2,029 research outputs found
Non-perturbative model for the half-off-shell \gamma N N vertex
Form factors in the nucleon-photon vertex with one off-shell nucleon are
calculated by dressing the vertex with pion loops up to infinite order. Cutting
rules and dispersion relations are implemented in the model. Using the
prescription of minimal substitution we construct a \gamma \pi N N vertex and
show that it has to be included in the model in order that the Ward-Takahashi
identity for the \gamma N N vertex be fulfilled. The vertex is to be applied in
a coupled-channel K-matrix formalism for Compton scattering, pion
photoproduction and pion scattering. The form factors show a pronounced cusp
structure at the pion threshold. As an illustration of a consistent application
of the model, we calculate the cross section of Compton scattering. To provide
gauge invariance in Compton scattering, a four-point \gamma \gamma N N contact
term is constructed using minimal substitution.Comment: 51 pages, 12 Postscript figures, using REVTeX. Submitted to Phys.
Rev. C, added reference
On electromagnetic off-shell effects in proton-proton bremsstrahlung
We study the influence of the off-shell structure of the nucleon
electromagnetic vertex on proton-proton bremsstrahlung observables. Realistic
choices for these off-shell effects are found to have considerable influences
on observables such as cross sections and analyzing powers. The rescattering
contribution diminishes the effects of off-shell modifications in
negative-energy states.Comment: 7 pages, 3 figure
Density resummation of perturbation series in a pion gas to leading order in chiral perturbation theory
The mean field (MF) approximation for the pion matter, being equivalent to
the leading ChPT order, involves no dynamical loops and, if self-consistent,
produces finite renormalizations only. The weight factor of the Haar measure of
the pion fields, entering the path integral, generates an effective Lagrangian
which is generally singular in the continuum limit.
There exists one parameterization of the pion fields only, for which the weight
factor is equal to unity and , respectively. This
unique parameterization ensures selfconsistency of the MF approximation. We use
it to calculate thermal Green functions of the pion gas in the MF approximation
as a power series over the temperature. The Borel transforms of thermal
averages of a function of the pion
fields with respect to the scalar pion density are found to be
. The perturbation series over the scalar
pion density for basic characteristics of the pion matter such as the pion
propagator, the pion optical potential, the scalar quark condensate
, the in-medium pion decay constant , and the
equation of state of pion matter appear to be asymptotic ones. These series are
summed up using the contour-improved Borel resummation method. The quark scalar
condensate decreases smoothly until MeV. The temperature
is the maximum temperature admissible for thermalized non-linear
sigma model at zero pion chemical potentials. The estimate of is
above the chemical freeze-out temperature MeV at RHIC and above
the phase transition to two-flavor quark matter MeV,
predicted by lattice gauge theories.Comment: Replaced with revised and extended version. Results are compared to
lattice gauge theories. 16 pages REVTeX, 13 eps 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
Contribution of spin 1/2 and 3/2 resonances to two-photon exchange effects in elastic electron-proton scattering
We calculate contributions of hadron resonances to two-photon exchange
effects in electron-proton scattering. In addition to the nucleon and P33
resonance, the following heavier resonances are included as intermediate states
in the two-photon exchange diagrams: D13, D33, P11, S11 and S31. We show that
the corrections due to the heavier resonances are smaller that the dominant
nucleon and P33 contributions. We also find that there is a partial
cancellation between the contributions from the spin 1/2 and spin 3/2
resonances, which results in a further suppression of their aggregate
two-photon exchange effect.Comment: 6 pages, 1 figure; additional comparison with data, results
unchanged; to be published in Phys. Rev.
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.
Medium effects in the production and decay of vector mesons in pion-nucleus reactions
The -, - and -resonance production and their dileptonic
decay in reactions at 1.1 - 1.7 GeV/c are calculated within the
intranuclear cascade (INC) approach. The invariant mass distribution of the
dilepton pair for each resonance can be decomposed in two components which
correspond to their decay 'inside' the target nucleus and in the vacuum,
respectively. The first components are strongly distorted by the nuclear medium
due to resonance-nucleon scattering and a possible mass shift at finite baryon
density. These medium modifications are compared to background sources in the
dilepton spectrum from bremsstrahlung as well as the Dalitz decays of
and mesons produced in the reaction. Detailed predictions for
reactions at 1.3 and 1.7 GeV/c are made within several momentum bins
for the lepton pair.Comment: 29 pages, LaTeX, including 12 postscript figures, UGI-97-07, Nucl.
Phys. A., in pres
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