arXiv subjects

G. Ripka

Publications and source records attributed to G. Ripka.

8 recordsLinked to original sources

arXiv2005

Are There Diquarks in the Nucleon?

This work is devoted to the study of diquark correlations inside the nucleon. We analyze some matrix elements which encode information about the non-perturbative forces, in different color anti-triplet diquark channels. We suggest a lattice calculation to check the quark-diquark picture and clarify the role of instanton-mediated interactions. We study in detail the physical properties of the 0+ diquark, using the Random Instanton Liquid Model. We find that instanton forces are sufficiently strong to form a diquark bound-state, with a mass of ~500 MeV, which is compatible with earlier estimates. We also compute its electro-magnetic form factor and find that the diquark is a broad object, with a size comparable with that of the proton.

hep-ph↗

arXiv2003

Dual superconductor models of color confinement

The lectures, delivered at ECT (European Centre for Theoretical Studies in Nuclear Physics and Related Areas) in Trento (Italy) in 2002 and 2003, are addressed to physicists who wish to acquire a minimal background to understand present day attempts to model the confinement of quantum chromo-dynamics (QCD) in terms of dual superconductors. The lectures focus more on the models than on attempts to derive them from QCD. They discuss the Dirac theory of magnetic monopoles, the world sheet swept out by Dirac strings, deformations of Dirac strings and charge quantization, gauge fields associated to the field tensor and to the dual field tensor, the Landau-Ginzburg (Abelian Higgs) model of a dual superconductor, the flux tube joining two equal and opposite color-electric charges, the Abrikosov-Nielsen-Olesen vortex, the divergencies of the London limit, the comparison of the calculated flux tube and string tension with lattice data, duality transformations and the use of Kalb-Ramond fields, the two-potential Zwanziger formalism, abelian gauge fixing and abelian projection, the occurence of monopoles in an abelian gauge, the maximal abelian gauge, and the confinement of static SU(3) color charges.

hep-ph↗

arXiv2002

Baryons as Solitons in Chiral Quark Models

We describe the formation of solitons in NJL-type models and discuss the influence of the regularization scheme on the stability of the solution. We concentrate on models with non-local regulators in which stable solutions exist without introducing additional constraints.

hep-ph↗

arXiv2000

Vacuum properties in the presence of quantum fluctuations of the quark condensate

The quantum fluctuations of the quark condensate are calculated using a regulated Nambu Jona-Lasinio model. The corresponding quantum fluctuations of the chiral fields are compared to those which are predicted by an ''equivalent'' sigma model. They are found to be large and comparable in size but they do not restore chiral symmetry. The restoration of chiral symmetry is prevented by an ''exchange term'' of the pion field which does not appear in the equivalent sigma model. A vacuum instability is found to be dangerously close when the model is regulated with a sharp 4-momentum cut-off.

hep-ph↗

arXiv2000

Quantum fluctuations of the quark condensate

The quantum fluctuations of the quark condensate are studied in a Nambu Jona-Lasinio model. Two Lorenz invariant regularizations are considered: a sharp 4-momentum cut-off and a soft gaussian regulator. The quantum fluctuations of the quark condensate are found to be large although chiral symmetry is not restored. Instabilities of the ground state appear when the system is probed by a source term proportional to the squared quark condensate. The instabilities are traced to unphysical poles introduced by the regulator and their effect is greatly enhanced when a sharp cut-off is used.

hep-ph↗

arXiv1996

Meson loops in the Nambu-Jona-Lasinio model

The effects of meson loops in the vacuum sector of the Nambu--Jona-Lasinio model are calculated. Using the effective action formalism we take consistently all next-to-leading-order $1 \over \Nc$ terms into account. This leads to a symmetry-conserving approach, in which all features of spontaneously broken chiral symmetry, such as the Goldstone theorem, the Gold\-ber\-ger--Treiman and the Gell-Mann--Oakes--Renner relations are preserved. Contributions to $\langle \overline{q}q \rangle$ and $F_π$ are calculated, and are shown to be substantial, at the level of $\sim 30\%$, consistent with the $1 \over \Nc$ expansion. The leading nonanalytic terms in the chiral expansion of $\langle \overline{q}q \rangle$, $F_π$ and $m_π$ agree have the same form as the one-loop results of chiral perturbation theory.

hep-ph↗

arXiv1996

Analytic structure of meson propagators in the proper-time regularized Nambu--Jona-Lasinio model

We analyze the analytic structure of meson propagators in the Nambu--Jona-Lasinio model with a proper-time regulator. We show that the regulator produces unphysical complex singularities. As a result the naive use of the Wick rotation is no longer allowed. Formulas involving integration over mesonic momenta, such as meson-loop contributions or dispersion relations for meson Green's functions, cannot be written in usual forms.

hep-ph↗

arXiv1993

The Regularization of the Fermion Determinant in Chiral Quark Models

The momentum dependence of the quark self energy gives a physically motivated and consistent regularization of both the real and imaginary parts of the quark loop contribution to the meson action. We show that the amplitudes for anomalous processes are always reproduced correctly.

hep-ph↗