296 research outputs found
Nucleon Form Factors in Point-Form Spectator-Model Constructions
We discuss electromagnetic currents in the point-form formulation of
relativistic quantum mechanics. The construction is along a spectator model and
implies that only one quark is explicitly coupled to the photon, but
nevertheless many-body contributions are present in the current operator. Such
effects are unavoidable in relativistic constructions and resulting ambiguities
are notably reduced by imposing charge normalization and time-reversal
invariance. The residual theoretical indetermination introduces small but
sizeable changes in the nucleon form-factors, particularly at higher Q-squared
values, with the data generally centered in the middle of the theoretical band.Comment: 8 pages, 3 figures, prepared for XI Convegno su Problemi di Fisica
Nucleare Teorica, Cortona 2006, 11-14 October 200
Covariant calculation of mesonic baryon decays
We present covariant predictions for pi and eta decay modes of N and Delta
resonances from relativistic constituent-quark models based on
one-gluon-exchange and Goldstone-boson-exchange dynamics. The results are
calculated within the point-form approach to Poincare-invariant relativistic
quantum mechanics applying a spectator-model decay operator. The direct
predictions of the constituent-quark models for covariant pi and eta decay
widths show a behaviour completely different from previous ones calculated in
nonrelativistic or so-called semirelativistic approaches. It is found that the
present theoretical results agree with experiment only in a few cases but
otherwise always remain smaller than the experimental data (as compiled by the
Particle Data Group). Possible reasons for this behaviour are discussed with
regard to the quality of both the quark-model wave functions and the mesonic
decay operator.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.
Strong Decays of N and Delta Resonances in the Point-Form Formalism
We present covariant predictions of relativistic constituent quark models for
pi and eta decay widths of N and Delta resonances. The results are calculated
for a model decay operator within the point-form spectator approximation. It is
found that most theoretical values underestimate the experimental data
considerably.Comment: 4 pages; Proceedings for N*2004, Grenoble, March 24-27 (2004
Meson Dynamics and the resulting "3-Nucleon-Force" diagrams: Results from a simplified test case
A simplified 1D (one-dimensional) model for a generalized 3N system is
considered, as a testing ground for the explicit treatment of the meson
dynamics in this system. We focus attention on the irreducible diagrams
generated by the pion dynamics in the 3N system, and in particular to a new
type of three-nucleon force discussed recently in the literature, and generated
by the one-pion-exchange mechanism in presence of a nucleon-nucleon
correlation. It is found that these new terms in the simplified model have an
approximately 30% effect compared to the standard three-nucleon force terms in
a `Triton' binding energy calculation. It is suggested that this effect should
also not be ignored in realistic calculations.Comment: 4 pages, Contribution to "Mesons and Light Nuclei 2001", Prague 2-6,
200
Covariant calculation of strange decays of baryon resonances
We present results for kaon decay widths of baryon resonances from a
relativistic study with constituent quark models. The calculations are done in
the point-form of Poincare-invariant quantum mechanics with a spectator-model
decay operator. We obtain covariant predictions of the Goldstone-boson-exchange
and a variant of the one-gluon-exchange constituent quark models for all kaon
decay widths of established baryon resonances. They are generally characterized
by underestimating the available experimental data. In particular, the widths
of kaon decays with increasing strangeness in the baryon turn out to be
extremely small. We also consider the nonrelativistic limit, leading to the
familiar elementary emission model, and demonstrate the importance of
relativistic effects. It is found that the nonrelativistic approach evidently
misses sensible influences from Lorentz boosts and some essential spin-coupling
terms.Comment: 6 pages, 3 table
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