4,248 research outputs found
Preferential attachment with choice
We consider the degree distributions of preferential attachment random graph
models with choice similar to those considered in recent work by Malyshkin and
Paquette and Krapivsky and Redner. In these models a new vertex chooses
vertices according to a preferential rule and connects to the vertex in the
selection with the th highest degree. For meek choice, where , we show
that both double exponential decay of the degree distribution and
condensation-like behaviour are possible, and provide a criterion to
distinguish between them. For greedy choice, where , we confirm that the
degree distribution asympotically follows a power law with logarithmic
correction when and shows condensation-like behaviour when .Comment: 17 pages, 1 figure. Accepted for publication in Random Structures and
Algorithm
Impact of Resonance on Thermal Targets for Invisible Dark Photon Searches
Dark photons in the MeV to GeV mass range are important targets for
experimental searches. We consider the case where dark photons decay
invisibly to hidden dark matter through . For generic masses,
proposed accelerator searches are projected to probe the thermal target region
of parameter space, where the particles annihilate through in the early universe and freeze out with the correct relic density.
However, if , dark matter annihilation is resonantly
enhanced, shifting the thermal target region to weaker couplings. For degeneracies, we find that the annihilation cross section is generically
enhanced by four (two) orders of magnitude for scalar (pseudo-Dirac) dark
matter. For such moderate degeneracies, the thermal target region drops to weak
couplings beyond the reach of all proposed accelerator experiments in the
scalar case and becomes extremely challenging in the pseudo-Dirac case.
Proposed direct detection experiments can probe moderate degeneracies in the
scalar case. For greater degeneracies, the effect of the resonance can be even
more significant, and both scalar and pseudo-Dirac cases are beyond the reach
of all proposed accelerator and direct detection experiments. For scalar dark
matter, we find an absolute minimum that sets the ultimate experimental
sensitivity required to probe the entire thermal target parameter space, but
for pseudo-Dirac fermions, we find no such thermal target floor.Comment: 17 pages, 2 figures; v2: improved agreement with existing
non-resonant results, added extensive discussion of implications for direct
detection experiment
The spectra of the laplacians of fractal graphs not satisfying spectral decimation
We consider the spectra of the Laplacians of two sequences of fractal graphs in the context of the general theory introduced by Sabot in 2003. For the sequence of graphs associated with the pentagasket, we give a description of the eigenvalues in terms of the iteration of a map from (C-2)(3) to itself. For the sequence of graphs introduced in a previous paper by the author, we show that the results found therein can be related to Sabot's theory
Undermining and Strengthening Social Networks through Network Modification
Social networks have well documented effects at the individual and aggregate
level. Consequently it is often useful to understand how an attempt to
influence a network will change its structure and consequently achieve other
goals. We develop a framework for network modification that allows for
arbitrary objective functions, types of modification (e.g. edge weight
addition, edge weight removal, node removal, and covariate value change), and
recovery mechanisms (i.e. how a network responds to interventions). The
framework outlined in this paper helps both to situate the existing work on
network interventions but also opens up many new possibilities for intervening
in networks. In particular use two case studies to highlight the potential
impact of empirically calibrating the objective function and network recovery
mechanisms as well as showing how interventions beyond node removal can be
optimised. First, we simulate an optimal removal of nodes from the Noordin
terrorist network in order to reduce the expected number of attacks (based on
empirically predicting the terrorist collaboration network from multiple types
of network ties). Second, we simulate optimally strengthening ties within
entrepreneurial ecosystems in six developing countries. In both cases we
estimate ERGM models to simulate how a network will endogenously evolve after
intervention
Dark Photons from the Center of the Earth: Smoking-Gun Signals of Dark Matter
Dark matter may be charged under dark electromagnetism with a dark photon
that kinetically mixes with the Standard Model photon. In this framework, dark
matter will collect at the center of the Earth and annihilate into dark
photons, which may reach the surface of the Earth and decay into observable
particles. We determine the resulting signal rates, including Sommerfeld
enhancements, which play an important role in bringing the Earth's dark matter
population to their maximal, equilibrium value. For dark matter masses 100 GeV - 10 TeV, dark photon masses MeV - GeV, and kinetic
mixing parameters , the resulting
electrons, muons, photons, and hadrons that point back to the center of the
Earth are a smoking-gun signal of dark matter that may be detected by a variety
of experiments, including neutrino telescopes, such as IceCube, and space-based
cosmic ray detectors, such as Fermi-LAT and AMS. We determine the signal rates
and characteristics, and show that large and striking signals---such as
parallel muon tracks---are possible in regions of the
plane that are not probed by direct detection, accelerator experiments, or
astrophysical observations.Comment: 26 pages, 10 figures. v2: minor revisions to match published version;
v3: updated direct detection and CMB constraints and corrected decay length
in code, moving the region of experimental sensitivity to values of epsilon
that are lower by an order of magnitud
Solar Hydrogen Production by Solar Thermal Decoupled Electrolysis: Analysis of FE3O4 in Solution
The Valparaiso University Solar Research Project utilizes a solar thermal decoupled electrolysis process for the production of H2 from water. This study focuses on the electrochemical conversion of magnetite to hematite during the H2 production process. Laboratory experiments show that the expected amount of H2 gas forms at the cathode of the electrolytic cell, but we have encountered difficulties recovering the expected amount of solid hematite at the anode. An intensive study of the complex solution chemistry using cyclic voltammetry, Mossbauer spectroscopy, and solubility modeling showed that several dissolved iron species potentially exist in solution. Further work must be done to clarify which species are key participants in the overall electrochemical reaction. This information will help us to determine and implement reaction conditions that are favorable for precipitation of hematite or other oxidized iron species
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