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 $W$'s and their decays at high energy $e^+e^-$ colliders. {\tt WOPPER} includes the effects from finite $W$ 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: $e^+e^-$ 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