72,656 research outputs found
Using survey participants to estimate the impact of nonparticipation
The authors evaluate the effectiveness of two models often used to measure the extent of nonparticipation bias in survey estimates. The first model establishes a "continuum of resistance" to being surveyed, placing people who were interviewed after one phone call on one end and nonparticipants on the other. The second assumes that there are "classes" of nonparticipants and that similar classes can be found among participants; it identifies groups of participants thought to be like nonparticipants and uses them as "proxies" to estimate the characteristics of nonparticipants. The authors use these models to examine how accurately they estimate the characteristics of nonparticipants and the impact of nonparticipation on survey estimates of means of child support awards and payments in Wisconsin. They find that neither model detects the true extent of nonparticipation bias.
Mass Spectrum and Bounds on the Couplings in Yukawa Models With Mirror-Fermions
The symmetric Yukawa model with mirror-fermions
in the limit where the mirror-fermion is decoupled is studied both analytically
and numerically. The bare scalar self-coupling is fixed at zero and
infinity. The phase structure is explored and the relevant phase transition is
found to be consistent with a second order one. The fermionic mass spectrum
close to that transition is discussed and a first non-perturbative estimate of
the influence of fermions on the upper and lower bounds on the renormalized
scalar self-coupling is given. Numerical results are confronted with
perturbative predictions.Comment: 7 (Latex) page
Quantitative rescattering theory for laser-induced high-energy plateau photoelectron spectra
A comprehensive quantitative rescattering (QRS) theory for describing the
production of high-energy photoelectrons generated by intense laser pulses is
presented. According to the QRS, the momentum distributions of these electrons
can be expressed as the product of a returning electron wave packet with the
elastic differential cross sections (DCS) between free electrons with the
target ion. We show that the returning electron wave packets are determined
mostly by the lasers only, and can be obtained from the strong field
approximation. The validity of the QRS model is carefully examined by checking
against accurate results from the solution of the time-dependent Schr\"odinger
equation for atomic targets within the single active electron approximation. We
further show that experimental photoelectron spectra for a wide range of laser
intensity and wavelength can be explained by the QRS theory, and that the DCS
between electrons and target ions can be extracted from experimental
photoelectron spectra. By generalizing the QRS theory to molecular targets, we
discuss how few-cycle infrared lasers offer a promising tool for dynamic
chemical imaging with temporal resolution of a few femtoseconds.Comment: 19 pages, 19 figure
An alternative model for the origin of gaps in circumstellar disks
Motivated by recent observational and numerical studies suggesting that
collapsing protostellar cores may be replenished from the local environment, we
explore the evolution of protostellar cores submerged in the external
counter-rotating environment. These models predict the formation of
counter-rotating disks with a deep gap in the gas surface density separating
the inner disk (corotating with the star) and the outer counter-rotating disk.
The properties of these gaps are compared to those of planet-bearing gaps that
form in disks hosting giant planets. We employ numerical hydrodynamics
simulations of collapsing cores that are replenished from the local
counter-rotating environment, as well as numerical hydrodynamic simulations of
isolated disks hosting giant planets, to derive the properties of the gaps that
form in both cases. Our numerical simulations demonstrate that counter-rotating
disks can form for a wide range of mass and angular momentum available in the
local environment. The gap that separates both disks has a depletion factor
smaller than 1%, can be located at a distance from ten to over a hundred AU
from the star, and can propagate inward with velocity ranging from 1 AU/Myr to
>100 AU/Myr. Unlike our previous conclusion, the gap can therefore be a
long-lived phenomenon, comparable in some cases to the lifetime of the disk
itself. For a proper choice of the planetary mass, the viscous \alpha-parameter
and the disk mass, the planet-bearing gaps and the gaps in counter-rotating
disks may show a remarkable similarity in the gas density profile and depletion
factor, which may complicate their observational differentiation.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
The Effects of Driving Restrictions on Air Quality: SĂŁo Paulo, BogotĂĄ, