303 research outputs found
On the massless contributions to the vacuum polarization of heavy quarks
Recently Groote and Pivovarov have given notice of a possible fault in the
use of sum rules involving two-point correlation functions to extract
information on heavy quark parameters, due to the presence of massless
contributions that invalidate the construction of moments of the spectral
densities. Here we show how to circumvent this problem through a new definition
of the moments, providing an infrared safe and consistent procedure.Comment: 1+9 pages, 3 figures. Discussion on QCD sum rules applications added.
Conclusions unchanged. Version to be published in Journal of Physics
Odd Parity Light Baryon Resonances
We use a consistent SU(6) extension of the meson-baryon chiral Lagrangian
within a coupled channel unitary approach in order to calculate the T-matrix
for meson-baryon scattering in s-wave. The building blocks of the scheme are
the pion and nucleon octets, the rho nonet and the Delta decuplet. We identify
poles in this unitary T-matrix and interpret them as resonances. We study here
the non exotic sectors with strangeness S=0,-1,-2,-3 and spin J=1/2, 3/2 and
5/2. Many of the poles generated can be associated with known N, Delta, Sigma,
Lambda and Xi resonances with negative parity. We show that most of the
low-lying three and four star odd parity baryon resonances with spin 1/2 and
3/2 can be related to multiplets of the spin-flavor symmetry group SU(6). This
study allows us to predict the spin-parity of the Xi(1620), Xi(1690), Xi(1950),
Xi(2250), Omega(2250) and Omega(2380) resonances, which have not been
determined experimentally yet.Comment: New appendix and references adde
Multichannel parametrization of \pi N scattering amplitudes and extraction of resonance parameters
We present results of a new multichannel partial-wave analysis for \pi N
scattering in the c.m. energy range 1080 to 2100 MeV. This work explicitly
includes \eta N and K \Lambda channels and the single pion photoproduction
channel. Resonance parameters were extracted by fitting partial-wave amplitudes
from all considered channels using a multichannel parametrization that is
consistent with S-matrix unitarity. The resonance parameters so obtained are
compared to predictions of quark models
Non-perturbative Landau gauge and infrared critical exponents in QCD
We discuss Faddeev-Popov quantization at the non-perturbative level and show
that Gribov's prescription of cutting off the functional integral at the Gribov
horizon does not change the Schwinger-Dyson equations, but rather resolves an
ambiguity in the solution of these equations. We note that Gribov's
prescription is not exact, and we therefore turn to the method of stochastic
quantization in its time-independent formulation, and recall the proof that it
is correct at the non-perturbative level. The non-perturbative Landau gauge is
derived as a limiting case, and it is found that it yields the Faddeev-Popov
method in Landau gauge with a cut-off at the Gribov horizon, plus a novel term
that corrects for over-counting of Gribov copies inside the Gribov horizon.
Non-perturbative but truncated coupled Schwinger-Dyson equations for the gluon
and ghost propagators and in Landau gauge are solved
asymptotically in the infrared region. The infrared critical exponents or
anomalous dimensions, defined by and are obtained in space-time dimensions . Two
possible solutions are obtained with the values, in dimensions, , or .Comment: 26 pages. Modified 2.25.02 to update references and to clarify
Introduction and Conclusio
Effective boost and "point-form" approach
Triangle Feynman diagrams can be considered as describing form factors of
states bound by a zero-range interaction. These form factors are calculated for
scalar particles and compared to point-form and non-relativistic results. By
examining the expressions of the complete calculation in different frames, we
obtain an effective boost transformation which can be compared to the
relativistic kinematical one underlying the present point-form calculations, as
well as to the Galilean boost. The analytic expressions obtained in this simple
model allow a qualitative check of certain results obtained in similar studies.
In particular, a mismatch is pointed out between recent practical applications
of the point-form approach and the one originally proposed by Dirac.Comment: revised version as accepted for publicatio
Field diffeomorphisms and the algebraic structure of perturbative expansions
We consider field diffeomorphisms in the context of real scalar field
theories. Starting from free field theories we apply non-linear field
diffeomorphisms to the fields and study the perturbative expansion for the
transformed theories. We find that tree level amplitudes for the transformed
fields must satisfy BCFW type recursion relations for the S-matrix to remain
trivial. For the massless field theory these relations continue to hold in loop
computations. In the massive field theory the situation is more subtle. A
necessary condition for the Feynman rules to respect the maximal ideal and
co-ideal defined by the core Hopf algebra of the transformed theory is that
upon renormalization all massive tadpole integrals (defined as all integrals
independent of the kinematics of external momenta) are mapped to zero.Comment: 8 pages, 2 figure
Gauge equivalence in QCD: the Weyl and Coulomb gauges
The Weyl-gauge ( QCD Hamiltonian is unitarily transformed to a
representation in which it is expressed entirely in terms of gauge-invariant
quark and gluon fields. In a subspace of gauge-invariant states we have
constructed that implement the non-Abelian Gauss's law, this unitarily
transformed Weyl-gauge Hamiltonian can be further transformed and, under
appropriate circumstances, can be identified with the QCD Hamiltonian in the
Coulomb gauge. We demonstrate an isomorphism that materially facilitates the
application of this Hamiltonian to a variety of physical processes, including
the evaluation of -matrix elements. This isomorphism relates the
gauge-invariant representation of the Hamiltonian and the required set of
gauge-invariant states to a Hamiltonian of the same functional form but
dependent on ordinary unconstrained Weyl-gauge fields operating within a space
of ``standard'' perturbative states. The fact that the gauge-invariant
chromoelectric field is not hermitian has important implications for the
functional form of the Hamiltonian finally obtained. When this nonhermiticity
is taken into account, the ``extra'' vertices in Christ and Lee's Coulomb-gauge
Hamiltonian are natural outgrowths of the formalism. When this nonhermiticity
is neglected, the Hamiltonian used in the earlier work of Gribov and others
results.Comment: 25 page
The Rarita--Schwinger field: renormalization and phenomenology
We discuss renormalization of propagator of interacting Rarita--Schwinger
field. Spin-3/2 contribution after renormalization takes usual resonance form.
For non-leading spin-1/2 terms we found procedure, which guarantees absence of
poles in energy plane. The obtained renormalized propagator has one free
parameter and is a straight generalization of the famous free propagator of
Moldauer and Case. Application of this propagator for production of
in \pi^{+}\particle{p}\to \pi^{+}\particle{p} leads to
good description of total cross-section and to reasonable agreement with
results of partial wave analysis.Comment: 19 pages, 3 figures, revtex4; misprints, min editorial change
Electromagnetic Meson Form Factors in the Salpeter Model
We present a covariant scheme to calculate mesonic transitions in the
framework of the Salpeter equation for -states. The full Bethe
Salpeter amplitudes are reconstructed from equal time amplitudes which were
obtained in a previous paper\cite{Mue} by solving the Salpeter equation for a
confining plus an instanton induced interaction. This method is applied to
calculate electromagnetic form factors and decay widths of low lying
pseudoscalar and vector mesons including predictions for CEBAF experiments. We
also describe the momentum transfer dependence for the processes
.Comment: 22 pages including 10 figure
The energies and residues of the nucleon resonances N(1535) and N(1650)
We extract pole positions for the N(1535) and N(1650) resonances using two
different models. The positions are determined from fits to different subsets
of the existing , and data
and found to be 1515(10)--i85(15)MeV and 1660(10)--i65(10)MeV, when the data is
described in terms of two poles. Sensitivity to the choice of fitted data is
explored. The corresponding and residues of these poles
are also extracted.Comment: 9 page
- …