99,786 research outputs found
Langevin Equation for the Density of a System of Interacting Langevin Processes
We present a simple derivation of the stochastic equation obeyed by the
density function for a system of Langevin processes interacting via a pairwise
potential. The resulting equation is considerably different from the
phenomenological equations usually used to describe the dynamics of non
conserved (Model A) and conserved (Model B) particle systems. The major feature
is that the spatial white noise for this system appears not additively but
multiplicatively. This simply expresses the fact that the density cannot
fluctuate in regions devoid of particles. The steady state for the density
function may however still be recovered formally as a functional integral over
the coursed grained free energy of the system as in Models A and B.Comment: 6 pages, latex, no figure
Dramatic robustness of a multiple delay dispersed interferometer to spectrograph errors: how mixing delays reduces or cancels wavelength drift
We describe demonstrations of remarkable robustness to instrumental noises by using a multiple delay externally dispersed interferometer (EDI) on stellar observations at the Hale telescope. Previous observatory EDI demonstrations used a single delay. The EDI (also called “TEDI”) boosted the 2,700 resolution of the native TripleSpec NIR spectrograph (950-2450 nm) by as much as 10x to 27,000, using 7 overlapping delays up to 3 cm. We observed superb rejection of fixed pattern noises due to bad pixels, since the fringing signal responds only to changes in multiple exposures synchronous to the applied delay dithering. Remarkably, we observed a ~20x reduction of reaction in the output spectrum to PSF shifts of the native spectrograph along the dispersion direction, using our standard processing. This allowed high resolution observations under conditions of severe and irregular PSF drift otherwise not possible without the interferometer. Furthermore, we recently discovered an improved method of weighting and mixing data between pairs of delays that can theoretically further reduce the net reaction to PSF drift to zero. We demonstrate a 350x reduction in reaction to a native PSF shift using a simple simulation. This technique could similarly reduce radial velocity noise for future EDI’s that use two delays overlapped in delay space (or a single delay overlapping the native peak). Finally, we show an extremely high dynamic range EDI measurement of our ThAr lamp compared to a literature ThAr spectrum, observing weak features (~0.001x height of nearest strong line) that occur between the major lines. Because of individuality of each reference lamp, accurate knowledge of its spectrum between the (unfortunately) sparse major lines is important for precision radial velocimetry
Spatially Resolved Stellar Populations of Eight GOODS-South Active Galactic Nuclei at z ~ 1
We present a pilot study of the stellar populations of eight active galactic nucleus (AGN) hosts at z ~ 1 and compare with (1) lower redshift samples and (2) a sample of nonactive galaxies of similar redshift. We utilize K' images in the Great Observatories Origins Deep Survey South field obtained with the laser guide star adaptive optics system at Keck Observatory. We combine these K' data with B, V, i, and z imaging from the Advanced Camera for Surveys on Hubble Space Telescope to give multicolor photometry at a matched spatial resolution better than 100 mas in all bands. The hosts harbor AGNs as inferred from their high X-ray luminosities (LX > 10^42 erg s^–1) or mid-IR colors. We find a correlation between the presence of younger stellar populations and the strength of the AGN, as measured with [O III] line luminosity or X-ray (2-10 keV) luminosity. This finding is consistent with similar studies at lower redshift. Of the three Type II galaxies, two are disk galaxies and one is of irregular type, while in the Type I sample there are only one disk-like source and four sources with smooth, elliptical/spheroidal morphologies. In addition, the mid-IR spectral energy distributions of the strong Type II AGNs indicate that they are excited to Luminous InfraRed Galaxy (LIRG) status via galactic starbursting, while the strong Type I AGNs are excited to LIRG status via hot dust surrounding the central AGN. This supports the notion that the obscured nature of Type II AGNs at z ~ 1 is connected with global starbursting and that they may be extincted by kpc-scale dusty features that are by-products of this starbursting
Aspects of emergent geometry in the AdS/CFT context
We study aspects of emergent geometry for the case of orbifold superconformal
field theories in four dimensions, where the orbifolds are abelian within the
AdS/CFT proposal. In particular, we show that the realization of emergent
geometry starting from the N=4 SYM theory in terms of a gas of particles in the
moduli space of vacua of a single D3 brane in flat space gets generalized to a
gas of particles on the moduli space of the corresponding orbifold conformal
field theory (a gas of D3 branes on the orbifold space). Our main purpose is to
show that this can be analyzed using the same techniques as in the N=4 SYM case
by using the method of images, including the measure effects associated to the
volume of the gauge orbit of the configurations. This measure effect gives an
effective repulsion between the particles that makes them condense into a
non-trivial vacuum configuration, and it is exactly these configurations that
lead to the geometry of X in the AdS x X dual field theoryComment: 24 page
The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
A model is presented in the philosophy of the "String Axiverse" of Arvanitaki
et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight
axions to their corresponding moduli through the mass term. The light fields
roll in their potentials at late times and contribute to the dark sector energy
densities in the cosmological expansion. The addition of a coupling and extra
field greatly enrich the possible phenomenology of the axiverse. There are a
number of interesting phases where the axion and modulus components behave as
Dark Matter or Dark Energy and can have considerable and distinct effects on
the expansion history of the universe by modifying the equation of state in the
past or causing possible future collapse of the universe. In future such a
coupling may help to alleviate fine tuning problems for cosmological axions. We
motivate and present the model, and briefly explore its cosmological
consequences numerically.Comment: 13 pages, 17 figures, published in PRD. v3: corrected SUSY
interpretation of axion potential scal
Calculation of the heavy-hadron axial couplings g_1, g_2, and g_3 using lattice QCD
In a recent letter [Phys. Rev. Lett. 108, 172003 (2012), arXiv:1109.2480] we
have reported on a lattice QCD calculation of the heavy-hadron axial couplings
, , and . These quantities are low-energy constants of
heavy-hadron chiral perturbation theory (HHPT) and are related to the
, , and couplings.
