624 research outputs found
Baryons and baryonic matter in the large Nc and heavy quark limits
This paper explores properties of baryons and finite density baryonic matter
in an artificial world in which Nc, the number of colors, is large and the
quarks of all species are degenerate and much larger than {\Lambda}_QCD. It has
long been known that in large Nc QCD, baryons composed entirely of heavy quarks
are accurately described in the mean-field approximation. However, the detailed
properties of baryons in the combined large Nc and heavy quark limits have not
been fully explored. Here some basic properties of baryons are computed using a
variational approach. At leading order in both the large Nc and heavy quark
expansions the baryon mass is computed explicitly as is the baryon form factor.
Baryonic matter, the analog of nuclear matter in this artificial world, should
also be well described in the mean-field approximation. In the special case
where all baryons have an identical spin flavor structure, it is shown that in
the formal heavy quark and large Nc limit interactions between baryons are
strictly repulsive at low densities. The energy per baryon is computed in this
limit and found to be exponentially small. It is shown that when the
restriction to baryons with an identical spin-flavor structure is dropped, a
phase of baryonic matter exists with a density of 2Nf times that for the
restricted case but with the same energy (where Nf is the number of degenerate
flavors). It is shown that this phase is at least metastable.Comment: 19 page
Is There a Peccei-Quinn Phase Transition?
The nature of axion cosmology is usually said to depend on whether the
Peccei-Quinn (PQ) symmetry breaks before or after inflation. The PQ symmetry
itself is believed to be an accident, so there is not necessarily a symmetry
during inflation at all. We explore these issues in some simple models, which
provide examples of symmetry breaking before and after inflation, or in which
there is no symmetry during inflation and no phase transition at all. One
effect of these observations is to relax the constraints from isocurvature
fluctuations due to the axion during inflation. We also observe new
possibilities for evading the constraints due to cosmic strings and domain
walls, but they seem less generic.Comment: 14 pages. Several references adde
Ultraviolet singularities in classical brane theory
We construct for the first time an energy-momentum tensor for the
electromagnetic field of a p-brane in arbitrary dimensions, entailing finite
energy-momentum integrals. The construction relies on distribution theory and
is based on a Lorentz-invariant regularization, followed by the subtraction of
divergent and finite counterterms supported on the brane. The resulting
energy-momentum tensor turns out to be uniquely determined. We perform the
construction explicitly for a generic flat brane. For a brane in arbitrary
motion our approach provides a new paradigm for the derivation of the,
otherwise divergent, self-force of the brane. The so derived self-force is
automatically finite and guarantees, by construction, energy-momentum
conservation.Comment: 41 pages, no figures, minor change
Symmetric Skyrmions
We present candidates for the global minimum energy solitons of charge one to
nine in the Skyrme model, generated using sophisticated numerical algorithms.
Assuming the Skyrme model accurately represents the low energy limit of QCD,
these configurations correspond to the classical nuclear ground states of the
light elements. The solitons found are particularly symmetric, for example, the
charge seven skyrmion has icosahedral symmetry, and the shapes are shown to fit
a remarkable sequence defined by a geometric energy minimization (GEM) rule. We
also calculate the energies and sizes to within at least a few percent
accuracy. These calculations provide the basis for a future investigation of
the low energy vibrational modes of skyrmions and hence the possibility of
testing the Skyrme model against experiment.Comment: latex, 9 pages, 1 figure (fig1.gif
Twisted Vortices in a Gauge Field Theory
We inspect a particular gauge field theory model that describes the
properties of a variety of physical systems, including a charge neutral
two-component plasma, a Gross-Pitaevskii functional of two charged Cooper pair
condensates, and a limiting case of the bosonic sector in the Salam-Weinberg
model. It has been argued that this field theory model also admits stable
knot-like solitons. Here we produce numerical evidence in support for the
existence of these solitons, by considering stable axis-symmetric solutions
that can be thought of as straight twisted vortex lines clamped at the two
ends. We compute the energy of these solutions as a function of the amount of
twist per unit length. The result can be described in terms of a energy
spectral function. We find that this spectral function acquires a minimum which
corresponds to a nontrivial twist per unit length, strongly suggesting that the
model indeed supports stable toroidal solitons.Comment: 10 pages, 5 figures, title changed, minor revisions, and more
references adde
Dual neutral variables and knot solitons in triplet superconductors
In this paper we derive a dual presentation of free energy functional for
spin-triplet superconductors in terms of gauge-invariant variables. The
resulting equivalent model in ferromagnetic phase has a form of a version of
the Faddeev model. This allows one in particular to conclude that spin-triplet
superconductors allow formation of stable finite-length closed vortices (the
knotted solitons).Comment: Replaced with version published in PRL (added a discussion of the
effect of the coupling of the fields {\vec s} and {\vec C} on knot
stability). Latest updates of the paper and miscellaneous links related to
knotted solitons are also available at the homepage of the author
http://www.teorfys.uu.se/PEOPLE/egor/ . Animations of knotted solitons by
Hietarinta and Salo are available at
http://users.utu.fi/h/hietarin/knots/c45_p2.mp
Numerical simulations of fragmentation of the Affleck-Dine condensate
We present numerical simulations of fragmentation of the Affleck-Dine
condensate in two spatial dimensions. We argue analytically that the final
state should consist of both Q-balls and anti-Q-balls in a state of maximum
entropy, with most of the balls small and relativistic. Such a behaviour is
found in simulations on a 100x100 lattice with cosmologically realistic
parameter values. During fragmentation process, we observe filament-like
texture in the spatial distribution of charge. The total charge in Q-balls is
found to be almost equal to the charge in anti-Q-balls and typically orders of
magnitude larger than charge asymmetry. Analytical considerations indicate
that, apart from geometrical factors, the results of the simulated two
dimensional case should apply also to the fully realistic three dimensional
case.Comment: 28 pages, 39 figure
Hopf instantons in Chern-Simons theory
We study an Abelian Chern-Simons and Fermion system in three dimensions. In
the presence of a fixed prescribed background magnetic field we find an
infinite number of fully three-dimensional solutions. These solutions are
related to Hopf maps and are, therefore, labelled by the Hopf index. Further we
discuss the interpretation of the background field.Comment: one minor error corrected, discussion of gauge fixing added, some
references adde
Constraints on Axion Models from
We explore a new class of axion models in which some, but not all, of the
left-handed quarks have a Peccei-Quinn symmetry. These models are potentially
afflicted by flavour changing neutral currents. We derive the bounds on the
Peccei-Quinn symmetry-breaking scale from bounds on the
branching ratio, showing that in some cases they are even stronger than the
astrophysical ones, but still not strong enough to kill off the models.Comment: 15pp RevTeX, 1 eps fig, uses graphics style. Expanded discussion on
massive scalars and pseudoscalars, typos. To appear in Physical Review
Axion Radiation from Strings
This paper revisits the problem of the string decay contribution to the axion
cosmological energy density. We show that this contribution is proportional to
the average relative increase when axion strings decay of a certain quantity
which we define. We carry out numerical simulations of the
evolution and decay of circular and non-circular string loops, of bent strings
with ends held fixed, and of vortex-antivortex pairs in two dimensions. In the
case of string loops and of vortex-antivortex pairs, decreases by
approximately 20%. In the case of bent strings, remains constant
or increases slightly. Our results imply that the string decay contribution to
the axion energy density is of the same order of magnitude as the
well-understood contribution from vacuum realignment.Comment: 29 pages, 10 figure
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