613 research outputs found
Position-space cuts for Wilson line correlators
We further develop the formalism for taking position-space cuts of eikonal
diagrams introduced in [Phys.Rev.Lett. 114 (2015), no. 18 181602,
arXiv:1410.5681]. These cuts are applied directly to the position-space
representation of any such diagram and compute its discontinuity to the leading
order in the dimensional regulator. We provide algorithms for computing the
position-space cuts and apply them to several two- and three-loop eikonal
diagrams, finding agreement with results previously obtained in the literature.
We discuss a non-trivial interplay between the cutting prescription and
non-Abelian exponentiation. We furthermore discuss the relation of the
imaginary part of the cusp anomalous dimension to the static interquark
potential.Comment: 39+18 pages, 16 figures; elaborated the discussion of the comparison
of numerical and analytic results for the three-gluon vertex diagram in the
caption of fig. 16; version to be published in JHE
Imaginary parts and discontinuities of Wilson line correlators
We introduce a notion of position-space cuts of eikonal diagrams, the set of
diagrams appearing in the perturbative expansion of the correlator of a set of
straight semi-infinite Wilson lines. The cuts are applied directly to the
position-space representation of any such diagram and compute its imaginary
part to the leading order in the dimensional regulator. Our cutting
prescription thus defines a position-space analog of the standard
momentum-space Cutkosky rules. Unlike momentum-space cuts which put internal
lines on shell, position-space cuts constrain a number of the gauge bosons
exchanged between the energetic partons to be lightlike, leading to a vanishing
and a non-vanishing imaginary part for space- and timelike kinematics,
respectively.Comment: 5 pages, 2 figures; minor changes; version published in PR
A systematic search for close supermassive black hole binaries in the Catalina Real-Time Transient Survey
Hierarchical assembly models predict a population of supermassive black hole
(SMBH) binaries. These are not resolvable by direct imaging but may be
detectable via periodic variability (or nanohertz frequency gravitational
waves). Following our detection of a 5.2 year periodic signal in the quasar PG
1302-102 (Graham et al. 2015), we present a novel analysis of the optical
variability of 243,500 known spectroscopically confirmed quasars using data
from the Catalina Real-time Transient Survey (CRTS) to look for close (< 0.1
pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least
1.5 cycles over a baseline of nine years, we find a sample of 111 candidate
objects. This is in conservative agreement with theoretical predictions from
models of binary SMBH populations. Simulated data sets, assuming stochastic
variability, also produce no equivalent candidates implying a low likelihood of
spurious detections. The periodicity seen is likely attributable to either jet
precession, warped accretion disks or periodic accretion associated with a
close SMBH binary system. We also consider how other SMBH binary candidates in
the literature appear in CRTS data and show that none of these are equivalent
to the identified objects. Finally, the distribution of objects found is
consistent with that expected from a gravitational wave-driven population. This
implies that circumbinary gas is present at small orbital radii and is being
perturbed by the black holes. None of the sources is expected to merge within
at least the next century. This study opens a new unique window to study a
population of close SMBH binaries that must exist according to our current
understanding of galaxy and SMBH evolution.Comment: 29 pages, 10 figures, accepted for publication in MNRAS - this
version contains extended table and figur
Sampling Bias Exaggerates a Textbook Example of a Trophic Cascade
Understanding trophic cascades in terrestrial wildlife communities is a major challenge because these systems are difficult to sample properly. We show how a tradition of non-random sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore [Canis lupus (wolf)] recovery is associated with a trophic cascade involving changes in herbivore [Cervus canadensis (elk)] behaviour and density that promote plant regeneration. Long-term data indicate a practice of sampling only the tallest young plants overestimated regeneration of overstory aspen (Populus tremuloides) by a factor of 4–7 compared to random sampling because it favoured plants taller than the preferred browsing height of elk and overlooked non-regenerating aspen stands. Random sampling described a trophic cascade, but it was weaker than the one that non-random sampling described. Our findings highlight the critical importance of basic sampling principles (e.g. randomisation) for achieving an accurate understanding of trophic cascades in terrestrial wildlife systems
Non-Random Sampling Measures the Occurrence but not the Strength of a Textbook Trophic Cascade
Although sampling the five tallest young aspen in a stand is useful for detecting the occurrence of any aspen recruitment, this technique overestimates the population response of aspen to wolf reintroduction. Our original conclusion that random sampling described a trophic cascade that was weaker than the one described by non-random sampling is unchanged
