting of the H and A bosons cannot be accounted for
in the absence of CP-mixing, and/or the detailed energy dependence of the
produced lineshape is clearly different from the one obtained by redefining the
masses, but not allowing any mixing. This analysis suggests that the detailed
study of the lineshape of this Higgs system may provide valuable information on
the CP nature of the underlying theory.Comment: 16 pages, 13 figures; v2: added one reference; v3: radiative
corrections taken into account, agreement now with CP-SuperH, conclusions
unchanged. v3 matches the paper version accepted for publication in JHE
A Pilot Study on effects of vaccination on immunity of broiler chickens
A pilot study was carried out with the aim of highlighting the effects of NDV vaccine on the immune responses of broiler chickens challenged with NDV. Twenty (20) broilers of day-old were used for the study. They were grouped into five of four per group. During the study they were fed with standard feeds and clean water ad libitum. Both vaccinated and unvaccinated groups were challenged with 0.2 saline suspension of 106 ELD50 intradermal inoculation of NDV challenged strain. The vaccinated groups showed neither clinical signs nor symptoms of NDV infections while unvaccinated group showed 100% mortality after 48hr. This result indicate that vaccines is still very important in the prevention, management and control of poultry diseases as maternal immunity passed on to the young chicks at precocial stage could not be relied on to fight against infectious disease in broiler chickens. Therefore, the use of locally produced vaccines should be encouraged among farmers for the prevention, control and management of outbreaks of viral infections in our community. Key: Challenged, Poultry birds- broilers, Newcastle disease virus, Vaccinatio
The Pinch Technique to All Orders
The generalization of the pinch technique to all orders in perturbation
theory is presented. The effective Green's functions constructed with this
procedure are singled out in a unique way through the full exploitation of the
underlying Becchi-Rouet-Stora-Tyutin symmetry. A simple all-order
correspondence between the pinch technique and the background field method in
the Feynman gauge is established.Comment: 10 pages, 4 figures; one reference added, typos corrected; final
version to match the pubblished on
Direct observation of incommensurate magnetism in Hubbard chains
The interplay between magnetism and doping is at the origin of exotic
strongly correlated electronic phases and can lead to novel forms of magnetic
ordering. One example is the emergence of incommensurate spin-density waves
with a wave vector that does not match the reciprocal lattice. In one dimension
this effect is a hallmark of Luttinger liquid theory, which also describes the
low energy physics of the Hubbard model. Here we use a quantum simulator based
on ultracold fermions in an optical lattice to directly observe such
incommensurate spin correlations in doped and spin-imbalanced Hubbard chains
using fully spin and density resolved quantum gas microscopy. Doping is found
to induce a linear change of the spin-density wave vector in excellent
agreement with Luttinger theory predictions. For non-zero polarization we
observe a decrease of the wave vector with magnetization as expected from the
Heisenberg model in a magnetic field. We trace the microscopic origin of these
incommensurate correlations to holes, doublons and excess spins which act as
delocalized domain walls for the antiferromagnetic order. Finally, when
inducing interchain coupling we observe fundamentally different spin
correlations around doublons indicating the formation of a magnetic polaron
Asymptotic properties of Born-improved amplitudes with gauge bosons in the final state
For processes with gauge bosons in the final state we show how to
continuously connect with a single Born-improved amplitude the resonant region,
where resummation effects are important, with the asymptotic region far away
from the resonance, where the amplitude must reduce to its tree-level form.
While doing so all known field-theoretical constraints are respected, most
notably gauge-invariance, unitarity and the equivalence theorem. The
calculations presented are based on the process , mediated by a
possibly resonant Higgs boson; this process captures all the essential
features, and can serve as a prototype for a variety of similar calculations.
By virtue of massive cancellations the resulting closed expressions for the
differential and total cross-sections are particularly compact.Comment: 23 pages, Latex, 4 Figures, uses axodra
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