5,360 research outputs found
Measurement of Dielectric Suppression of Bremsstrahlung
In 1953, Ter-Mikaelian predicted that the bremsstrahlung of low energy
photons in a medium is suppressed because of interactions between the produced
photon and the electrons in the medium. This suppression occurs because the
emission takes place over on a long distance scale, allowing for destructive
interference between different instantaneous photon emission amplitudes. We
present here measurements of bremsstrahlung cross sections of 200 keV to 20 MeV
photons produced by 8 and 25 GeV electrons in carbon and gold targets. Our data
shows that dielectric suppression occurs at the predicted level, reducing the
cross section up to 75 percent in our data.Comment: 11 pages, format is postscript file, gzip-ed, uuencode-e
Large Deviations of the Maximum Eigenvalue for Wishart and Gaussian Random Matrices
We present a simple Coulomb gas method to calculate analytically the
probability of rare events where the maximum eigenvalue of a random matrix is
much larger than its typical value. The large deviation function that
characterizes this probability is computed explicitly for Wishart and Gaussian
ensembles. The method is quite general and applies to other related problems,
e.g. the joint large deviation function for large fluctuations of top
eigenvalues. Our results are relevant to widely employed data compression
techniques, namely the principal components analysis. Analytical predictions
are verified by extensive numerical simulations.Comment: 4 pages, 3 .eps figures include
Bremsstrahlung Suppression due to the LPM and Dielectric Effects in a Variety of Materials
The cross section for bremsstrahlung from highly relativistic particles is
suppressed due to interference caused by multiple scattering in dense media,
and due to photon interactions with the electrons in all materials. We present
here a detailed study of bremsstrahlung production of 200 keV to 500 MeV
photons from 8 and 25 GeV electrons traversing a variety of target materials.
For most targets, we observe the expected suppressions to a good accuracy. We
observe that finite thickness effects are important for thin targets.Comment: 52 pages, 13 figures (incorporated in the revtex LaTeX file
Inferring Human Colonization History Using a Copying Model
Genome-wide scans of genetic variation can potentially provide detailed information on how modern humans colonized the world but require new methods of analysis. We introduce a statistical approach that uses Single Nucleotide Polymorphism (SNP) data to identify sharing of chromosomal segments between populations and uses the pattern of sharing to reconstruct a detailed colonization scenario. We apply our model to the SNP data for the 53 populations of the Human Genome Diversity Project described in Conrad et al. (Nature Genetics 38,1251-60, 2006). Our results are consistent with the consensus view of a single âOut-of-Africaâ bottleneck and serial dilution of diversity during global colonization, including a prominent East Asian bottleneck. They also suggest novel details including: (1) the most northerly East Asian population in the sample (Yakut) has received a significant genetic contribution from the ancestors of the most northerly European one (Orcadian). (2) Native South Americans have received ancestry from a source closely related to modern North-East Asians (Mongolians and Oroquen) that is distinct from the sources for native North Americans, implying multiple waves of migration into the Americas. A detailed depiction of the peopling of the world is available in animated form
On the volume of the set of mixed entangled states II
The problem of of how many entangled or, respectively, separable states there
are in the set of all quantum states is investigated. We study to what extent
the choice of a measure in the space of density matrices describing
N--dimensional quantum systems affects the results obtained. We demonstrate
that the link between the purity of the mixed states and the probability of
entanglement is not sensitive to the measure chosen. Since the criterion of
partial transposition is not sufficient to distinguish all separable states for
N > 6, we develop an efficient algorithm to calculate numerically the
entanglement of formation of a given mixed quantum state, which allows us to
compute the volume of separable states for N=8 and to estimate the volume of
the bound entangled states in this case.Comment: 14 pages in Latex, Revtex + epsf; 7 figures in .ps included (one new
figure in the revised version, several minor changes
How humans transmit language:horizontal transmission matches word frequencies among peers on Twitter
Network Connectivity Dynamics, Cognitive Biases, and the Evolution of Cultural Diversity in RoundâRobin Interactive MicroâSocieties
Crossing scales, crossing disciplines: collective motion and collective action in the Global Commonsâ
Two conflicting tendencies can be seen throughout the biological world: individuality and collective behaviour. Natural selection operates on differences among individuals, rewarding those who perform better. Nonetheless, even within this milieu, cooperation arises, and the repeated emergence of multicellularity is the most striking example. The same tendencies are played out at higher levels, as individuals cooperate in groups, which compete with other such groups. Many of our environmental and other global problems can be traced to such conflicts, and to the unwillingness of individual agents to take account of the greater good. One of the great challenges in achieving sustainability will be in understanding the basis of cooperation, and in taking multicellularity to yet a higher level, finding the pathways to the level of cooperation that is the only hope for the preservation of the planet
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