63 research outputs found
On the Isovector Channels in Relativistic Point Coupling Models within the Hartree and Hartree-Fock Approximations
We investigate the consequences of Fierz transformations acting upon the
contact interactions for nucleon fields occurring in relativistic point
coupling models in Hartree approximation, which yield the same models but in
Hartree-Fock approximation instead. We find for four-fermion interactions
occurring in two existing relativistic point coupling phenomenologies that
whereas in Hartree the isovector-scalar strength, corresponding to delta-meson
exchange, is unnaturally small, indicating a possible new symmetry, in
Hartree-Fock it is instead comparable to the isovector-vector strength
corresponding to rho-meson exchange, but the sum of the two isovector coupling
constants appears to be preserved in both approaches. Furthermore, in
Hartree-Fock approximation, both QCD-scaled isovector coupling constants are
natural (dimensionless and of order 1) whereas in Hartree approximation only
that of the isovector-vector channel is natural. This indicates that it is not
necessary to search for a new symmetry and, moreover, that the role of the
delta-meson should be reexamined.Comment: 10 pages; accepted for publication in Nuclear Physics
Nuclear Ground State Observables and QCD Scaling in a Refined Relativistic Point Coupling Model
We present results obtained in the calculation of nuclear ground state
properties in relativistic Hartree approximation using a Lagrangian whose
QCD-scaled coupling constants are all natural (dimensionless and of order 1).
Our model consists of four-, six-, and eight-fermion point couplings (contact
interactions) together with derivative terms representing, respectively, two-,
three-, and four-body forces and the finite ranges of the corresponding mesonic
interactions. The coupling constants have been determined in a self-consistent
procedure that solves the model equations for representative nuclei
simultaneously in a generalized nonlinear least-squares adjustment algorithm.
The extracted coupling constants allow us to predict ground state properties of
a much larger set of even-even nuclei to good accuracy. The fact that the
extracted coupling constants are all natural leads to the conclusion that QCD
scaling and chiral symmetry apply to finite nuclei.Comment: 44 pages, 13 figures, 9 tables, REVTEX, accepted for publication in
Phys. Rev.
A Monte Carlo Event Generator for W Off-shell Pair Production including Higher Order Electromagnetic Radiative Corrections
We present the Monte Carlo event generator {\tt WOPPER} for pair production
of 's and their decays at high energy colliders. {\tt WOPPER}
includes the effects from finite width and focusses on the calculation of
higher order electromagnetic corrections in the leading log approximation
including soft photon exponentiation and explicit generation of exclusive hard
photons.Comment: Contribution to the Second Workshop -- Munich, Annecy, Hamburg:
Collisions at 500~GeV: The Physics Potential, November 20, 1992, to
April 3, 1993. LaTeX, 6 pages + 4 uuencoded EPS figures, IKDA 93/28, SI-93-
Relativistic nuclear model with point-couplings constrained by QCD and chiral symmetry
We derive a microscopic relativistic point-coupling model of nuclear
many-body dynamics constrained by in-medium QCD sum rules and chiral symmetry.
The effective Lagrangian is characterized by density dependent coupling
strengths, determined by chiral one- and two-pion exchange and by QCD sum rule
constraints for the large isoscalar nucleon self-energies that arise through
changes of the quark condensate and the quark density at finite baryon density.
This approach is tested in the analysis of the equations of state for symmetric
and asymmetric nuclear matter, and of bulk and single-nucleon properties of
finite nuclei. In comparison with purely phenomenological mean-field
approaches, the built-in QCD constraints and the explicit treatment of pion
exchange restrict the freedom in adjusting parameters and functional forms of
density dependent couplings. It is shown that chiral (two-pion exchange)
fluctuations play a prominent role for nuclear binding and saturation, whereas
strong scalar and vector fields of about equal magnitude and opposite sign,
induced by changes of the QCD vacuum in the presence of baryonic matter,
generate the large effective spin-orbit potential in finite nuclei.Comment: 46 pages, 12 figures, uses elsart.cls, revised version, to appear in
Nucl.Phys. A735 (2004) 449-48
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