690 research outputs found
Towers of Gravitational Theories
In this essay we introduce a theoretical framework designed to describe black
hole dynamics. The difficulties in understanding such dynamics stems from the
proliferation of scales involved when one attempts to simultaneously describe
all of the relevant dynamical degrees of freedom. These range from the modes
that describe the black hole horizon, which are responsible for dissipative
effects, to the long wavelength gravitational radiation that drains mechanical
energy from macroscopic black hole bound states. We approach the problem from a
Wilsonian point of view, by building a tower of theories of gravity each of
which is valid at different scales. The methodology leads to multiple new
results in diverse topics including phase transitions of Kaluza-Klein black
holes and the interactions of spinning black hole in non-relativistic orbits.
Moreover, our methods tie together speculative ideas regarding dualities for
black hole horizons to real physical measurements in gravitational wave
detectors.Comment: Awarded second prize for 2006 Gravity Research Foundation essay
contes
Critical States in a Dissipative Sandpile Model
A directed dissipative sandpile model is studied in the two-dimension.
Numerical results indicate that the long time steady states of this model are
critical when grains are dropped only at the top or, everywhere. The critical
behaviour is mean-field like. We discuss the role of infinite avalanches of
dissipative models in periodic systems in determining the critical behaviour of
same models in open systems.Comment: 4 pages (Revtex), 5 ps figures (included
Progress in QCD next-to-leading order calculations
I review progress related to the calculation of QCD jet cross sections at the
NLO accuracy. After a short introduction into the theory of NLO calculations, I
discuss two recent developments: the calculation of two- and three-jet
leptoproduction at the NLO accuracy and the extension of the dipole subtraction
method for computing NLO corrections for processes involving massive partons.Comment: 5 pages, 4 figures, LaTeX using JHEP3.cls, Invited talk at the
International Europhysics Conference on High-Energy Physics (HEP 2001
Two-loop Anomalous Dimensions of Heavy Baryon Currents in Heavy Quark Effective Theory
We present results on the two-loop anomalous dimensions of the heavy baryon
HQET currents with arbitrary Dirac matrices
and . From our general result we obtain the two-loop
anomalous dimensions for currents with quantum numbers of the ground state
heavy baryons , and . As a by-product of our
calculation and as an additional check we rederive the known two-loop anomalous
dimensions of mesonic scalar, pseudoscalar, vector, axial vector and tensor
currents in massless QCD as well as in HQET.Comment: 21 pages, LaTeX, 2 figures are included in PostScript forma
Scaling in a Nonconservative Earthquake Model of Self-Organised Criticality
We numerically investigate the Olami-Feder-Christensen model for earthquakes
in order to characterise its scaling behaviour. We show that ordinary finite
size scaling in the model is violated due to global, system wide events.
Nevertheless we find that subsystems of linear dimension small compared to the
overall system size obey finite (subsystem) size scaling, with universal
critical coefficients, for the earthquake events localised within the
subsystem. We provide evidence, moreover, that large earthquakes responsible
for breaking finite size scaling are initiated predominantly near the boundary.Comment: 6 pages, 6 figures, to be published in Phys. Rev. E; references
sorted correctl
The three-dimensional random field Ising magnet: interfaces, scaling, and the nature of states
The nature of the zero temperature ordering transition in the 3D Gaussian
random field Ising magnet is studied numerically, aided by scaling analyses. In
the ferromagnetic phase the scaling of the roughness of the domain walls,
, is consistent with the theoretical prediction .
