276 research outputs found
Quark Mass Dependence of Nucleon Properties and Extrapolation from Lattice QCD
We summarize developments concerning the quark mass dependence of nucleon
magnetic moments and the axial-vector coupling constant g_A. The aim is to
explore the feasibility of chiral effective field theory methods for the
extrapolation of lattice QCD results, from the relatively large quark masses
that can be handled in such computations down to the physically relevant range.Comment: 9 pages, Latex, 4ps figures, uses World Scientific style file;
presented at International School ``Quarks in Hadrons and Nuclei'', Erice,
Sicily, September 200
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
Radiative pion capture by a nucleon
The differential cross sections for and are computed up to in heavy baryon chiral perturbation
theory (HBChPT). The expressions at and have no free
parameters. There are three unknown parameters at , low energy
constants of the HBChPT Lagrangian, which are determined by fitting to
experimental data. Two acceptable fits are obtained, which can be separated by
comparing with earlier dispersion relation calculations of the inverse process.
Expressions for the multipoles, with emphasis on the p-wave multipoles, are
obtained and evaluated at threshold. Generally the results obtained from the
best of the two fits are in good agreement with the dispersion relation
predictions.Comment: 24 pages, Latex, using RevTe
Quark mass dependence of nucleon mass and axial-vector coupling constant
We present an updated analysis of the quark mass dependence of the nucleon
mass and nucleon axial-vector coupling g_A, comparing different formulations of
SU(2) Baryon Chiral Effective Field Theory, with and without explicit delta
(1232) degrees of freedom. We discuss the outcome of the corresponding
interpolations between lattice QCD data and the physical values for these two
nucleon observables. It turns out that in order to obtain successful
interpolating functions at one-loop order, the inclusion of explicit delta
(1232) degrees of freedom is not decisive for the nucleon mass but crucial for
g_A. A chiral extrapolation of recent lattice results by the LHP collaborations
is also shown.Comment: 5 pages, 7 figures, Talk given at the Workshop on Computational
Hadron Physics, Nicosia, Cyprus, 14-17 September 200
Chiral Perturbation Theory and Nucleon Polarizabilities
Compton scattering offers in principle an intriguing new window on nucleon
structure. Existing experiments and future programs are discussed and the state
of theoretical understanding of such measurements is explored.Comment: 15 page standard Latex file---invited talk at Chiral Dynamics
Workshop, Mainz, Germany---typos correcte
Generalized Polarizabilities of the Nucleon in Chiral Effective Theories
Using the techniques of chiral effective field theories we evaluate the so
called generalized polarizabilities of the nucleon, which characterize the
structure dependent components in virtual Compton scattering (VCS) as probed in
the electron scattering reaction e N \to e' N gamma. Results are given for both
spin-dependent and spin-independent structure effects to O(p^3) in SU(2) Heavy
Baryon Chiral Perturbation Theory and to O(epsilon^3) in the SU(2) Small Scale
Expansion. Finally we compare our calculations with results from the pioneering
VCS experiment on the proton from Mainz.Comment: 39 pages, 12 figures, revte
Complete one-loop analysis of the nucleon's spin polarizabilities
We present a complete one-loop analysis of the four nucleon spin
polarizabilities in the framework of heavy baryon chiral perturbation theory.
The first non-vanishing contributions to the isovector and first corrections to
the isoscalar spin polarizabilities are calculated. No unknown parameters enter
these predictions. We compare our results to various dispersive analyses. We
also discuss the convergence of the chiral expansion and the role of the delta
isobar.Comment: 4 pp, REVTE
Isospin violation and the proton's neutral weak magnetic form factor
The effects of isospin violation on the neutral weak magnetic form factor of
the proton are studied using two-flavour chiral perturbation theory. The first
nonzero contributions appear at O(p^4) in the small-momentum expansion, and the
O(p^5) corrections are also calculated. The leading contributions from an
explicit Delta(1232) isomultiplet are included as well. At such a high order in
the chiral expansion, one might have expected a large number of unknown
parameters to contribute. However, it is found that no unknown parameters can
appear within loop diagrams, and a single tree-level counterterm at O(p^4) is
sufficient to absorb all divergences. The momentum dependence of the neutral
weak magnetic form factor is not affected by this counterterm.Comment: 26 pages including 9 figure
Generalized polarizabilities and the chiral structure of the nucleon
We discuss the virtual Compton scattering reaction at
low energies. We present results for the generalized polarizabilities of the
nucleon obtained in heavy baryon chiral perturbation theory at .Comment: 5 pages, LaTex file, 1 postscript figure, uses ``espcrc1.sty'', talk
given by S. Scherer at the 15th International Conference on Few Body Problems
in Physics, Groningen, The Netherlands, 22-26 July 1997, to appear in the
proceedings (Nucl. Phys. A
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