arXiv subjects

Hilmar Forkel

Publications and source records attributed to Hilmar Forkel.

45 records · Page 3Linked to original sources

arXiv1996

Dispersion Analysis of the Strange Vector Form Factors of the Nucleon

We analyze the nucleon matrix element of the strange quark vector current in a nucleon--model independent dispersive approach with input from the current world data set for the isoscalar electromagnetic form factors. The update of Jaffe's minimal 3-pole ansatz for the spectral functions yields a 40% larger (Sachs) strangeness radius, $(r_s^2)_{Sachs} = 0.20 {\rm fm}^2$, and a by 20% reduced magnitude of the strangeness magnetic moment, $μ_s = -0.26$. In the pole approximation these values are shown to be upper bounds. After extending the ansatz in order to implement the asymptotic QCD momentum dependence (which the 3-pole form factors cannot reproduce), we find the magnitude of the 3-pole results reduced by up to a factor of 2.5. The signs of the leading moments originate primarily from the large $ϕ$-meson couplings and are generic in the pole approximation.

hep-ph↗

arXiv1995

Dynamical Higgs Mechanism without Elementary Scalars: A Lesson from Instantons

The generation of gauge--dependent fermion vacuum condensates in Yang--Mills theory would dynamically break the gauge symmetry and thus provide an alternative to the Higgs mechanism in unified theories. We explore a simple example: instanton-induced quark pair (diquark) condensation in QCD with 2 flavors and $N_c$ colors. We do find diquark condensates in the vacuum, but only for $N_c = 2$, where they are equivalent to the standard quark-antiquark condensate. At $N_c \geq 3$ diquark condensates exist as meta--stable saddle points of the instanton-induced effective QCD action and may strongly affect the properties of matter under extreme conditions. The scalar diquark excitation, which is a Goldstone boson at $N_c =2$, becomes unbound for $N_c \geq 3$.

hep-ph↗

arXiv1995

Strangeness in the Nucleon: The Strange Vector Form Factors

We discuss two descriptions of the nucleon's strange vector form factors in the framework of vector meson dominance. The first, an updated and extended version of Jaffe's dispersion analysis, approximates the spectral functions of the form factors as a sum of vector meson poles, whereas the second combines vector meson dominance in the $ω$ and $ϕ$ meson sector with an intrinsic strangeness distribution from a kaon cloud. (Plenary talk given at the International School for Nuclear Physics, 17th Course: Quarks and Hadrons in Nuclei, Erice, Sicily, Sept 19-27, 1995.)

hep-ph↗

arXiv1995

Chiral Phase Structure of Hot and Dense Nuclear Matter

The description of nuclear matter bulk properties in the framework of quark-based hadron models in curved unit cells is extended to finite temperature. We discuss the analogy and interplay of density and temperature effects in this formulation and study, as an application, the temperature and density dependence of chiral vacuum properties in the Nambu-Jona-Lasinio (NJL) model.

nucl-th↗

arXiv1994

Chiral Quark Dynamics in Dense Nuclear Matter

We consider a new approach to the description of dense nuclear matter in the framework of chirally symmetric, quark-based hadron models. As previously in the Skyrme model, the dense environment is described in terms of hyperspherical cells of unit baryon number. The intrinsic curvature of these cells generates a new gauge interaction for the quark fields which mediates interactions with the ambient matter. We apply this approach to the Nambu-Jona-Lasinio (NJL) model, construct its curved-space quark propagator and solve the ladder Bethe-Salpeter equation for the pion. We find a high-density phase transition to chiral restoration, discuss the density dependence of the chiral order parameter and of the pion properties, and compare with results of the conventional chemical-potential approach. The new approach can additionally describe baryon-density-free cavities in the dense medium.

nucl-th↗

arXiv1994

The Electromagnetic Pion Form Factor and Instantons

We calculate the electromagnetic pion form factor at intermediate space-like momentum transfer from the QCD sum rule for the correlation function of two {\it pseudoscalar } interpolating fields and the electromagnetic current. This correlator receives essential contributions from direct ({\it i.e.} small-scale) instantons, which we evaluate under the assumption of an instanton size distribution consistent with instanton liquid and lattice simulations. The resulting form factor is in good agreement both with the sum rule based on the axial-current correlator and with experiment.

hep-ph↗

arXiv1994

Stranger in the Light: The Strange Vector Form Factors of the Nucleon

A model combining vector meson dominance, $ω- ϕ$ mixing and a kaon cloud contribution is used to estimate the strangeness vector form factors of the nucleon in the momentum range of the planned measurements at MIT-Bates and CEBAF. We compare our results with some other theoretical estimates and discuss the nucleon strangeness radius in models based on dynamically dressed, extended constituent quarks. For a quantitative estimate in the latter framework we use the Nambu-Jona-Lasinio model.

hep-ph↗

arXiv1993

Direct Instantons in QCD Nucleon Sum Rules

We study the role of direct (i.e. small-scale) instantons in QCD correlation functions for the nucleon. They generate sizeable, nonperturbative corrections to the conventional operator product expansion, which improve the quality of both QCD nucleon sum rules and cure the long-standing stability problem, in particular, of the chirally odd sum-rule.

hep-ph↗

arXiv1993

Just How Strange? Loops, Poles and the Strangeness Radius of the Nucleon

We consider a simple model for the strangeness radius of the nucleon. The model is based on vector meson dominance (VMD) and $ω- ϕ$ mixing in addition to a kaon cloud contribution. We find that the VMD contribution is similar in magnitude and of the same sign as the kaon contribution to the Sachs strangeness radius and is significantly larger than the kaon contribution to the Dirac radius. Two figures (not included) can be obtained through the authors (marina@quark.umd.edu).

hep-ph↗