31 research outputs found
Relativistic predictions of spin observables for exclusive proton knockout reactions
Within the framework of the relativistic distorted wave impulse approximation
(DWIA), we investigate the sensitivity of complete sets of polarization
transfer observables for exclusive proton knockout from the 3s,
2d and 2d states in Pb, at an incident laboratory
kinetic energy of 202 MeV, and for coincident coplanar scattering angles
(, ), to different distorting optical potentials,
finite-range (FR) versus zero-range (ZR) approximations to the DWIA, as well as
medium-modified meson-nucleon coupling constants and meson masses. Results are
also compared to the nonrelativistic DWIA predictions based on the
Schr\"{o}dinger equation.Comment: Submitted for publication to Physicical Review C, 23 pages, 7 figure
Non-perturbative momentum dependence of the coupling constant and hadronic models
Models of hadron structure are associated with a hadronic scale which allows
by perturbative evolution to calculate observables in the deep inelastic
region. The resolution of Dyson-Schwinger equations leads to the freezing of
the QCD running coupling (effective charge) in the infrared, which is best
understood as a dynamical generation of a gluon mass function, giving rise to a
momentum dependence which is free from infrared divergences. We use this new
development to understand why perturbative treatments are working reasonably
well despite the smallness of the hadronic scale.Comment: Changes in Acknowledgments and PACS number
Relativistic predictions of exclusive analyzing powers at an incident energy of 202 MeV
Within the framework of the relativistic distorted wave impulse approximation
(DWIA), we investigate the sensitivity of the analyzing power - for exclusive
proton knockout from the 3s, 2d and 2d states in
Pb, at an incident laboratory kinetic energy of 202 MeV, and for
coincident coplanar scattering angles (, ) - to
different distorting optical potentials, finite-range (FR) versus zero-range
(ZR) approximations to the DWIA, as well as medium-modified coupling constants
and meson masses. Results are also compared to the nonrelativistic DWIA
predictions based on the Schr\"{o}dinger equation. Whereas the nonrelativistic
model fails severely, both ZR and FR relativistic DWIA models provide an
excellent description of the data. For the FR predictions, it is necessary to
invoke a 20% reduction of sigma-nucleon and omega-nucleon coupling constants as
well as for -, - and -meson masses, by the nuclear
medium. On the other hand, the ZR predictions suggest that the strong
interaction in the nuclear medium is adequately represented by the free
nucleon-nucleon interaction associated with the impulse approximation. We also
demonstrate that, although the analyzing power is relatively insensitive to the
use different relativistic global optical potential parameter sets, the
prominent oscillatory behavior of this observable is largely attributed to
distortion of the scattering wave functions relative to their plane wave
values.Comment: 16 pages, 3 figures, submitted to Phys. Rev.
On the Behavior of the Effective QCD Coupling alpha_tau(s) at Low Scales
The hadronic decays of the tau lepton can be used to determine the effective
charge alpha_tau(m^2_tau') for a hypothetical tau-lepton with mass in the range
0 < m_tau' < m_tau. This definition provides a fundamental definition of the
QCD coupling at low mass scales. We study the behavior of alpha_tau at low mass
scales directly from first principles and without any renormalization-scheme
dependence by looking at the experimental data from the OPAL Collaboration. The
results are consistent with the freezing of the physical coupling at mass
scales s = m^2_tau' of order 1 GeV^2 with a magnitude alpha_tau ~ 0.9 +/- 0.1.Comment: 15 pages, 4 figures, submitted to Physical Review D, added
references, some text added, no results nor figures change
Neutrino Cooling of Neutron Stars. Medium effects
This review demonstrates that neutrino emission from dense hadronic component
in neutron stars is subject of strong modifications due to collective effects
in the nuclear matter. With the most important in-medium processes incorporated
in the cooling code an overall agreement with available soft X ray data can be
easily achieved. With these findings so called "standard" and "non-standard"
cooling scenarios are replaced by one general "nuclear medium cooling scenario"
which relates slow and rapid neutron star coolings to the star masses (interior
densities). In-medium effects take important part also at early hot stage of
neutron star evolution decreasing the neutrino opacity for less massive and
increasing for more massive neutron stars. A formalism for calculation of
neutrino radiation from nuclear matter is presented that treats on equal
footing one-nucleon and multiple-nucleon processes as well as reactions with
resonance bosons and condensates. Cooling history of neutron stars with quark
cores is also discussed.Comment: To be published in "Physics of Neutron Star Interiors", Eds. D.
Blaschke, N.K. Glendenning, A. Sedrakian, Springer, Heidelberg (2001