10,078 research outputs found
A season of saints: sermons for festivals and commemorations after Pentecost
Reviewed Book: Rossing, John P. A season of saints: sermons for festivals and commemorations after Pentecost. Lima, Ohio: CSS Pub, 1992
Infrared Renormalons and Finite Volume
We analyze the perturbative expansion of a condensate in the O(N) non-linear
sigma model for large N on a two dimensional finite lattice. On an infinite
volume this expansion is affected by an infrared renormalon. We extrapolate
this analysis to the case of the gluon condensate of Yang-Mills theory and
argue that infrared renormalons can be detected by performing perturbative
studies even on relatively small lattices.Comment: LaTeX file, 6 figures in postscrip
RENORMALONS FROM EIGHT LOOP EXPANSION OF THE GLUON CONDENSATE IN LATTICE GAUGE THEORY,
We use a numerical method to obtain the weak coupling perturbative
coefficients of local operators with lattice regularization. Such a method
allows us to extend the perturbative expansions obtained so far by analytical
Feynman diagrams calculations. In SU(3) lattice gauge theory in four dimensions
we compute the first eight coefficients of the expectation value of the Wilson
loop on the elementary plaquette which is related to the gluon condensate. The
computed eight coefficients grow with the order much faster than predicted by
the presence of the infrared renormalon associated to the dimension of the
gluon condensate. However the renormalon behaviour for large order is quite
well reproduced if one considers the expansion coefficients in a new coupling
related to the lattice coupling by large perturbative corrections. This is
expected since the lattice and continuum Lambda scales differ by almost two
orders of magnitude.Comment: 11 pages, LaTe
On The Difficulty of Computing Higher-Twist Corrections
We discuss the evaluation of power corrections to hard scattering and decay
processes for which an operator product expansion is applicable. The Wilson
coefficient of the leading-twist operator is the difference of two perturbative
series, each of which has a renormalon ambiguity of the same order as the power
corrections themselves, but which cancel in the difference. We stress the
necessity of calculating this coefficient function to sufficiently high orders
in perturbation theory so as to make the uncertainty of the same order or
smaller than the relevant power corrections. We investigate in some simple
examples whether this can be achieved. Our conclusion is that in most of the
theoretical calculations which include power corrections, the uncertainties are
at least comparable to the power corrections themselves, and that it will be a
very difficult task to improve the situation.Comment: 27 pages, uuencoded file containing latex source and axodraw.sty fil
Gravitational Wave Emission from the Single-Degenerate Channel of Type Ia Supernovae
The thermonuclear explosion of a C/O white dwarf as a Type Ia supernova (SN
Ia) generates a kinetic energy comparable to that released by a massive star
during a SN II event. Current observations and theoretical models have
established that SNe Ia are asymmetric, and therefore--like SNe II--potential
sources of gravitational wave (GW) radiation. We perform the first detailed
calculations of the GW emission for a SN Ia of any type within the
single-degenerate channel. The gravitationally-confined detonation (GCD)
mechanism predicts a strongly-polarized GW burst in the frequency band around 1
Hz. Third-generation spaceborne GW observatories currently in planning may be
able to detect this predicted signal from SNe Ia at distances up to 1 Mpc. If
observable, GWs may offer a direct probe into the first few seconds of the SNe
Ia detonation.Comment: 8 pages, 4 figures, Accepted by Physical Review Letter
Austenite Formation and Manganese Partitioning during Double Soaking of an Ultralow Carbon Medium-Manganese Steel
Double soaking (DS) is a thermal processing route intended to produce austeniteâmartensite microstructures in steels containing austenite-stabilizing additions and consists of intercritical annealing (primary soaking), followed by heating and brief isothermal holding at an increased temperature (secondary soaking), and quenching. Herein, experimental dilatometry during DS of a medium-manganese (Mn) steel with nominally 7 wt% Mn and an ultralow residual carbon concentration, in combination with phase-field simulations of austenite formation during secondary soaking, is presented. The feasibility of maintaining heterogeneous Mn distributions during DS is demonstrated and insight is provided on the effects of the secondary soaking temperature and prior Mn distribution on the ferrite-to-austenite phase transformation during the secondary soaking portion of the DS treatment
Infrared Lightcurves of Near Earth Objects
We present lightcurves and derive periods and amplitudes for a subset of 38
near earth objects (NEOs) observed at 4.5 microns with the IRAC camera on the
the Spitzer Space Telescope, many of them having no previously reported
rotation periods. This subset was chosen from about 1800 IRAC NEO observations
as having obvious periodicity and significant amplitude. For objects where the
period observed did not sample the full rotational period, we derived lower
limits to these parameters based on sinusoidal fits. Lightcurve durations
ranged from 42 to 544 minutes, with derived periods from 16 to 400 minutes. We
discuss the effects of lightcurve variations on the thermal modeling used to
derive diameters and albedos from Spitzer photometry. We find that both
diameters and albedos derived from the lightcurve maxima and minima agree with
our previously published results, even for extreme objects, showing the
conservative nature of the thermal model uncertainties. We also evaluate the
NEO rotation rates, sizes, and their cohesive strengths.Comment: 16 pages, 4 figures, 3 tables, to appear in the Astrophysical Journal
Supplement Serie
Brewster-angle measurements of sea-surface reflectance using a high resolution spectroradiometer
This paper describes the design, construction and testing of a ship-borne spectroradiometer based on an imaging spectrograph and cooled CCD array with a wavelength range of 350-800 nm and 4 nm spectral sampling. The instrument had a minimum spectral acquisition time of 0.1 s, but in practice data were collected over periods of 10 s to allow averaging of wave effects. It was mounted on a ship's superstructure so that it viewed the sea surface from a height of several metres at the Brewster angle (53 degrees) through a linear polarizing filter. Comparison of sea-leaving spectra acquired with the polarizer oriented horizontally and vertically enabled estimation of the spectral composition of sky light reflected directly from the sea surface. A semi-empirical correction procedure was devised for retrieving water-leaving radiance spectra from these measurements while minimizing the influence of reflected sky light. Sea trials indicated that reflectance spectra obtained by this method were consistent with the results of radiance transfer modelling of case 2 waters with similar concentrations of chlorophyll and coloured dissolved organic matter. Surface reflectance signatures measured at three locations containing blooms of different phytoplankton species were easily discriminated and the instrument was sufficiently sensitive to detect solar-stimulated fluorescence from surface chlorophyll concentrations down to 1 mg mâ3
Nonperturbative Effects from the Resummation of Perturbation Theory
Using the general argument in Borel resummation of perturbation theory that
links the divergent perturbation theory to the nonperturbative effect we argue
that the nonperturbative effect associated with the perturbation theory should
have a branch cut only along the positive real axis in the complex coupling
plane. The component in the weak coupling expansion of the nonperturbative
amplitude, which usually includes the leading term in the weak coupling
expansion, that gives rise to the branch cut can be calculated in principle
from the perturbation theory combined with some exactly calculable properties
of the nonperturbative effect. The realization of this mechanism is
demonstrated in the double well potential and the two-dimensional O(N)
nonlinear sigma model. In these models the leading term in weak coupling of the
nonperturbative effect can be obtained with good accuracy from the first terms
of the perturbation theory. Applying this mechanism to the infrared renormalon
induced nonperturbative effect in QCD, we suggest some of the QCD condensate
effects can be calculated in principle from the perturbation theory.Comment: 21 Pages, 1 Figure; To appear in Phys Rev
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