1,606 research outputs found
BRST-driven cancellations and gauge invariant Green's functions
We study a fundamental, all order cancellation operating between graphs of
distinct kinematic nature, which allows for the construction of
gauge-independent effective self-energies, vertices, and boxes at arbitrary
order.Comment: 4 pages, 3 figures. Contributed to QCD 03: High-Energy Physics
International Conference in Quantum Chromodynamics, Montpellier, France, 2-9
July 200
On the definition and observability of the neutrino charge radius
We present a brief summary of recent results concerning the unambiguous
definition and experimental extraction of the gauge-invariant and
process-independent neutrino charge radius.Comment: 5 pages, no figures, talk presented at the XXX International Meeting
on Fundamental Physics, IMFP2002, Jaca (Huesca), January 28th -- February
1st, 200
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
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 neutrino charge radius in the presence of fermion masses
We show how the crucial gauge cancellations leading to a physical definition
of the neutrino charge radius persist in the presence of non-vanishing fermion
masses. An explicit one-loop calculation demonstrates that, as happens in the
massless case, the pinch technique rearrangement of the Feynman amplitudes,
together with the judicious exploitation of a fundamental current relation
leads to a completely gauge independent definition of the effective neutrino
charge radius. Using the formalism of the Nielsen identities it is further
proved that the same cancellation mechanism operates unaltered to all orders in
perturbation theory.Comment: 26 pages, 8 figure
Chiral symmetry breaking with lattice propagators
We study chiral symmetry breaking using the standard gap equation,
supplemented with the infrared-finite gluon propagator and ghost dressing
function obtained from large-volume lattice simulations. One of the most
important ingredients of this analysis is the non-abelian quark-gluon vertex,
which controls the way the ghost sector enters into the gap equation.
Specifically, this vertex introduces a numerically crucial dependence on the
ghost dressing function and the quark-ghost scattering amplitude. This latter
quantity satisfies its own, previously unexplored, dynamical equation, which
may be decomposed into individual integral equations for its various form
factors. In particular, the scalar form factor is obtained from an approximate
version of the "one-loop dressed" integral equation, and its numerical impact
turns out to be rather considerable. The detailed numerical analysis of the
resulting gap equation reveals that the constituent quark mass obtained is
about 300 MeV, while fermions in the adjoint representation acquire a mass in
the range of (750-962) MeV.Comment: 32 pages, 13 figure
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