102 research outputs found
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
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
Nucleon generalized polarizabilities within a relativistic Constituent Quark Model
Nucleon generalized polarizabilities are investigated within a relativistic
framework, defining such quantities through a Lorentz covariant multipole
expansion of the amplitude for virtual Compton scattering. The key physical
ingredients in the calculation of the nucleon polarizabilities are the Lorentz
invariant reduced matrix elements of the electromagnetic transition current,
which can be evaluated from off-energy-shell helicity amplitudes. The evolution
of the proton paramagnetic polarizability, , as a function of
the virtual-photon three-momentum transfer is explicitly evaluated within
a relativistic constituent quark model by adopting transition form factors
obtained in the light-front formalism. The discussion is focussed on the role
played by the effects due to the relativistic approach and to the transition
form factors, derived within different models.Comment: 14 pages and three figures (included), to appear in Phys. Rev. C (May
1998
Structure analysis of the virtual Compton scattering amplitude at low energies
We analyze virtual Compton scattering off the nucleon at low energies in a
covariant, model-independent formalism.
We define a set of invariant functions which, once the irregular nucleon pole
terms have been subtracted in a gauge-invariant fashion, is free of poles and
kinematical zeros.
The covariant treatment naturally allows one to implement the constraints due
to Lorentz and gauge invariance, crossing symmetry, and the discrete
symmetries.
In particular, when applied to the reaction,
charge-conjugation symmetry in combination with nucleon crossing generates four
relations among the ten originally proposed generalized polarizabilities of the
nucleon.Comment: 19 pages, LaTeX2e/RevTeX, no figures, original sections IV.-VI.
removed, to be discussed in a separate publication, none of the conclusions
change
Nucleon Polarizabilities from Deuteron Compton Scattering within a Green's-Function Hybrid Approach
We examine elastic Compton scattering from the deuteron for photon energies
ranging from zero to 100 MeV, using state-of-the-art deuteron wave functions
and NN-potentials. Nucleon-nucleon rescattering between emission and absorption
of the two photons is treated by Green's functions in order to ensure gauge
invariance and the correct Thomson limit. With this Green's-function hybrid
approach, we fulfill the low-energy theorem of deuteron Compton scattering and
there is no significant dependence on the deuteron wave function used.
Concerning the nucleon structure, we use Chiral Effective Field Theory with
explicit \Delta(1232) degrees of freedom within the Small Scale Expansion up to
leading-one-loop order. Agreement with available data is good at all energies.
Our 2-parameter fit to all elastic data leads to values for the
static isoscalar dipole polarizabilities which are in excellent agreement with
the isoscalar Baldin sum rule. Taking this value as additional input, we find
\alpha_E^s= (11.3+-0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and \beta_M^s =
(3.2-+0.7(stat)+-0.6(Baldin)) x 10^{-4} fm^3 and conclude by comparison to the
proton numbers that neutron and proton polarizabilities are essentially the
same.Comment: 47 pages LaTeX2e with 20 figures in 59 .eps files, using graphicx.
Minor modifications; extended discussion of theoretical uncertainties of
polarisabilities extraction. Version accepted for publication in EPJ
Baryon magnetic moments and sigma terms in lattice-regularized chiral perturbation theory
An SU(3) chiral Lagrangian for the lightest decuplet of baryons is
constructed on a discrete lattice of spacetime points, and is added to an
existing lattice Lagrangian for the lightest octets of mesons and baryons. A
nonzero lattice spacing renders all loop integrations finite, and the continuum
limit of any physical observable is identical to the result obtained from
dimensional regularization. Chiral symmetry and gauge invariance are preserved
even at nonzero lattice spacing. Specific calculations discussed here include
the non-renormalization of a conserved vector current, the magnetic moments of
octet baryons, and the pi N and KN sigma terms that relate to the nucleon's
strangeness content. The quantitative difference between physics at a nonzero
lattice spacing and physics in the continuum limit is easily computed, and it
represents an expectation for the size of discretization errors in
corresponding lattice QCD simulations.Comment: 19 pages, 5 figures, one paragraph added to introduction, to appear
in Phys Rev
coupling constant
We calculate the coupling
using light cone QCD sum rule. Our result is
.Comment: RevTex, 5 pages + 1 PS figur
Generalized dipole polarizabilities and the spatial structure of hadrons
We present a phenomenological discussion of spin-independent, generalized
dipole polarizabilities of hadrons entering the virtual Compton scattering
process gamma* h -> gamma h. We introduce a new method of obtaining a tensor
basis with appropriate Lorentz-invariant amplitudes which are free from
kinematical singularities and constraints. We then motivate a gauge-invariant
separation into a generalized Born term containing ground-state properties
only, and a residual contribution describing the model-dependent internal
structure. The generalized dipole polarizabilities are defined in terms of
Lorentz-invariant residual amplitudes. Particular emphasis is laid on a
physical interpretation of these quantities as characterizing the spatial
distributions of the induced electric polarization and magnetization of
hadrons. It is argued that three dipole polarizabilities, namely the
longitudinal electric alpha_L(q^2), the transverse electric alpha_T(q^2), and
the magnetic beta(q^2) ones are required in order to fully reconstruct local
polarizations induced by soft external fields in a hadron. One of these
polarizabilities, alpha_T(q^2), describes an effect of higher order in the soft
final-photon momentum q'. We argue that the associated spatial distributions
obtained via the Fourier transforms in the Breit frame are meaningful even for
such a light particle as the pion. The spatial distributions are determined at
large distances r ~ 1/m_pi for pions, kaons, and octet baryons by use of ChPT.Comment: 41 pages, 5 figures, RevTex fil
Parity Violation in gamma proton Compton Scattering
A measurement of parity-violating spin-dependent gamma proton Compton
scattering will provide a theoretically clean determination of the
parity-violating pion-nucleon coupling constant . We
calculate the leading parity-violating amplitude arising from one-loop pion
graphs in chiral perturbation theory. An asymmetry of ~5 10^{-8} is estimated
for Compton scattering of 100 MeV photons.Comment: 6 pages, 1 figure, latex. Reference adde
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