1,710 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
B NMR detection of the magnetic field distribution in the mixed superconducting state of MgB
The temperature dependence of the magnetic field distribution in the mixed
superconducting phase of randomly oriented MgB powder was probed by B NMR spectroscopy. Below the temperature of the second critical () field, K, our spectra reveal two NMR signal
components, one mapping the magnetic field distribution in the mixed
superconducting state and the other one arising from the normal state. The
complementary use of bulk magnetization and NMR measurements reveals that
MgB is an anisotropic superconductor with a Tesla
anisotropy parameter
On the charge radius of the neutrino
Using the pinch technique we construct at one-loop order a neutrino charge
radius, which is finite, depends neither on the gauge-fixing parameter nor on
the gauge-fixing scheme employed, and is process-independent. This definition
stems solely from an effective proper photon-neutrino one-loop vertex, with no
reference to box or self-energy contributions. The role of the box in this
construction is critically examined. In particular it is shown that the
exclusion of the effective WW box from the definition of the neutrino charge
radius is not a matter of convention but is in fact dynamically realized when
the target-fermions are right-handedly polarized. In this way we obtain a
unique decomposition of effective self-energies, vertices, and boxes, which
separately respect electroweak gauge invariance. We elaborate on the tree-level
origin of the mechanism which enforces at one-loop level massive cancellations
among the longitudinal momenta appearing in the Feynman diagrams, and in
particular those associated with the non-abelian character of the theory.
Various issues related to the known connection between the pinch technique and
the Background Field Method are further clarified. Explicit closed expressions
for the neutrino charge radius are reported.Comment: 26 pages, plain Latex, 7 Figures in a separate ps fil
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
The t->WZb decay in the Standard Model: A Critical Reanalysis
We compute the t->WZb decay rate, in the Standard Model, at the leading order
in perturbation theory, with special attention to the effects of the finite
widths of the W and Z bosons. These effects are extremely important, since the
t->WZb decay occurs near its kinematical threshold. They increase the value of
the decay rate by orders of magnitude near threshold or allow it below the
nominal threshold. We discuss a procedure to take into account the finite-width
effects and compare the results with previous studies of this decay. Within the
Standard Model, for a top quark mass in the range between 170 and 180 GeV, we
find BR(t->WZb) ~ 2 x 10^{-6}, which makes the observation at the LHC very
difficult if at all possible.Comment: 10 pages, 4 eps figures, LaTeX. Few references added and minor
changes in the text. Results unchanged. Final version to appear on PL
- …