322 research outputs found
Extended Goldstone-Boson-Exchange Constituent Quark Model
We present an extension of the Goldstone-boson-exchange constituent quark
model including additional interactions beyond the ones used hitherto. For the
hyperfine interaction between the constituent quarks we assume pseudoscalar,
vector, and scalar meson exchanges and consider all relevant force components
produced by these types of exchanges. The resulting model, which corresponds to
a relativistic Poincare-invariant Hamiltonian (or equivalently mass operator),
provides a unified framework for a covariant description of all light and
strange baryons. The ground states and resonances up to an excitation energy of
about 2 GeV are reproduced in fair agreement with phenomenology, with the
exception of the first excitations above the Lambda and Xi ground states.Comment: 11 pages, 4 figures, 3 tables; substantial revisions, additional
author, bibliography extended and update
Spectator-model operators in point-form relativistic quantum mechanics
We address the construction of transition operators for electromagnetic,
weak, and hadronic reactions of relativistic few-quark systems along the
spectator model. While the problem is of relevance for all forms of
relativistic quantum mechanics, we specifically adhere to the point form, since
it preserves the spectator character of the corresponding transition operators
in any reference frame. The conditions imposed on the construction of
point-form spectator-model operators are discussed and their implications are
exemplified for mesonic decays of baryon resonances within a relativistic
constituent quark model.Comment: 10 pages, 4 figures, updated version accepted for publication in
Europ. Phys. J.
Effective Q-Q Interactions in Constituent Quark Models
We study the performance of some recent potential models suggested as
effective interactions between constituent quarks. In particular, we address
constituent quark models for baryons with hybrid Q-Q interactions stemming from
one-gluon plus meson exchanges. Upon recalculating two of such models we find
them to fail in describing the N and \Delta spectra. Our calculations are based
on accurate solutions of the three-quark systems in both a variational
Schr\"odinger and a rigorous Faddeev approach. It is argued that hybrid {Q-Q}
interactions encounter difficulties in describing baryon spectra due to the
specific contributions from one-gluon and pion exchanges together. In contrast,
a chiral constituent quark model with a Q-Q interaction solely derived from
Goldstone-boson exchange is capable of providing a unified description of both
the N and \Delta spectra in good agreement with phenomenology.Comment: 21 pages, LaTe
Is there diquark clustering in the nucleon?
It is shown that the instanton-induced interaction in qq pairs, iterated in
t-channel, leads to a meson-exchange interactions between quarks. In this way
one can achieve a simultaneous understanding of low-lying mesons, baryons and
the nuclear force. The discussion is general and does not necessarily rely on
the instanton-induced interaction. Any nonperturbative gluonic interaction
between quarks, which is a source of the dynamical chiral symmetry breaking and
explains the - mass splitting, will imply an effective meson
exchange picture in baryons. Due to the (anti)screening there is a big
difference between the initial 't Hooft interaction and the effective
meson-exchange interaction. It is demonstrated that the effective
meson-exchange interaction, adjusted to the baryon spectrum, does not bind the
scalar diquark and does not induce any significant quark-diquark clustering in
the nucleon because of the nontrivial role played by the Pauli principle.Comment: Final version to appear in Phys. Rev. D; typos have been corrected;
some formulae have been written in a more detailed form; some references have
been update
Using pi_2(1670) -> b_1(1235) pi to Constrain Hadronic Models
We show that current analyses of experimental data indicate that the strong
decay mode pi_2 -> b_1 pi is anomalously small. Non-relativistic quark models
with spin-1 quark pair creation, such as ^3P_0, ^3S_1 and ^3D_1 models, as well
as instanton and lowest order one-boson (in this case pi) emission models, can
accommodate the analyses of experimental data, because of a quark-spin
selection rule. Models and effects that violate this selection rule, such as
higher order one-boson emission models, as well as mixing with other Fock
states, may be constrained by the small pi_2 -> b_1 pi decay. This can provide
a viability check on newly proposed decay mechanisms. We show that for mesons
made up of a heavy quark and anti-quark, the selection rule is exact to all
orders of Quantum Chromodynamics (QCD) perturbation theory.Comment: 11 pages, LaTeX, 3 encapsulated Postscript figures, small change
Electroweak properties of baryons in a covariant chiral quark model
The proton and neutron electromagnetic form factors and the nucleon axial
form factor have been calculated in the Goldstone-boson exchange
constituent-quark model within the point-form approach to relativistic quantum
mechanics. The results, obtained without any adjustable parameter nor quark
form factors, are, due to the dramatic effects of the boost required by the
covariant treatment, in striking agreement with the data.Comment: Proceedings of the Conference N*2001, Mainz; 4 pages, 3 figures
included in eps format; World Scientific style file include
Chirally symmetric but confining dense and cold matter
The folklore tradition about the QCD phase diagram is that at the chiral
restoration phase transition at finite density hadrons are deconfined and there
appears the quark matter. We address this question within the only known
exactly solvable confining and chirally symmetric model. It is postulated
within this model that there exists linear Coulomb-like confining interaction.
The chiral symmetry breaking and the quark Green function are obtained from the
Schwinger-Dyson (gap) equation while the color-singlet meson spectrum results
from the Bethe-Salpeter equation. We solve this model at T=0 and finite
chemical potential and obtain a clear chiral restoration phase transition
at the critical value \mu_{cr}. Below this value the spectrum is similar to the
previously obtained one at \mu = 0. At \mu > \mu_{cr} the quarks are still
confined and the physical spectrum consists of bound states which are arranged
into a complete set of exact chiral multiplets. This explicitly demonstrates
that a chirally symmetric matter consisting of confined but chirally symmetric
hadrons at finite chemical potential is also possible in QCD. If so, there must
be nontrivial implications for astrophysics.Comment: 7 pp; the paper has been expanded to make some technical details more
clear; 3 new figures have been added. To appear in PR
Effective restoration of chiral symmetry in excited mesons
A fast restoration of chiral symmetry in excited mesons is demonstrated. A minimal "realistic" chirally symmetric confining model is used, where the only interaction between quarks is the linear instantaneous Lorentz-vector confining potential. The model is the 3+1 generalization of the 1+1 dimensional `t Hooft model. Chiral symmetry breaking is generated via the nonperturbative resummation of valence quarks self-energy loops and the meson bound states are obtained from the Bethe-Salpeter equation. The excited mesons fall into approximate chiral multiplets and lie on the approximately linear radial and angular Regge trajectories, though a significant deviation from the linearity of the angular trajectory is observed
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