As the randomness is increased through the transition, the probability
distribution of the interfacial tension of domain walls scales as for a single
second order transition. At the critical point, the fractal dimensions of
domain walls and the fractal dimension of the outer surface of spin clusters
are investigated: there are at least two distinct physically important fractal
dimensions. These dimensions are argued to be related to combinations of the
energy scaling exponent, , which determines the violation of
hyperscaling, the correlation length exponent , and the magnetization
exponent . The value is derived from the
magnetization: this estimate is supported by the study of the spin cluster size
distribution at criticality. The variation of configurations in the interior of
a sample with boundary conditions is consistent with the hypothesis that there
is a single transition separating the disordered phase with one ground state
from the ordered phase with two ground states. The array of results are shown
to be consistent with a scaling picture and a geometric description of the
influence of boundary conditions on the spins. The details of the algorithm
used and its implementation are also described.Comment: 32 pp., 2 columns, 32 figure
Noise-Induced Phase Separation: Mean-Field Results
We present a study of a phase-separation process induced by the presence of
spatially-correlated multiplicative noise. We develop a mean-field approach
suitable for conserved-order-parameter systems and use it to obtain the phase
diagram of the model. Mean-field results are compared with numerical
simulations of the complete model in two dimensions. Additionally, a comparison
between the noise-driven dynamics of conserved and nonconserved systems is made
at the level of the mean-field approximation.Comment: 12 pages (including 6 figures) LaTeX file. Submitted to Phys. Rev.
2-loop Functional Renormalization Group Theory of the Depinning Transition
We construct the field theory which describes the universal properties of the
quasi-static isotropic depinning transition for interfaces and elastic periodic
systems at zero temperature, taking properly into account the non-analytic form
of the dynamical action. This cures the inability of the 1-loop flow-equations
to distinguish between statics and quasi-static depinning, and thus to account
for the irreversibility of the latter. We prove two-loop renormalizability,
obtain the 2-loop beta-function and show the generation of "irreversible"
anomalous terms, originating from the non-analytic nature of the theory, which
cause the statics and driven dynamics to differ at 2-loop order. We obtain the
roughness exponent zeta and dynamical exponent z to order epsilon^2. This
allows to test several previous conjectures made on the basis of the 1-loop
result. First it demonstrates that random-field disorder does indeed attract
all disorder of shorter range. It also shows that the conjecture zeta=epsilon/3
is incorrect, and allows to compute the violations, as zeta=epsilon/3 (1 +
0.14331 epsilon), epsilon=4-d. This solves a longstanding discrepancy with
simulations. For long-range elasticity it yields zeta=epsilon/3 (1 + 0.39735
epsilon), epsilon=2-d (vs. the standard prediction zeta=1/3 for d=1), in
reasonable agreement with the most recent simulations. The high value of zeta
approximately 0.5 found in experiments both on the contact line depinning of
liquid Helium and on slow crack fronts is discussed.Comment: 32 pages, 17 figures, revtex
B-->pi and B-->K transitions in standard and quenched chiral perturbation theory
We study the effects of chiral logs on the heavy-->light pseudoscalar meson
transition form factors by using standard and quenched chiral perturbation
theory combined with the static heavy quark limit. The resulting expressions
are used to indicate the size of uncertainties due to the use of the quenched
approximation in the current lattice studies. They may also be used to assess
the size of systematic uncertainties induced by missing chiral log terms in
extrapolating toward the physical pion mass. We also provide the coefficient
multiplying the quenched chiral log, which may be useful if the quenched
lattice studies are performed with very light mesons.Comment: 33 pages, 8 PostScript figures, version to appear in PR
Long-Range Forces of QCD
We consider the scattering of two color dipoles (e.g., heavy quarkonium
states) at low energy - a QCD analog of Van der Waals interaction. Even though
the couplings of the dipoles to the gluon field can be described in
perturbation theory, which leads to the potential proportional to
(N_c^2-1)/R^{7}, at large distances R the interaction becomes totally
non-perturbative. Low-energy QCD theorems are used to evaluate the leading
long-distance contribution \sim (N_f^2-1)/(11N_c - 2N_f)^2 R^{-5/2} exp(-2 \mu
R) (\mu is the Goldstone boson mass), which is shown to arise from the
correlated two-boson exchange. The sum rule which relates the overall strength
of the interaction to the energy density of QCD vacuum is derived.
Surprisingly, we find that when the size of the dipoles shrinks to zero (the
heavy quark limit in the case of quarkonia), the non-perturbative part of the
interaction vanishes more slowly than the perturbative part as a consequence of
scale anomaly. As an application, we evaluate elastic \pi J/\psi and \pi J/\psi
\to \pi \psi' cross sections.Comment: 16pages, 9 eps figures; discussion extended, 2 new references added,
to appear in Phys.Rev.
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