18,602 research outputs found
Analytical model for flux saturation in sediment transport
The transport of sediment by a fluid along the surface is responsible for
dune formation, dust entrainment and for a rich diversity of patterns on the
bottom of oceans, rivers, and planetary surfaces. Most previous models of
sediment transport have focused on the equilibrium (or saturated) particle
flux. However, the morphodynamics of sediment landscapes emerging due to
surface transport of sediment is controlled by situations out-of-equilibrium.
In particular, it is controlled by the saturation length characterizing the
distance it takes for the particle flux to reach a new equilibrium after a
change in flow conditions. The saturation of mass density of particles
entrained into transport and the relaxation of particle and fluid velocities
constitute the main relevant relaxation mechanisms leading to saturation of the
sediment flux. Here we present a theoretical model for sediment transport
which, for the first time, accounts for both these relaxation mechanisms and
for the different types of sediment entrainment prevailing under different
environmental conditions. Our analytical treatment allows us to derive a closed
expression for the saturation length of sediment flux, which is general and can
thus be applied under different physical conditions
Kinematical and Dynamical Aspects of Higher-Spin Bound-State Equations in Holographic QCD
In this paper we derive holographic wave equations for hadrons with arbitrary
spin starting from an effective action in a higher-dimensional space asymptotic
to anti-de Sitter (AdS) space. Our procedure takes advantage of the local
tangent frame, and it applies to all spins, including half-integer spins. An
essential element is the mapping of the higher-dimensional equations of motion
to the light-front Hamiltonian, thus allowing a clear distinction between the
kinematical and dynamical aspects of the holographic approach to hadron
physics. Accordingly, the non-trivial geometry of pure AdS space encodes the
kinematics, and the additional deformations of AdS space encode the dynamics,
including confinement. It thus becomes possible to identify the features of
holographic QCD which are independent of the specific mechanisms of conformal
symmetry breaking. In particular, we account for some aspects of the striking
similarities and differences observed in the systematics of the meson and
baryon spectra.Comment: 43 page
Hadron Spectroscopy and Dynamics from Light-Front Holography and Conformal Symmetry
To a first semiclassical approximation one can reduce the multi-parton
light-front problem in QCD to an effective one-dimensional quantum field
theory, which encodes the fundamental conformal symmetry of the classical QCD
Lagrangian. This procedure leads to a relativistic light-front wave equation
for arbitrary spin which incorporates essential spectroscopic and
non-perturbative dynamical features of hadron physics. The mass scale for
confinement and higher dimensional holographic mapping to AdS space are also
emergent properties of this framework.Comment: 11 pages, 6 figures. Invited plenary talk, presented by GdT at the
13th International Conference on Meson-Nucleon Physics and the Structure of
the Nucleon (MENU 2013), Rome, September 30 - October 4, 201
Self-Assembly of Monatomic Complex Crystals and Quasicrystals with a Double-Well Interaction Potential
For the study of crystal formation and dynamics we introduce a simple
two-dimensional monatomic model system with a parametrized interaction
potential. We find in molecular dynamics simulations that a surprising variety
of crystals, a decagonal and a dodecagonal quasicrystal are self-assembled. In
the case of the quasicrystals the particles reorder by phason flips at elevated
temperatures. During annealing the entropically stabilized decagonal
quasicrystal undergoes a reversible phase transition at 65% of the melting
temperature into an approximant, which is monitored by the rotation of the de
Bruijn surface in hyperspace.Comment: 4 pages, 6 figures. Physical Review Letters, in Press (April 2007
Threefold Complementary Approach to Holographic QCD
A complementary approach, derived from (a) higher dimensional anti--de Sitter
(AdS) space, (b) light-front quantization and (c) the invariance properties of
the full conformal group in one dimension leads to a nonperturbative
relativistic light-front wave equation which incorporates essential
spectroscopic and dynamical features of hadron physics. The fundamental
conformal symmetry of the classical QCD Lagrangian in the limit of massless
quarks is encoded in the resulting effective theory. The mass scale for
confinement emerges from the isomorphism between the conformal group and
SO(2,1). This scale appears in the light-front Hamiltonian by mapping to the
evolution operator in the formalism of de Alfaro, Fubini and Furlan, which
retains the conformal invariance of the action. Remarkably, the specific form
of the confinement interaction and the corresponding modification of AdS space
are uniquely determined in this procedure.Comment: Reference added. Article accepted for publication in PL
Light-Front Holographic Quantum Chromodynamics
Anti-de Sitter space in five dimensions provides an exact geometrical
representation of the conformal group. Remarkably, gravity in AdS space is
holographically dual to frame-independent light-front Hamiltonian theory,
derived from the quantization of the QCD Lagrangian at fixed light-front time
. Light-front holography also leads to a precise relation
between the bound-state amplitudes in the fifth dimension of AdS space and
the variable , where is the argument of the
boost-invariant light-front wavefunctions describing the internal structure of
hadrons in physical space-time. The holographic mapping of AdS space with a
specific "soft-wall" dilaton yields a confining potential for the
light-front Schr\"odinger equation for hadrons with arbitrary spin .
Remarkably, has a unique form of a harmonic oscillator potential
if one requires that the chiral QCD action remains conformally invariant. One
thus obtains an effective light-front effective theory for general spin which
respects the conformal symmetry of the four-dimensional classical QCD
Lagrangian. The predictions of the LF equations of motion include a zero-mass
pion in the chiral limit, and linear Regge trajectories with the same slope in the radial quantum number and the
orbital angular momentum . The light-front AdS/QCD holographic approach
gives a frame-independent representation of color-confining dynamics, Regge
spectroscopy, as well as the excitation spectra of relativistic light-quark
meson and baryon bound states in QCD in terms of a single mass parameter. We
also briefly discuss the implications of the underlying conformal template of
QCD for renormalization scale-setting, and the implications of light-front
quantization for the value of the cosmological constant.Comment: Presented by SJB at the International Conference on Nuclear Theory in
the Supercomputing Era (NTSE 2013) in honor of James Vary, May 13 - May 17,
2013, Iowa State University, Ames, Iow
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