710 research outputs found
Instantons, Fluxons and Open Gauge String Theory
We use the exact instanton expansion to illustrate various string
characteristics of noncommutative gauge theory in two dimensions. We analyse
the spectrum of the model and present some evidence in favour of Hagedorn and
fractal behaviours. The decompactification limit of noncommutative torus
instantons is shown to map in a very precise way, at both the classical and
quantum level, onto fluxon solutions on the noncommutative plane. The
weak-coupling singularities of the usual Gross-Taylor string partition function
for QCD on the torus are studied in the instanton representation and its double
scaling limit, appropriate for the mapping onto noncommutative gauge theory, is
shown to be a generating function for the volumes of the principal moduli
spaces of holomorphic differentials. The noncommutative deformation of this
moduli space geometry is described and appropriate open string interpretations
are proposed in terms of the fluxon expansion.Comment: 70 pages, 6 figure
Large-scale anisotropy in scalar turbulence
The effect of anisotropy on the statistics of a passive tracer transported by
a turbulent flow is investigated. We show that under broad conditions an
arbitrarily small amount of anisotropy propagates to the large scales where it
eventually dominates the structure of the concentration field. This result is
obtained analytically in the framework of an exactly solvable model and
confirmed by numerical simulations of scalar transport in two-dimensional
turbulence
On shape dependence of holographic entanglement entropy in AdS4/CFT3 with Lifshitz scaling and hyperscaling violation
We study the divergent terms and the finite term in the expansion of the holographic entanglement entropy as the ultraviolet cutoff vanishes for smooth spatial regions having arbitrary shape, when the gravitational background is a four dimensional asymptotically Lifshitz spacetime with hyperscaling violation, in a certain range of the hyperscaling parameter. Both static and time dependent backgrounds are considered. For the coefficients of the divergent terms and for the finite term, analytic expressions valid for any smooth entangling curve are obtained. The analytic results for the finite terms are checked through a numerical analysis focussed on disks and ellipses
Granular size segregation in underwater sand ripples
We report an experimental study of a binary sand bed under an oscillating
water flow. The formation and evolution of ripples is observed. The appearance
of a granular segregation is shown to strongly depend on the sand bed
preparation. The initial wavelength of the mixture is measured. In the final
steady state, a segregation in volume is observed instead of a segregation at
the surface as reported before. The correlation between this phenomenon and the
fluid flow is emphasised. Finally, different ``exotic'' patterns and their
geophysical implications are presented.Comment: 8 page
Fermi-Walker gauge in 2+1 dimensional gravity.
It is shown that the Fermi-Walker gauge allows the general solution of
determining the metric given the sources, in terms of simple quadratures. We
treat the general stationary problem providing explicit solving formulas for
the metric and explicit support conditions for the energy momentum tensor. The
same type of solution is obtained for the time dependent problem with circular
symmetry. In both cases the solutions are classified in terms of the invariants
of the Wilson loops outside the sources. The Fermi-Walker gauge, due to its
physical nature, allows to exploit the weak energy condition and in this
connection it is proved that, both for open and closed universes with
rotational invariance, the energy condition imply the total absence of closed
time like curves. The extension of this theorem to the general stationary
problem, in absence of rotational symmetry is considered. At present such
extension is subject to some assumptions on the behavior of the determinant of
the dreibein in this gauge. PACS number: 0420Comment: 28 pages, RevTex, no figure
Anyons and Chiral Solitons on a Line
We show that excitations in a recently proposed gauge theory for anyons on a
line in fact do not obey anomalous statistics. On the other hand, the theory
supports novel chiral solitons. Also we construct a field-theoretic description
of lineal anyons, but gauge fields play no role.Comment: 8 pages, revtex, no figure
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