764 research outputs found
Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations
Using molecular dynamics simulations, we report a study of the dynamics of
two-dimensional vortex lattices driven over a disordered medium. In strong
disorder, when topological order is lost, we show that the depinning transition
is analogous to a second order critical transition: the velocity-force response
at the onset of motion is continuous and characterized by critical exponents.
Combining studies at zero and nonzero temperature and using a scaling analysis,
two critical expo- nents are evaluated. We find v\sim (F-F_c)^\beta with
\beta=1.3\pm0.1 at T=0 and F>F_c, and v\sim T^{1/\delta} with
\delta^{-1}=0.75\pm0.1 at F=F_c, where F_c is the critical driving force at
which the lattice goes from a pinned state to a sliding one. Both critical
exponents and the scaling function are found to exhibit universality with
regard to the pinning strength and different disorder realizations.
Furthermore, the dynamics is shown to be chaotic in the whole critical region.Comment: 8 pages, 6 figure
Hiding relativistic degrees of freedom in the early universe
We quantify the extent to which extra relativistic energy density can be
concealed by a neutrino asymmetry without conflicting with the baryon asymmetry
measured by the Wilkinson Microwave Anisotropy Probe (WMAP). In the presence of
a large electron neutrino asymmetry, slightly more than seven effective
neutrinos are allowed by Big Bang Nucleosynthesis (BBN) and WMAP at 2\sigma.
The same electron neutrino degeneracy that reconciles the BBN prediction for
the primordial helium abundance with the observationally inferred value also
reconciles the LSND neutrino with BBN by suppressing its thermalization prior
to BBN.Comment: 5 pages, 5 figure
WMAPping out Neutrino Masses
Recent data from from the Wilkinson Microwave Anisotropy Probe (WMAP) place
important bounds on the neutrino sector. The precise determination of the
baryon number in the universe puts a strong constraint on the number of
relativistic species during Big-Bang Nucleosynthesis. WMAP data, when combined
with the 2dF Galaxy Redshift Survey (2dFGRS), also directly constrain the
absolute mass scale of neutrinos. These results impinge upon a neutrino
oscillation interpretation of the result from the Liquid Scintillator Neutrino
Detector (LSND). We also note that the Heidelberg--Moscow evidence for
neutrinoless double beta decay is only consistent with the WMAP+2dFGRS data for
the largest values of the nuclear matrix element.Comment: 5 pages, 2 figures. v2:Streamlined discussion of thermalization;
improved discussion of large lepton asymmetry. v3: Added References. v4: More
conservative treatment of bia
N=2 Current Algebras for Non-Semi-Simple Groups
We examine the problem of constructing N=2 superconformal algebras out of N=1
non-semi-simple affine Lie algebras. These N=2 superconformal theories share
the property that the super Virasoro central charge depends only on the
dimension of the Lie algebra. We find, in particular, a construction having a
central charge c=9. This provides a possible internal space for string
compactification and where mirror symmetry might be explored.Comment: 10 pages, BONN-HE-94-0
Superstring Gravitational Wave Backgrounds with Spacetime Supersymmetry
We analyse the stringy gravitational wave background based on the current
algebra . We determine its exact spectrum and construct the modular
invariant vacuum energy. The corresponding N=1 extension is also constructed.
The algebra is again mapped to free bosons and fermions and we show that this
background has N=4 (N=2) unbroken spacetime supersymmetry in the type II
(heterotic case).Comment: 14 pages, CERN-TH.7218/94, HUN-IEP-94/5, LPTENS-94/10 (references
updated
Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy
We consider the effect of inhomogeneous neutrino degeneracy on Big Bang
nucleosynthesis for the case where the distribution of neutrino chemical
potentials is given by a Gaussian. The chemical potential fluctuations are
taken to be isocurvature, so that only inhomogeneities in the electron chemical
potential are relevant. Then the final element abundances are a function only
of the baryon-photon ratio , the effective number of additional neutrinos
, the mean electron neutrino degeneracy parameter , and
the rms fluctuation of the degeneracy parameter, . We find that for
fixed , , and , the abundances of helium-4,
deuterium, and lithium-7 are, in general, increasing functions of .
Hence, the effect of adding a Gaussian distribution for the electron neutrino
degeneracy parameter is to decrease the allowed range for . We show that
this result can be generalized to a wide variety of distributions for .Comment: 9 pages, 3 figures, added discussion of neutrino oscillations,
altered presentation of figure
Integrable N=2 Supersymmetric Field Theories
Some additional references are included on the last 3 pages.Comment: 14 pages, OUTP-92-12
Exact four dimensional string solutions and Toda-like sigma models from `null-gauged' WZNW theories
We construct a new class of exact string solutions with a four dimensional
target space metric of signature () by gauging the independent left
and right nilpotent subgroups with `null' generators of WZNW models for rank 2
non-compact groups . The `null' property of the generators () implies the consistency of the gauging and the absence of
\a'-corrections to the semiclassical backgrounds obtained from the gauged
WZNW models. In the case of the maximally non-compact groups () the construction corresponds to gauging some of the
subgroups generated by the nilpotent `step' operators in the Gauss
decomposition. The rank 2 case is a particular example of a general
construction leading to conformal backgrounds with one time-like direction. The
conformal theories obtained by integrating out the gauge field can be
considered as sigma model analogs of Toda models (their classical equations of
motion are equivalent to Toda model equations). The procedure of `null gauging'
applies also to other non-compact groups.Comment: 33 p., harvmac, Imperial/TP/93-94/17 (minor changes: few explanations
added
Supersymmetric Toda Field Theories
We present new supersymmetric extensions of Conformal Toda and
Affine Toda field theories. These new theories are constructed using methods
similar to those that have been developed to find supersymmetric extensions of
two-dimensional bosonic sigma models with a scalar potential. In particular, we
show that the Conformal Toda field theory admits a (1,1)-supersymmetric
extension, and the Affine Toda field admits a (1,0)-supersymmetric
extension.Comment: 13 page
Topological charges and the genus of surfaces
We show that the topological charge of the n-soliton solution of the
sine-Gordon equation n is related to the genus g > 1 of a constant negative
curvature compact surface described by this configuration. The relation is
n=2(g-1), where n is even. The moduli space of complex dimension B(g)=3(g-1)
corresponds precisely to the freedom to choosing the configuration with n
solitons of arbitrary positions and velocities. We speculate also that the odd
soliton states will describe the unoriented surfaces.Comment: 8 pages, Latex. To be published in Journal of Geommetry and Physic
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