1,328 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
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
Electroweak pinch technique to all orders
The generalization of the pinch technique to all orders in the electroweak
sector of the Standard Model within the class of the renormalizable 't Hooft
gauges, is presented. In particular, both the all-order PT gauge-boson-- and
scalar--fermions vertices, as well as the diagonal and mixed gauge-boson and
scalar self-energies are explicitly constructed. This is achieved through the
generalization to the Standard Model of the procedure recently applied to the
QCD case, which consist of two steps: (i) the identification of special Green's
functions, which serve as a common kernel to all self-energy and vertex
diagrams, and (ii) the study of the (on-shell) Slavnov-Taylor identities they
satisfy. It is then shown that the ghost, scalar and scalar--gauge-boson
Green's functions appearing in these identities capture precisely the result of
the pinching action at arbitrary order. It turns out that the aforementioned
Green's functions play a crucial role, their net effect being the non-trivial
modification of the ghost, scalar and scalar--gauge-boson diagrams of the
gauge-boson-- or scalar--fermions vertex we have started from, in such a way as
to dynamically generate the characteristic ghost and scalar sector of the
background field method. The pinch technique gauge-boson and scalar
self-energies are also explicitly constructed by resorting to the method of the
background-quantum identities.Comment: 48 pages, 8 figures; v2: typos correcte
Tau anomalous magnetic moment form factor at Super B/Flavor factories
The proposed high-luminosity B/Flavor factories offer new opportunities for
the improved determination of the fundamental physical parameters of standard
heavy leptons. Compared to the electron or the muon case, the magnetic
properties of the lepton are largely unexplored. We show that the
electromagnetic properties of the , and in particular its magnetic form
factor, may be measured competitively in these facilities, using unpolarized or
polarized electron beams. Various observables of the 's produced on top
of the resonances, such as cross-section and normal polarization for
unpolarized electrons or longitudinal and transverse asymmetries for polarized
beams, can be combined in order to increase the sensitivity on the magnetic
moment form factor. In the case of polarized electrons, we identify a special
combination of transverse and longitudinal polarizations able to
disentangle this anomalous magnetic form factor from both the charge form
factor and the interference with the Z-mediating amplitude. For an integrated
luminosity of one could achieve a sensitivity of
about , which is several orders of magnitude below any other existing
high- or low-energy bound on the magnetic moment. Thus one may obtain a QED
test of this fundamental quantity to a few % precision.Comment: 20 pages, 4 figure
Strangeness contribution to the vector and axial form factors of the nucleon
The strangeness contribution to the vector and axial form factors of the
nucleon is presented for momentum transfers in the range
GeV. The results are obtained via a combined analysis of forward-scattering
parity-violating elastic asymmetry data from the and HAPPEx
experiments at Jefferson Lab, and elastic and scattering
data from Experiment 734 at Brookhaven National Laboratory. The
parity-violating asymmetries measured in elastic scattering at
forward angles establish a relationship between the strange vector form factors
and , with little sensitivity to the strange axial form factor
. On the other hand, elastic neutrino scattering at low is
dominated by the axial form factor, with still some significant sensitivity to
the vector form factors as well. The combination of the two data sets allows
the simultaneous extraction of , , and over a significant
range of for the very first time.Comment: 3 pages, 1 figure, will appear in AIP Conference Proceedings for
PANIC 200
Radiative Corrections to W and Quark Propagators in the Resonance Region
We discuss radiative corrections to W and quark propagators in the resonance
region, |s-M^2| \lsim M*Gamma. We show that conventional mass renormalization,
when applied to photonic or gluonic corrections, leads in next to leading order
(NLO) to contributions proportional to [M*Gamma/(s-M^2)]^n, (n=1,2...), i.e. to
a non-convergent series in the resonance region, a difficulty that affects all
unstable particles coupled to massless quanta. A solution of this problem,
based on the concepts of pole mass and width, is presented. It elucidates the
issue of renormalization of amplitudes involving unstable particles and
automatically circumvents the problem of apparent on-shell singularities. The
roles of the Fried-Yennie gauge and the Pinch Technique prescription are
discussed. Because of special properties of the photonic and gluonic
contributions, and in contrast with the Z case, the gauge dependence of the
conventional on-shell definition of mass is unbounded in NLO. The evaluations
of the width in the conventional and pole formulations are compared and shown
to agree in NLO but not beyond.Comment: 19 pages, 7 figures, LaTeX (uses epsfig). Slight rewording of the
abstract and one of the sentences of the text. Minor misprints corrected. To
appear in Phys. Rev.
The Two-Loop Pinch Technique in the Electroweak Sector
The generalization of the two-loop Pinch Technique to the Electroweak Sector
of the Standard Model is presented. We restrict ourselves to the case of
conserved external currents, and provide a detailed analysis of both the
charged and neutral sectors. The crucial ingredient for this construction is
the identification of the parts discarded during the pinching procedure with
well-defined contributions to the Slavnov-Taylor identity satisfied by the
off-shell one-loop gauge-boson vertices; the latter are nested inside the
conventional two-loop self-energies. It is shown by resorting to a set of
powerful identities that the two-loop effective Pinch Technique self-energies
coincide with the corresponding ones computed in the Background Feynman gauge.
The aforementioned identities are derived in the context of the
Batalin-Vilkovisky formalism, a fact which enables the individual treatment of
the self-energies of the photon and the -boson. Some possible
phenomenological applications are briefly discussed.Comment: 50 pages, uses axodra
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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