304 research outputs found
Loop Calculus in Statistical Physics and Information Science
Considering a discrete and finite statistical model of a general position we
introduce an exact expression for the partition function in terms of a finite
series. The leading term in the series is the Bethe-Peierls (Belief
Propagation)-BP contribution, the rest are expressed as loop-contributions on
the factor graph and calculated directly using the BP solution. The series
unveils a small parameter that often makes the BP approximation so successful.
Applications of the loop calculus in statistical physics and information
science are discussed.Comment: 4 pages, submitted to Phys.Rev.Lett. Changes: More general model,
Simpler derivatio
Universality, Scaling and Topology with a Modified Lattice Action
We examined the effect of a complete suppression of a lattice artifact, the
negative plaquettes, on physical quantities, such as the critical temperature,
the string tension, the topological charge, glueball masses, and their ratios.Comment: 3 pages, self unpacking uuencoded PostScript file, contribution to
conference LATTICE '9
Low Energy Excitations in Spin Glasses from Exact Ground States
We investigate the nature of the low-energy, large-scale excitations in the
three-dimensional Edwards-Anderson Ising spin glass with Gaussian couplings and
free boundary conditions, by studying the response of the ground state to a
coupling-dependent perturbation introduced previously. The ground states are
determined exactly for system sizes up to 12^3 spins using a branch and cut
algorithm. The data are consistent with a picture where the surface of the
excitations is not space-filling, such as the droplet or the ``TNT'' picture,
with only minimal corrections to scaling. When allowing for very large
corrections to scaling, the data are also consistent with a picture with
space-filling surfaces, such as replica symmetry breaking. The energy of the
excitations scales with their size with a small exponent \theta', which is
compatible with zero if we allow moderate corrections to scaling. We compare
the results with data for periodic boundary conditions obtained with a genetic
algorithm, and discuss the effects of different boundary conditions on
corrections to scaling. Finally, we analyze the performance of our branch and
cut algorithm, finding that it is correlated with the existence of
large-scale,low-energy excitations.Comment: 18 Revtex pages, 16 eps figures. Text significantly expanded with
more discussion of the numerical data. Fig.11 adde
Scaling in the Positive Plaquette Model and Universality in SU(2) Lattice Gauge Theory
We investigate universality, scaling, the beta-function and the topological
charge in the positive plaquette model for SU(2) lattice gauge theory.
Comparing physical quantities, like the critical temperature, the string
tension, glueball masses, and their ratios, we explore the effect of a complete
suppression of a certain lattice artifact, namely the negative plaquettes, for
SU(2) lattice gauge theory. Our result is that this modification does not
change the continuum limit, i.e., the universality class. The positive
plaquette model and the standard Wilson formulation describe the same physical
situation. The approach to the continuum limit given by the beta-function in
terms of the bare lattice coupling, however, is rather different: the
beta-function of the positive plaquette model does not show a dip like the
model with standard Wilson action.Comment: 35 pages, preprint numbers FSU-SCRI-94-71 and HU Berlin-IEP-94/1
A Conditional Strategy for Cell-Type-Specific Labeling of Endogenous Excitatory Synapses in Drosophila
Chemical neurotransmission occurs at specialized contacts where neurotransmitter release machinery apposes neurotransmitter receptors to underlie circuit function. A series of complex events underlies preand postsynaptic protein recruitment to neuronal connections. To better study synaptic development in individual neurons, we need cell-type-specific strategies to visualize endogenous synaptic proteins. Although presynaptic strategies exist, postsynaptic proteins remain less studied because of a paucity of cell-type-specific reagents. To study excitatory postsynapses with cell-type specificity, we engineered dlg1[4K], a conditionally labeled marker of Drosophila excitatory postsynaptic densities. With binary expression systems, dlg1[4K] labels central and peripheral postsynapses in larvae and adults. Using dlg1[4K], we find that distinct rules govern postsynaptic organization in adult neurons, multiple binary expression systems can concurrently label pre- and postsynapse in a cell-type-specific manner, and neuronal DLG1 can sometimes localize presynaptically. These results validate our strategy for conditional postsynaptic labeling and demonstrate principles of synaptic organization
Critical Exponents of the Superconducting Phase Transition
We study the critical exponents of the superconducting phase transition in
the context of renormalization group theory starting from a dual formulation of
the Ginzburg-Landau theory. The dual formulation describes a loop gas of
Abrikosov flux tubes which proliferate when the critical temperature is
approached from below. In contrast to the Ginzburg-Landau theory, it has a
spontaneously broken global symmetry and possesses an infrared stable fixed
point. The exponents coincide with those of a superfluid with reversed
temperature axis.Comment: Postscript file. For related work see www adress
http://www.physik.fu-berlin.de/kleiner_re.html in our homepage
http://www.physik.fu-berlin.de/kleinert.htm
Four-quark flux distribution and binding in lattice SU(2)
The full spatial distribution of the color fields of two and four static
quarks is measured in lattice SU(2) field theory at separations up to 1 fm at
beta=2.4. The four-quark case is equivalent to a qbar q qbar q system in SU(2)
and is relevant to meson-meson interactions. By subtracting two-body flux tubes
from the four-quark distribution we isolate the flux contribution connected
with the four-body binding energy. This contribution is further studied using a
model for the binding energies. Lattice sum rules for two and four quarks are
used to verify the results.Comment: 46 pages including 71 eps figures. 3D color figures are available at
www.physics.helsinki.fi/~ppennane/pics
Renormalons in Effective Field Theories
We investigate the high-order behavior of perturbative matching conditions in
effective field theories. These series are typically badly divergent, and are
not Borel summable due to infrared and ultraviolet renormalons which introduce
ambiguities in defining the sum of the series. We argue that, when treated
consistently, there is no physical significance to these ambiguities. Although
nonperturbative matrix elements and matching conditions are in general
ambiguous, the ambiguity in any physical observable is always higher order in
than the theory has been defined. We discuss the implications for the
recently noticed infrared renormalon in the pole mass of a heavy quark. We show
that a ratio of form factors in exclusive decays (which is related
to the pole mass) is free from renormalon ambiguities regardless of the mass
used as the expansion parameter of HQET. The renormalon ambiguities also cancel
in inclusive heavy hadron decays. Finally, we demonstrate the cancellation of
renormalons in a four-Fermi effective theory obtained by integrating out a
heavy colored scalar.Comment: Minor changes mad
An Extended Variational Principle for the SK Spin-Glass Model
The recent proof by F. Guerra that the Parisi ansatz provides a lower bound
on the free energy of the SK spin-glass model could have been taken as offering
some support to the validity of the purported solution. In this work we present
a broader variational principle, in which the lower bound, as well as the
actual value, are obtained through an optimization procedure for which
ultrametic/hierarchal structures form only a subset of the variational class.
The validity of Parisi's ansatz for the SK model is still in question. The new
variational principle may be of help in critical review of the issue.Comment: 4 pages, Revtex
Maximal variance reduction for stochastic propagators with applications to the static quark spectrum
We study a new method -- maximal variance reduction -- for reducing the
variance of stochastic estimators for quark propagators. We find that while
this method is comparable to usual iterative inversion for light-light mesons,
a considerable improvement is achieved for systems containing at least one
infinitely heavy quark. Such systems are needed for heavy quark effective
theory. As an illustration of the effectiveness of the method we present
results for the masses of the ground state and excited states of
mesons and baryons. We compare these results with the experimental
spectra involving quarks.Comment: 31 pages with 7 postscript file
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