2,026 research outputs found
Mean- Field Approximation and a Small Parameter in Turbulence Theory
Numerical and physical experiments on two-dimensional (2d) turbulence show
that the differences of transverse components of velocity field are well
described by a gaussian statistics and Kolmogorov scaling exponents. In this
case the dissipation fluctuations are irrelevant in the limit of small
viscosity. In general, one can assume existence of critical
space-dimensionality , at which the energy flux and all odd-order
moments of velocity difference change sign and the dissipation fluctuations
become dynamically unimportant. At the flow can be described by the
``mean-field theory'', leading to the observed gaussian statistics and
Kolmogorov scaling of transverse velocity differences. It is shown that in the
vicinity of the ratio of the relaxation and translation
characteristic times decreases to zero, thus giving rise to a small parameter
of the theory. The expressions for pressure and dissipation contributions to
the exact equation for the generating function of transverse velocity
differences are derived in the vicinity of . The resulting equation
describes experimental data on two-dimensional turbulence and demonstrate onset
of intermittency as and in three-dimensional flows in
close agreement with experimental data. In addition, some new exact relations
between correlation functions of velocity differences are derived. It is also
predicted that the single-point pdf of transverse velocity difference in
developing as well as in the large-scale stabilized two-dimensional turbulence
is a gaussian.Comment: 25 pages, 1 figur
Effective Monopoles within Thick Branes
The monopole mass is revealed to be considerably modified in the thick
braneworld paradigm, and depends on the position of the monopole in the brane
as well. Accordingly, the monopole radius continuously increases, leading to an
unacceptable setting that can be circumvented when the brane thickness has an
upper limit. Despite such peculiar behavior, the quantum corrections accrued --
involving the classical monopole solution -- are shown to be still under
control. We analyze the monopole's peculiarities also taking into account the
localization of the gauge fields. Furthermore, some additional analysis in the
thick braneworld context and the similar behavior evinced by the topological
string are investigated.Comment: 7 pages, 1 figur
Wavy Wilson Line and AdS/CFT
Wilson loops which are small deviations from straight, infinite lines, called
wavy lines, are considered in the context of the AdS/CFT correspondence. A
single wavy line and the connected correlation function of a straight and wavy
line are considered. It is argued that, to leading order in ``waviness'', the
functional form of the loop is universal and the coefficient, which is a
function of the 't Hooft coupling, is found in weak coupling perturbation
theory and the strong coupling limit using the AdS/CFT correspondence.
Supersymmetric arguments are used to simplify the computation and to show that
the straight line obeys the Migdal-Makeenko loop equation
Photoproduction of Baryons Decaying into N pi and N eta
A combined analysis of photoproduction data on \gamma p to \pi N, eta N was
performed including the data on K Lambda and K Sigma. The data are interpreted
in an isobar model with s--channel baryon resonances and pi, rho,(omega), K,
and K^* exchange in the t--channel. Three baryon resonances have a substantial
coupling to eta N, the well known N(1535)S_{11}, N(1720)P_{13}, and
N(2070)D_{15}. The inclusion of data with open strangeness reveals the presence
of further new resonances, N(1840)P_{11}, N(1875)D_{13} and N(2170)D_{13}.Comment: 13 pages, 14 figure
Topological transition in a two-dimensional model of liquid crystal
Simulations of nematic-isotropic transition of liquid crystals in two
dimensions are performed using an O(2) vector model characterised by non linear
nearest neighbour spin interaction governed by the fourth Legendre polynomial
. The system is studied through standard Finite-Size Scaling and
conformal rescaling of density profiles of correlation functions. A topological
transition between a paramagnetic phase at high temperature and a critical
phase at low temperature is observed. The low temperature limit is discussed in
the spin wave approximation and confirms the numerical results
Pdf's of Derivatives and Increments for Decaying Burgers Turbulence
A Lagrangian method is used to show that the power-law with a -7/2 exponent
in the negative tail of the pdf of the velocity gradient and of velocity
increments, predicted by E, Khanin, Mazel and Sinai (1997 Phys. Rev. Lett. 78,
1904) for forced Burgers turbulence, is also present in the unforced case. The
theory is extended to the second-order space derivative whose pdf has power-law
tails with exponent -2 at both large positive and negative values and to the
time derivatives. Pdf's of space and time derivatives have the same
(asymptotic) functional forms. This is interpreted in terms of a "random Taylor
hypothesis".Comment: LATEX 8 pages, 3 figures, to appear in Phys. Rev.
4d-Flat Compactifications With Brane Vorticities
We present solutions in six-dimensional gravity coupled to a sigma model, in
the presence of three-brane sources. The space transverse to the branes is a
compact non-singular manifold. The example of O(3) sigma model in the presence
of two three-branes is worked out in detail. We show that the four-dimensional
flatness is obtained with a single condition involving the brane tensions,
which are in general different and may be both positive, and another
characteristic of the branes, vorticity. We speculate that the adjustment of
the effective four-dimensional cosmological constant may occur through the
exchange of vorticity between the branes. We then give exact instanton type
solutions for sigma models targeted on a general K\"ahler manifold, and
elaborate in this framework on multi-instantons of the O(3) sigma model. The
latter have branes, possibly with vorticities, at the instanton positions, thus
generalizing our two-brane solution.Comment: 8 pages. New references added and minor typos are correcte
Photon-graviton mixing in an electromagnetic field
Einstein-Maxwell theory implies the mixing of photons with gravitons in an
external electromagnetic field. This process and its possible observable
consequences have been studied at tree level for many years. We use the
worldline formalism for obtaining an exact integral representation for the
one-loop corrections to this amplitude due to scalars and fermions. We study
the structure of this amplitude, and obtain exact expressions for various
limiting cases.Comment: 13 pages, 1 figure, talk given by C. Schubert at QFEXT07, Leipzig,
17-21 Sep 2007, final published version (slightly extended
Fractional charge in transport through a 1D correlated insulator of finite length
Transport through a one channel wire of length confined between two leads
is examined when the 1D electron system has an energy gap : induced by the interaction in charge mode (: charge velocity in the
wire). In spinless case the transformation of the leads electrons into the
charge density wave solitons of fractional charge entails a non-trivial low
energy crossover from the Fermi liquid behavior below the crossover energy to the insulator one with the
fractional charge in current vs. voltage, conductance vs. temperature, and in
shot noise. Similar behavior is predicted for the Mott insulator of filling
factor .Comment: 5 twocolumn pages in RevTex, no figure
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