A pedagogical introduction to the concept of
"Chiral Soliton Models for Baryons"
is available as Springer Lecture Notes.
In the limit of infinitely many color degrees of freedom
quantum-chromo-dynamics (QCD) is equivalent to an effective theory
of weakly interacting mesons. In this effective meson theory baryons
then emerge as soliton solitons, i.e. field configurations which
solve the classical equations of motion and yield a localized energy density.
Based of this feature of QCD, we attempt to obtain a comprehensive
model for the physics of baryons.
Although the specific effective meson theory remains unknown many aspects
of baryons physics can be illuminated from this perspective. Since at
low energy only light mesons should be important the effective theory may
well be approximated via the symmetries of QCD in that energy regime
Besides Lorentz-covariance this is in particular chiral symmetry and
its spontaneous breaking. In the simplest case one is lead to the
Skyrme soliton model whose only degrees of freedom are the pions. The
latter being the Goldstone bosons of spontaneous chiral symmetry
breaking. Extensions of the Skyrme model can be obtained by applying
the rules of chiral symmetry to the meson dynamics. In such an approach many
static nucleon properties (magnetic moments, spin decomposition etc.) can
be described reasonably well. In particular we are interested in the
generalization to the three flavor case by incorporating strange
degrees of freedom [1]. This not only
allows us to discuss static properties of hyperons but also the effects
of virtual strange quark-antiquark pairs in the nucleon.
A key issue of our research project is to consider the
Nambu-Jona-Lasinio model as an approximation to the quark flavor
dynamics. Using functional integral techniques the effective
meson theory equivalent to the NJL model can be determined.
Remarkably the resulting effective meson theory contains soliton
solutions[2]. In contrast to
Skyrme type models the connection to the quark degrees is
known explicitly. Hence the NJL model soliton is well suited to
study issues which are directly related to the quark substructure
of the nucleon. Along this line we have recently succeeded to
compute nucleon structure function in this chiral soliton model
[3] [4]
[5]. More recently very exhaustive
numerical simulations on these ideas have been performed
[6].
The ultimate goal of these investigations is to describe the connection
between the soliton picture and the QCD parton model.
More recently the soliton picture has acquired renewed interest
in the context of pentaquark baryons. These are states that in
(non-relativistic) quark model are composed of four quarks and
an anti-quark. Consequently such models predict large pentaquark
masses. The opposite is the case in the soliton model. Here pentaquarks
arise as collective flavor excitations of the nucleon and hence should
be comparable in mass. Mostly our interest has been in the
mixing scenario between pentaquarks and radial excitations of
the nucleon
[7]
as well as a proper description the pentaquark width
(e.g. lifetime)
[8]. This proper description can
be verified in the limit of infinitely many color degrees of freedom,
where the small and large amplitude fluctuations are equivalent. For
the most prominent collective nucleon excitation, the Δ resonance,
this test is not available because nucleon and Δ are degenerate
and the Δ is hence stable in that limit. Thus, even though the
pentaquarks may not be as fascinating as earlier assumed, they have been
very fruitful in understanding the lifetimes of collective excitations
in the continuum[9].
The chiral soliton model description has been extend to heavy baryons
that, in a valence quark picture, contain strange and charm (or bottom)
baryons[10].
Interestingly enough, any self-contained description of both exotic and
and heavy baryons requires considerations beyond the mean field approach
to chiral soliton models[11].
Some selected research publications:
Energies that arise from quantum fluctations about a non-trivial
background configuration can be computed with spectal methods that
utilize scattering data associated with this background. A pedagogical
introduction to
"Spectral Methods in Quantum Field Theory" is available as Springer
Lecture Notes. A more recent
review has been published as a chapter in
book on the future of the Casimir energy.
In renormalizable models one-loop quantum corrections are considered in
the construction of the soliton configuration. In order to uniquely
determine the finite parts of these corrections to the energy it is
important to cancel the ultra-violet divergences by the same counterterms as
in the perturbative sector. This is accomplished by identifying the
Born approximants in the phase shift expression for the Casimir energy
and perturbatively computed Feynman diagrams. Consequently also the scale
dependence of the regularization procedure drops out. Finally the
minimum of the energy functional is constructed by varying those
parameters which characterize the localized field configuration. In
some models those quantum corrections can actually stabilize a
soliton which classically is unstable.
[1].
Furthermore it is shown that the scattering phase shifts
together with Levinson's theorem may be utilized to determine
the charge of the vacuum that is polarized by the background
soliton configuration. This is in particular the case for
non-integer charges as for example carried by the
chiral bag-model in 3+1 dimensions
[2].
Furthermore this formalism can be generalized to geometric configurations
with the background field dwelling only in a subset of the coordinates.
In the other coordinates the background field equals the vacuum configuration.
Renormalizability of the considered quantum field theory then yields
novel sum rules between the phase shifts and the bound state energies.
These sum rules can be interpreted as generalizations of Levinson's
theorem [3].
These techniques are successfully applied to gain deeper understanding of the Casimir effect because it allows for the important distinction between divergences inherited from the quantum field theory and those that are induced by singular backgrounds. The former are curred by standard renormalization techniques but the latter can only be removed by proper consideration of material properties. Fortunately, these properties do not affect the standard results for the Casimir force between rigid bodies. Yet the similar approach to the Casimir self-tension of single objects must be questioned [4], [5].
Using the methods described in ref. [1] the vacuum polarization energies of magnetic vortices have been computed [6]. In particular, the obstacles that arise for regularization from the Feynman series not being gauge invariant term by term have been solved for string type configurations in the standard model [7]. We have found that the fermion contribution to the vacuum polarization energies of these configurations is much smaller than their classical energies unless the fermion mass is about ten times as large as that of the top quark [8]. This is a step towards investigating the existence of such structures that eventually have significant impact on our understanding of cosmology. On the technical side, the studies of ref. [7] provide further support for the use of spectral methods to compute the vacuum polarization energies as they verify the equality of the finite parts of the Born approximants and the Feynman diagrams. Recently we have shown [9] that by populating the bound fermion levels induced by a cosmic string dynamically stabilizes the string if the fermion mass parameter is only slightly larger than suggested by the top quark mass.
The calculation of the vacuum polarization energies for cosmic strings involves a number of complicated (numerical) computations that individually are not gauge invariant. It has been shown that, when collecting all pieces, gauge invariance is maintained[10]. The quantum stabilization of closed abelian strings has been discussed in [11].
Some recent work focuses on vacuum polarization energies of systems with mass gaps [12]. Here the conjecture emerges that quantum corrections destabilze a classically stable soliton when the field potential has degenerate minima with different curvatures.
Also some conceptual questions of the Casmir energy have been explored [13].
Abelian strings (vortices) are characterized by singular gauge potentials. The standard approach to find the vacuum polarization energy therefore requires modification that is derived and applied in [14]. The picture emerges that the quantum corrections favor a lump of vortices with the certain topological charge of isolated ones with the same total charge.
Some selected research publications:
More recent work focuses on collective corrdinates in kink-antikink scattering [1] in soliton models with one space dimension. In particular the question was considered under which circumstance bouncing configurations emerge and which mechanism(s) drives these bounces. The approach can be also applied to lattices by transforming the differential field equations to difference equation, i.e. avoiding the continuum limit [2].
Some selected research publications: