8,187 research outputs found
Two-particle irreducible effective actions versus resummation: analytic properties and self-consistency
Approximations based on two-particle irreducible (2PI) effective actions
(also known as -derivable, Cornwall-Jackiw-Tomboulis or Luttinger-Ward
functionals depending on context) have been widely used in condensed matter and
non-equilibrium quantum/statistical field theory because this formalism gives a
robust, self-consistent, non-perturbative and systematically improvable
approach which avoids problems with secular time evolution. The strengths of
2PI approximations are often described in terms of a selective resummation of
Feynman diagrams to infinite order. However, the Feynman diagram series is
asymptotic and summation is at best a dangerous procedure. Here we show that,
at least in the context of a toy model where exact results are available, the
true strength of 2PI approximations derives from their self-consistency rather
than any resummation. This self-consistency allows truncated 2PI approximations
to capture the branch points of physical amplitudes where adjustments of
coupling constants can trigger an instability of the vacuum. This, in effect,
turns Dyson's argument for the failure of perturbation theory on its head. As a
result we find that 2PI approximations perform better than Pad\'e approximation
and are competitive with Borel-Pad\'e resummation. Finally, we introduce a
hybrid 2PI-Pad\'e method.Comment: Version accepted for publication in Nuclear Physics B. 31 pages, 16
figures. Uses feynm
The use of some Gaelic songs and poetry in <i>The Cheviot, the Stag and the Black Black Oil</i>
No abstract available
The bimodal color distribution of small Kuiper Belt objects
We conducted a two-night photometric survey of small Kuiper Belt objects
(KBOs) near opposition using the wide-field Hyper Suprime-Cam instrument on the
8.2 m Subaru Telescope. The survey covered about 90 deg^2 of sky, with each
field imaged in the g and i bands. We detected 356 KBOs, ranging in absolute
magnitude from 6.5 to 10.4. Filtering for high-inclination objects within the
hot KBO population, we show that the g-i color distribution is strongly
bimodal, indicative of two color classes - the red and very red subpopulations.
After categorizing objects into the two subpopulations by color, we present the
first dedicated analysis of the magnitude distributions of the individual color
subpopulations and demonstrate that the two distributions are roughly identical
in shape throughout the entire size range covered by our survey. Comparing the
color distribution of small hot KBOs with that of Centaurs, we find that they
have similar bimodal shapes, thereby providing strong confirmation of previous
explanations for the attested bimodality of Centaurs. We also show that the
magnitude distributions of the two KBO color subpopulations and the two color
subpopulations observed in the Jupiter Trojans are statistically
indistinguishable. Finally, we discuss a hypothesis describing the origin of
the KBO color bimodality based on our survey results.Comment: 9 pages, 6 figures, accepted for publication in AJ, supplemental
table accessible on online journa
Multiband photometry of a Patroclus-Menoetius mutual event: Constraints on surface heterogeneity
We present the first complete multiband observations of a binary asteroid
mutual event. We obtained high-cadence, high-signal-to-noise photometry of the
UT 2018 April 9 inferior shadowing event in the Jupiter Trojan binary system
Patroclus-Menoetius in four Sloan bands , , , and . We use
an eclipse lightcurve model to fit for a precise mid-eclipse time and estimate
the minimum separation of the two eclipsing components during the event. Our
best-fit mid-eclipse time of is 19
minutes later than the prediction of Grundy et al. (2018); the minimum
separation between the center of Menoetius' shadow and the center of Patroclus
is km slightly larger than the predicted 69.5 km. Using the
derived lightcurves, we find no evidence for significant albedo variations or
large-scale topographic features on the Earth-facing hemisphere and limb of
Patroclus. We also apply the technique of eclipse mapping to place an upper
bound of 0.15 mag on wide-scale surface color variability across
Patroclus.Comment: 5 pages, 3 figures, accepted for publication in A
Groups of type via graphical small cancellation
We construct an uncountable family of groups of type . In contrast to
every previous construction of non-finitely presented groups of type we do
not use Morse theory on cubical complexes; instead we use Gromov's graphical
small cancellation theory.Comment: 3 figures. Second version: two paragraphs added emphasizing the
difference between our construction and Morse theoretic one
A hypothesis for the color bimodality of Jupiter Trojans
One of the most enigmatic and hitherto unexplained properties of Jupiter
Trojans is their bimodal color distribution. This bimodality is indicative of
two sub-populations within the Trojans, which have distinct size distributions.
In this paper, we present a simple, plausible hypothesis for the origin and
evolution of the two Trojan color sub-populations. In the framework of
dynamical instability models of early Solar System evolution, which suggest a
common primordial progenitor population for both Trojans and Kuiper belt
objects, we use observational constraints to assert that the color bimodalities
evident in both minor body populations developed within the primordial
population prior to the onset of instability. We show that, beginning with an
initial composition of rock and ices, location-dependent volatile loss through
sublimation in this primordial population could have led to sharp changes in
the surface composition with heliocentric distance. We propose that the
depletion or retention of HS ice on the surface of these objects was the
key factor in creating an initial color bimodality. Objects that retained
HS on their surfaces developed characteristically redder colors upon
irradiation than those that did not. After the bodies from the primordial
population were scattered and emplaced into their current positions, they
preserved this primordial color bimodality to the present day. We explore
predictions of the volatile loss model - in particular, the effect of
collisions within the Trojan population on the size distributions of the two
sub-populations - and propose further experimental and observational tests of
our hypothesisComment: 8 pages, 3 figures, accepted for publication in A
SKA Engineering Change Proposal: Gridded Visibilities to Enable Precision Cosmology with Radio Weak Lensing
This document was submitted as supporting material to an Engineering Change
Proposal (ECP) for the Square Kilometre Array (SKA). This ECP requests gridded
visibilities as an extra imaging data product from the SKA, in order to enable
bespoke analysis techniques to measure source morphologies to the accuracy
necessary for precision cosmology with radio weak lensing. We also discuss the
properties of an SKA weak lensing data set and potential overlaps with other
cosmology science goals.Comment: Comments welcome. 4 pages, 3 figures. Progress can be tracked at the
SKA ECP register https://skaoffice.atlassian.net/wiki/display/EP/ECP+Registe
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