In the following, we discuss important details of the calculation and give
further results. To determine the axial couplings, we explicitly match the
matrix elements of the axial current in QCD with the corresponding matrix
elements in HHPT. We construct the ratios of correlation functions used
to calculate the matrix elements in lattice QCD, and study the contributions
from excited states. We present the complete numerical results and discuss the
data analysis in depth. In particular, we demonstrate the convergence of
HHPT for the axial-current matrix elements at pion masses up to
about 400 MeV and show the impact of the nonanalytic loop contributions.
Finally, we present additional predictions for strong and radiative decay
widths of charm and bottom baryons.Comment: 42 pages, 20 figures, updated calculation of Xi_b^{*0} width using
mass measurement from CMS, published versio
Quantizing Open Spin Chains with Variable Length: an example from Giant Gravitons
We study an XXX open spin chain with variable number of sites, where the
variability is introduced only at the boundaries. This model arises naturally
in the study of Giant Gravitons in the AdS/CFT correspondence. We show how to
quantize the spin chain by mapping its states to a bosonic lattice of finite
length with sources and sinks of particles at the boundaries. Using coherent
states, we show how the Hamiltonian for the bosonic lattice gives the correct
description of semiclassical open strings ending on Giant Gravitons.Comment: 4 pages. v2: updated reference
Functional imaging reveals working memory and attention interact to produce the attentional blink
Copyright @ 2012 Massachusetts Institute of Technology PressIf two centrally presented visual stimuli occur within approximately half a second of each other, the second target often fails to be reported correctly. This effect, called the attentional blink (AB; Raymond, J. E., Shapiro, K. L., & Arnell, K. M. Temporary suppression of visual processing in an RSVP task: An attentional blink? Journal of Experimental Psychology, Human Perception and Performance, 18, 849-860, 1992], has been attributed to a resource "bottleneck," likely arising as a failure of attention during encoding into or retrieval from visual working memory (WM). Here we present participants with a hybrid WM-AB study while they undergo fMRI to provide insight into the neural underpinnings of this bottleneck. Consistent with a WM-based bottleneck account, fronto-parietal brain areas exhibited a WM load-dependent modulation of neural responses during the AB task. These results are consistent with the view that WM and attention share a capacity-limited resource and provide insight into the neural structures that underlie resource allocation in tasks requiring joint use of WM and attention.This research was supported by a project grant (071944) from the Wellcome Trust to Kimron Shapiro
Gravitational waves from an early matter era
We investigate the generation of gravitational waves due to the gravitational
instability of primordial density perturbations in an early matter-dominated
era which could be detectable by experiments such as LIGO and LISA. We use
relativistic perturbation theory to give analytic estimates of the tensor
perturbations generated at second order by linear density perturbations. We
find that large enhancement factors with respect to the naive second-order
estimate are possible due to the growth of density perturbations on sub-Hubble
scales. However very large enhancement factors coincide with a breakdown of
linear theory for density perturbations on small scales. To produce a
primordial gravitational wave background that would be detectable with LIGO or
LISA from density perturbations in the linear regime requires primordial
comoving curvature perturbations on small scales of order 0.02 for Advanced
LIGO or 0.005 for LISA, otherwise numerical calculations of the non-linear
evolution on sub-Hubble scales are required.Comment: 23 pages, 2 figure
Policies drain the North China Plain: Agricultural policy and groundwater depletion in Luancheng County, 1949-2000
Agricultural production / Groundwater / Aquifers / Water shortage / Irigation efficiency / Agricultural policy / Crop production / Wastewaters / Water management / Hydrology / Economic development / Crop yield / Cotton / Wheat / Sprinkler irrigation / Water conservation / Water use efficiency / Pumping / Water balance / Vegetables / Rural economy / Irrigated framing
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