731 research outputs found

### The dynamics of coset dimensional reduction

The evolution of multiple scalar fields in cosmology has been much studied,
particularly when the potential is formed from a series of exponentials. For a
certain subclass of such systems it is possible to get `assisted` behaviour,
where the presence of multiple terms in the potential effectively makes it
shallower than the individual terms indicate. It is also known that when
compactifying on coset spaces one can achieve a consistent truncation to an
effective theory which contains many exponential terms, however, if there are
too many exponentials then exact scaling solutions do not exist. In this paper
we study the potentials arising from such compactifications of eleven
dimensional supergravity and analyse the regions of parameter space which could
lead to scaling behaviour.Comment: 27 pages, 4 figures; added citation

### Renormalization Group Improved Optimized Perturbation Theory: Revisiting the Mass Gap of the O(2N) Gross-Neveu Model

We introduce an extension of a variationally optimized perturbation method,
by combining it with renormalization group properties in a straightforward
(perturbative) form. This leads to a very transparent and efficient procedure,
with a clear improvement of the non-perturbative results with respect to
previous similar variational approaches. This is illustrated here by deriving
optimized results for the mass gap of the O(2N) Gross-Neveu model, compared
with the exactly know results for arbitrary N. At large N, the exact result is
reproduced already at the very first order of the modified perturbation using
this procedure. For arbitrary values of N, using the original perturbative
information only known at two-loop order, we obtain a controllable percent
accuracy or less, for any N value, as compared with the exactly known result
for the mass gap from the thermodynamical Bethe Ansatz. The procedure is very
general and can be extended straightforwardly to any renormalizable Lagrangian
model, being systematically improvable provided that a knowledge of enough
perturbative orders of the relevant quantities is available.Comment: 18 pages, 1 figure, v2: Eq. (4.5) corrected, comments adde

### A monopole solution from noncommutative multi-instantons

We extend the relation between instanton and monopole solutions of the
selfduality equations in SU(2) gauge theory to noncommutative space-times.
Using this approach and starting from a noncommutative multi-instanton solution
we construct a U(2) monopole configuration which lives in 3 dimensional
ordinary space. This configuration resembles the Wu-Yang monopole and satisfies
the selfduality (Bogomol'nyi) equations for a U(2) Yang-Mills-Higgs system.Comment: 19 pages; title and abstract changed, brane interpretation corrected.
Version to appear in JHE

### Renormalon disappearance in Borel sum of the 1/N expansion of the Gross-Neveu model mass gap

The exact mass gap of the O(N) Gross-Neveu model is known, for arbitrary $N$,
from non-perturbative methods. However, a "naive" perturbative expansion of the
pole mass exhibits an infinite set of infrared renormalons at order 1/N,
formally similar to the QCD heavy quark pole mass renormalons, potentially
leading to large ${\cal O}(\Lambda)$ perturbative ambiguities. We examine the
precise vanishing mechanism of such infrared renormalons, which avoids this
(only apparent)contradiction, and operates without need of (Borel) summation
contour prescription, usually preventing unambiguous separation of perturbative
contributions. As a consequence we stress the direct Borel summability of the
(1/N) perturbative expansion of the mass gap. We briefly speculate on a
possible similar behaviour of analogous non-perturbative QCD quantities.Comment: 16 pp., 1 figure. v2: a few paragraphs and one appendix added, title
and abstract slightly changed, essential results unchange

### Quantum interface unbinding transitions

We consider interfacial phenomena accompanying bulk quantum phase transitions
in presence of surface fields. On general grounds we argue that the surface
contribution to the system free energy involves a line of singularities
characteristic of an interfacial phase transition, occurring below the bulk
transition temperature T_c down to T=0. This implies the occurrence of an
interfacial quantum critical regime extending into finite temperatures and
located within the portion of the phase diagram where the bulk is ordered. Even
in situations, where the bulk order sets in discontinuously at T=0, the
system's behavior at the boundary may be controlled by a divergent length scale
if the tricritical temperature is sufficiently low. Relying on an effective
interfacial model we compute the surface phase diagram in bulk spatial
dimensionality $d\geq 2$ and extract the values of the exponents describing the
interfacial singularities in $d\geq 3$

### New axially symmetric Yang-Mills-Higgs solutions with negative cosmological constant

We construct numerically new axially symmetric solutions of SU(2)
Yang-Mills-Higgs theory in $(3+1)$ anti-de Sitter spacetime. Two types of
finite energy, regular configurations are considered: multimonopole solutions
with magnetic charge $n>1$ and monopole-antimonopole pairs with zero net
magnetic charge. A somewhat detailed analysis of the boundary conditions for
axially symmetric solutions is presented. The properties of these solutions are
investigated, with a view to compare with those on a flat spacetime background.
The basic properties of the gravitating generalizations of these configurations
are also discussed.Comment: 18 pages, 7 figures; v2: typos correcte

### Soft Supersymmetry Breaking due to Dimensional Reduction over Non-Symmetric Coset Spaces

A ten-dimensional supersymmetric $E_8$ gauge theory is compactified over
six-dimensional coset spaces, establishing further our earlier conjecture that
the resulting four dimensional theory is a softly broken supersymmetric gauge
theory in the case that the used coset space is non-symmetric. The specific
non-symmetric six-dimensional spaces examined in the present study are
$Sp(4)/(SU(2) \times U(1))_{non-max.}$ and $SU(3)/U(1) \times U(1)$.Comment: 14 pages, Latex, Comments related to the Scherk-Schwarz mechanism and
relevant references adde

### Diffusion on a hypercubic lattice with pinning potential: exact results for the error-catastrophe problem in biological evolution

In the theoretical biology framework one fundamental problem is the so-called
error catastrophe in Darwinian evolution models. We reexamine Eigen's
fundamental equations by mapping them into a polymer depinning transition
problem in a ``genotype'' space represented by a unitary hypercubic lattice.
The exact solution of the model shows that error catastrophe arises as a direct
consequence of the equations involved and confirms some previous qualitative
results. The physically relevant consequence is that such equations are not
adequate to properly describe evolution of complex life on the Earth.Comment: 10 pages in LaTeX. Figures are available from the authors.
[email protected] (e-mail address

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