6,158 research outputs found
Multilevel compression of random walks on networks reveals hierarchical organization in large integrated systems
To comprehend the hierarchical organization of large integrated systems, we
introduce the hierarchical map equation, which reveals multilevel structures in
networks. In this information-theoretic approach, we exploit the duality
between compression and pattern detection; by compressing a description of a
random walker as a proxy for real flow on a network, we find regularities in
the network that induce this system-wide flow. Finding the shortest multilevel
description of the random walker therefore gives us the best hierarchical
clustering of the network, the optimal number of levels and modular partition
at each level, with respect to the dynamics on the network. With a novel search
algorithm, we extract and illustrate the rich multilevel organization of
several large social and biological networks. For example, from the global air
traffic network we uncover countries and continents, and from the pattern of
scientific communication we reveal more than 100 scientific fields organized in
four major disciplines: life sciences, physical sciences, ecology and earth
sciences, and social sciences. In general, we find shallow hierarchical
structures in globally interconnected systems, such as neural networks, and
rich multilevel organizations in systems with highly separated regions, such as
road networks.Comment: 11 pages, 5 figures. For associated code, see
http://www.tp.umu.se/~rosvall/code.htm
The map equation
Many real-world networks are so large that we must simplify their structure
before we can extract useful information about the systems they represent. As
the tools for doing these simplifications proliferate within the network
literature, researchers would benefit from some guidelines about which of the
so-called community detection algorithms are most appropriate for the
structures they are studying and the questions they are asking. Here we show
that different methods highlight different aspects of a network's structure and
that the the sort of information that we seek to extract about the system must
guide us in our decision. For example, many community detection algorithms,
including the popular modularity maximization approach, infer module
assignments from an underlying model of the network formation process. However,
we are not always as interested in how a system's network structure was formed,
as we are in how a network's extant structure influences the system's behavior.
To see how structure influences current behavior, we will recognize that links
in a network induce movement across the network and result in system-wide
interdependence. In doing so, we explicitly acknowledge that most networks
carry flow. To highlight and simplify the network structure with respect to
this flow, we use the map equation. We present an intuitive derivation of this
flow-based and information-theoretic method and provide an interactive on-line
application that anyone can use to explore the mechanics of the map equation.
We also describe an algorithm and provide source code to efficiently decompose
large weighted and directed networks based on the map equation.Comment: 9 pages and 3 figures, corrected typos. For associated Flash
application, see http://www.tp.umu.se/~rosvall/livemod/mapequation
Quantitative ultraviolet measurements on wetted thin-layer chromatography plates using a charge-coupled device camera
This paper presents the first study of the UV imaging of spots on thin-layer chromatographic plates whilst still wet with solvent. Imaging of spots of benzophenone during and after development was carried out using a charge-coupled device camera. Limits of detection were found to be 5 ng on a wetted plate and 3 ng for a dry plate and the relationship between peak area and sample loading was found to be linear in the low nanograrn range over an order of magnitude for both wet and dry modes with r(2) values > 0.99. It was found that UV measurements on wet glass-backed plates suffer from low sensitivity; however, the use of aluminium-backed plates gave increased sensitivity. The apparent absorption coefficient (epsilon(app)) of 10AU m(2) g(-1) at 254 nm is consistent with reflection of the light from the aluminium surface with a double pass through the sorbent layer, and suggests that use of aluminium-backed plates should enable monitoring of separations by UV absorbance during TLC development. (c) 2008 Elsevier B.V. All rights reserved
Long life stau in the minimal supersymmetric standard model
We study the stau lifetime in a scenario with the LSP taken to be a
neutralino and the NLSP being a stau, based on the minimal supersymmetric
Standard Model. The mass difference between the LSP and NLSP, , must
satisfy a few % or less for coannihilation to
occur, where is the neutralino mass. We calculate the stau
lifetime from the decay modes ,
, and and
discuss its dependence on various parameters. We find that the lifetime is in
the range -- sec for GeV. We
also discuss the connection with lepton flavor violation if there is mixing
between sleptons.Comment: 15 pages, 5 figure
Implications of a New Solar System Population of Neutralinos on Indirect Detection Rates
Recently, a new Solar System population of weakly interacting massive