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Aspen age structure and stand conditions on elk winter range in the northern Yellowstone ecosystem
Aerial photographs and field sampling were used to compare aspen (Populus tremuloides) age structure and stand conditions on elk winter range in the northern Yellowstone ecosystem. The elk winter ranges studied were the northern range in Yellowstone National Park (YNP) and the Gallatin National Forest and the Sunlight/Crandall elk winter range in the Shoshone National Forest. I found significant differences when comparing aspen stands inside and outside of YNP borders. The aspen stands in the Gallatin and Sunlight/Crandall areas had a greater incidence of tall aspen suckers and stems in the 1-4, 5-9, and 10-19 cm DBH classes. Aspen stems within YNP had a significantly higher percentage of stems with high levels of bark damage (>66% of bark surface damaged on the lowest 3 m of stem) than aspen stems in stands in the Gallatin or Sunlight/Crandall. An aspen age structure was developed using 598 increment cores. The aspen age structure in YNP was significantly different than the age structures of the Gallatin and Sunlight/Crandall elk winter ranges. The Gallatin and Sunlight/Crandall areas did not have significant differences in their age structures. The greatest differences between YNP and the National Forest areas was in the younger age classes, measured as aspen stems originating between 1920-1989. Within YNP, I found that the aspen age structure, size class distribution, incidence of tall suckers, and the percentage of browsed suckers of the scree habitat type was significantly different than the xeric and mesic habitat types. Scree forms a "natural exclosure" where ungulate browsing is reduced. Aspen stands have successfully recruited new stems into their overstories in all habitat types from 1880-1989 on the Sunlight/Crandall elk winter range and the Gallatin's portion of the northern range. Within YNP, aspen stands successfully recruited new overstory stems between 1860-1929 in all habitat types. Since 1930, YNP aspen have successfully recruited overstory stems mostly in scree habitat type stands and other areas of reduced browsing pressure. I discussed several potential ecological factors impacting aspen overstory recruitment and conclude that changes in ungulate browsing patterns best explains the spatial and temporal pattern I observed
Improving Usability of Social and Behavioral Sciences’ Evidence: A Call to Action for a National Infrastructure Project for Mining Our Knowledge
Over the last century, the social and behavioral sciences have accumulated a vast storehouse of knowledge with the potential to transform society and all its constituents. Unfortunately, this knowledge has accumulated in a form (e.g., journal papers) and scale that makes it extremely difficult to search, categorize, analyze, and integrate across studies. In this commentary based on a National Science Foundation-funded workshop, we describe the social and behavioral sciences’ knowledge-management problem. We discuss the knowledge-scale problem and how we lack a common language, a common format to represent knowledge, a means to analyze and summarize in an automated way, and approaches to visualize knowledge at a large scale. We then describe that we need a collaborative research program between information systems, information science, and computer science (IICS) researchers and social and behavioral science (SBS) researchers to develop information system artifacts to address the problem that many scientific disciplines share but that the social and behavioral sciences have uniquely not addressed
A Coupled Experimental and Computational Approach to Quantify Deleterious Hemodynamics, Vascular Alterations, and Mechanisms of Long-Term Morbidity in Response to Aortic Coarctati
Introduction
Coarctation of the aorta (CoA) is associated with morbidity despite treatment. Although mechanisms remain elusive, abnormal hemodynamics and vascular biomechanics are implicated. We present a novel approach that facilitates quantification of coarctation-induced mechanical alterations and their impact on vascular structure and function, without genetic or confounding factors. Methods
Rabbits underwent thoracic CoA at 10 weeks of age (~ 9 human years) to induce a 20 mm Hg blood pressure (BP) gradient using permanent or dissolvable suture thereby replicating untreated and corrected CoA. Computational fluid dynamics (CFD) was performed using imaging and BP data at 32 weeks to quantify velocity, strain and wall shear stress (WSS) for comparison to vascular structure and function as revealed by histology and myograph results. Results
Systolic and mean BP was elevated in CoA compared to corrected and control rabbits leading to vascular thickening, disorganization and endothelial dysfunction proximally and distally. Corrected rabbits had less severe medial thickening, endothelial dysfunction, and stiffening limited to the proximal region despite 12 weeks of normal BP (~ 4 human years) after the suture dissolved. WSS was elevated distally for CoA rabbits, but reduced for corrected rabbits. Discussion These findings are consistent with alterations in humans. We are now poised to investigate mechanical contributions to mechanisms of morbidity in CoA using these methods
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