8,909 research outputs found
Quark mass dependence of the nucleon axial-vector coupling constant
We study the quark mass expansion of the axial-vector coupling constant g_A
of the nucleon. The aim is to explore the feasibility of chiral effective field
theory methods for extrapolation of lattice QCD results - so far determined at
relatively large quark masses corresponding to pion masses larger than 0.6 GeV
- down to the physical value of the pion mass. We compare two versions of
non-relativistic chiral effective field theory: One scheme restricted to pion
and nucleon degrees of freedom only, and an alternative approach which
incorporates explicit Delta(1230) resonance degrees of freedom. It turns out
that, in order to approach the physical value of g_A in a leading-one-loop
calculation, the inclusion of the explicit Delta(1230) degrees of freedom is
crucial. With information on important higher order couplings constrained from
analyses of inelastic pion production processes, a chiral extrapolation
function for g_A is obtained, which works well from the chiral limit across the
physical point into the region of present lattice data. The resulting
enhancement of our extrapolation function near the physical pion mass is found
to arise from an interplay between long- and short- distance physics.Comment: 21 pages, LaTeX, 7 figure
Numerical study of the scaling properties of SU(2) lattice gauge theory in Palumbo non-compact regularization
In the framework of a non-compact lattice regularization of nonabelian gauge
theories we look, in the SU(2) case, for the scaling window through the
analysis of the ratio of two masses of hadronic states. In the two-dimensional
parameter space of the theory we find the region where the ratio is constant,
and equal to the one in the Wilson regularization. In the scaling region we
calculate the lattice spacing, finding it at least 20% larger than in the
Wilson case; therefore the simulated physical volume is larger.Comment: 24 pages, 7 figure
Chiral Magnetism of the Nucleon
We study the quark mass expansion of the magnetic moments of the nucleon in a
chiral effective field theory including nucleons, pions and delta resonances as
explicit degrees of freedom. We point out that the usual powercounting applied
so far to this problem misses important quark mass structures generated via an
intermediate isovector M1 nucleon-delta transition. We propose a modified
powercounting and compare the resulting chiral extrapolation function to
available (quenched) lattice data. The extrapolation is found to work
surprisingly well, given that the lattice data result from rather large quark
masses. Our calculation raises the hope that extrapolations of lattice data
utilizing chiral effective field theory might be applicable over a wider range
in quark masses than previously thought, and we discuss some open questions in
this context. Furthermore, we observe that within the current lattice data
uncertainties the extrapolations presented here are consistent with the Pade
fit ansatz introduced by the Adelaide group a few years ago.Comment: 30 pages, Latex, 7 figure
Non-perturbative renormalization of the quark condensate in Ginsparg-Wilson regularizations
We present a method to compute non-perturbatively the renormalization
constant of the scalar density for Ginsparg-Wilson fermions. It relies on
chiral symmetry and is based on a matching of renormalization group invariant
masses at fixed pseudoscalar meson mass, making use of results previously
obtained by the ALPHA Collaboration for O(a)-improved Wilson fermions. Our
approach is quite general and enables the renormalization of scalar and
pseudoscalar densities in lattice regularizations that preserve chiral symmetry
and of fermion masses in any regularization. As an application we compute the
non-perturbative factor which relates the renormalization group invariant quark
condensate to its bare counterpart, obtained with overlap fermions at beta=5.85
in the quenched approximation.Comment: 21 pages, 4 postscript files, LaTe
Regularization-independent study of renormalized non-perturbative quenched QED
A recently proposed regularization-independent method is used for the first
time to solve the renormalized fermion Schwinger-Dyson equation numerically in
quenched QED. The Curtis-Pennington vertex is used to illustrate the
technique and to facilitate comparison with previous calculations which used
the alternative regularization schemes of modified ultraviolet cut-off and
dimensional regularization. Our new results are in excellent numerical
agreement with these, and so we can now conclude with confidence that there is
no residual regularization dependence in these results. Moreover, from a
computational point of view the regularization independent method has enormous
advantages, since all integrals are absolutely convergent by construction, and
so do not mix small and arbitrarily large momentum scales. We analytically
predict power law behaviour in the asymptotic region, which is confirmed
numerically with high precision. The successful demonstration of this efficient
new technique opens the way for studies of unquenched QED to be undertaken in
the near future.Comment: 20 pages,5 figure
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