149 research outputs found
Two-photon form factors of the pi0, eta and eta-prime mesons in the chiral theory with resonances
We have developed a phenomenological approach which describes very well the
pi0, eta and eta-prime meson production in the two-photon interactions. The
simultaneous description of the pi0, eta and eta-prime meson two-photon form
factors is consistent with data in the space-like region. The obtained form
factors are implemented in the event generator EKHARA and the simulated cross
sections are presented. Uncertainties in the measured form factors coming from
the model dependence in Monte Carlo simulations are studied. The model
predictions for the form factor slopes at the origin are given and the high-Q2
limit is also discussed.Comment: 11 pages, 7 figures, 3 tables; two-column revtex4 styl
Comparing phenomenological recipes with a microscopic model for the electric amplitude in strangeness photoproduction
Corrections to the Born approximation in photo-induced strangeness production
off a proton are calculated in a semi-realistic microscopic model. The vertex
corrections and internal contributions to the amplitude of the reaction are included on the one-loop level. Different
gauge-invariant phenomenological prescriptions for the modification of the Born
contribution via the introduction of form factors and contact terms are
discussed. In particular, it is shown that the popular minimal-substitution
method of Ohta corresponds to a special limit of the more realistic approach.Comment: 10 pages, 6 figures in the tex
Compton scattering in a unitary approach with causality constraints
Pion-loop corrections for Compton scattering are calculated in a novel
approach based on the use of dispersion relations in a formalism obeying
unitarity. The basic framework is presented, including an application to
Compton scattering. In the approach the effects of the non-pole contribution
arising from pion dressing are expressed in terms of (half-off-shell) form
factors and the nucleon self-energy. These quantities are constructed through
the application of dispersion integrals to the pole contribution of loop
diagrams, the same as those included in the calculation of the amplitudes
through a K-matrix formalism. The prescription of minimal substitution is used
to restore gauge invariance. The resulting relativistic-covariant model
combines constraints from unitarity, causality, and crossing symmetry.Comment: 25 pages, 9 ps-figure
Low-energy Compton scattering on the nucleon and sum rules
The Gerasimov-Drell-Hearn and Baldin-Lapidus sum rules are evaluated in the
dressed K-matrix model for photon-induced reactions on the nucleon. For the
first time the sum of the electric and magnetic polarisabilities
and the forward spin polarisability are explicitly calculated in two
alternative ways -- from the sum rules and from the low-energy expansion of the
real Compton scattering amplitude -- within the {\em same} framework. The two
methods yield compatible values for but differ somewhat for
. Consistency between the two ways of determining the
polarisabilities is a measure of the extent to which basic symmetries of the
model are obeyed.Comment: 9 pages, 4 figures, using REVTeX. More concise version, results
unchanged. To appear in Phys. Rev.
Final-sate radiation in electron-positron annihilation into a pion pair
The process of annihilation into a pair with radiation
of a photon is considered. The amplitude of the reaction
consists of the model independent initial-state
radiation (ISR) and model-dependent final-state radiation (FSR). The general
structure of the FSR tensor is constructed from Lorentz covariance, gauge
invariance and discrete symmetries in terms of the three invariant functions.
To calculate these functions we apply Chiral Perturbation Theory (ChPT) with
vector and axial-vector mesons. The contribution of
process to the muon anomalous magnetic moment is evaluated, and results are
compared with the dominant contribution in the framework of a hybrid model,
consisting of VMD and point-like scalar eletrodynamics. The developed approach
allows us also to calculate the charge asymmetry.Comment: 21 pages, 8 figure
The Adler-Weisberger and Goldberger-Miyazawa-Oehme sum rules as probes of constraints from analyticity and chiral symmetry in dynamical models for pion-nucleon scattering
The Adler-Weisberger and Goldberger-Miyazawa-Oehme sum rules are calculated
within a relativistic, unitary and crossing symmetric dynamical model for
pion-nucleon scattering using two different methods: 1) by evaluating the
scattering amplitude at the corresponding low-energy kinematics and 2) by
evaluating the sum-rule integrals with the calculated total cross section. The
discrepancy between the results of the two methods provides a measure of the
breaking of analyticity and chiral symmetry in the model. The contribution of
the resonance, including its dressing with meson loops, is discussed
in some detail and found to be small.Comment: 12 pages, 6 figures, using RevTEX4. References added, discussion
extended, conclusions unchanged. To be published in Nuclear Physics
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