particle (WIMP) dark matter has been proposed to exist. We investigate the
implications of this population on indirect signals in neutrino telescopes (due
to WIMP annihilations in the Earth) for the case when the WIMP is the lightest
neutralino of the MSSM, the minimal supersymmetric extension of the standard
model. The velocity distribution and capture rate of this new population is
evaluated and the flux of neutrino-induced muons from the center of the Earth
in neutrino telescopes is calculated. The strength of the signal is very
sensitive to the velocity distribution of the new population. We analytically
estimate this distribution using the approximate conservation of the component
of the WIMP angular momentum orthogonal to the ecliptic plane. The non-linear
problem of combining a fixed capture rate from the standard galactic WIMP
population with one rising linearly with time from the new population to obtain
the present-day annihilation rate in the Earth is also solved analytically. We
show that the effects of the new population can be crucial for masses below
around 150 GeV, where enhancements of the predicted muon flux from the center
of the Earth by up to a factor of 100 compared to previously published
estimates occur. As a result of the new WIMP population, the next generation of
neutrino telescopes should be able to probe a much larger region of parameter
space in the mass range 60-130 GeV.Comment: 21 pages, 5 eps figures, uses JHEP.cls. Figures made more readable,
references updated. Matches published versio
Introducing "Surveylady": A Case for the Use of Avatars as Part of Gaming Research
The popularity explosion of Massively Multiplayer Online Role-Playing Games (MMOs) such as World of Warcraft provides researchers with a venue to reach a wider research subject base than ever before. But what is the best way to collect data about these virtual worlds? This paper illustrates the rich potential of using an avatar to interact with MMO participants while players are immersed in the game’s virtual environment. Rather than observing from the periphery, this paper makes the case for the researcher to ‘dive right in’ and interview gamers within their (virtual) environment. This paper will argue that this methodology acts as a means for collecting rich, nuanced data about the gaming community
A new population of WIMPs in the solar system and indirect detection rates
A new Solar System population of Weakly Interacting Massive Particle (WIMP)
dark matter has been proposed to exist. We investigate the implications of this
population on indirect signals in neutrino telescopes (due to WIMP
annihilations in the Earth) for the case when the WIMP is the lightest
neutralino of the MSSM, the minimal supersymmetric extension of the standard
model. The velocity distribution and capture rate of this new population is
evaluated and the flux of neutrino-induced muons from the center of the Earth
in neutrino telescopes is calculated. We show that the effects of the new
population can be crucial for masses around 60-120 GeV, where enhancements of
the predicted muon flux from the center of the Earth by up to a factor of 100
compared to previously published estimates occur. As a result of the new WIMP
population, neutrino telescopes should be able to probe a much larger region of
parameter space in this mass range.Comment: 6 pages, 4 figures. Contribution to the proceedings of the 3rd
International Workshop on the Identification of Dark Matter (IDM2000) in
York, in pres
QED radiative corrections to the decay pi^0 to e^+e^-
We reconsider QED radiative corrections (RC) to the
decay width. One kind of RC investigated earlier has a renormalization group
origin and can be associated with the final state interaction of electron and
positron. It determines the distribution of lepton pair invariant masses in the
whole kinematic region. The other type of RC has a double-logarithmic character
and is related to almost on-mass-shell behavior of the lepton form factors. The
total effect of RC for the decay is estimated to be
3.2% and for the decay is 4.3%.Comment: 12 pages, 3 figure
Maps of random walks on complex networks reveal community structure
To comprehend the multipartite organization of large-scale biological and
social systems, we introduce a new information theoretic approach that reveals
community structure in weighted and directed networks. The method decomposes a
network into modules by optimally compressing a description of information
flows on the network. The result is a map that both simplifies and highlights
the regularities in the structure and their relationships. We illustrate the
method by making a map of scientific communication as captured in the citation
patterns of more than 6000 journals. We discover a multicentric organization
with fields that vary dramatically in size and degree of integration into the
network of science. Along the backbone of the network -- including physics,
chemistry, molecular biology, and medicine -- information flows
bidirectionally, but the map reveals a directional pattern of citation from the
applied fields to the basic sciences.Comment: 7 pages and 4 figures plus supporting material. For associated source
code, see http://www.tp.umu.se/~rosvall
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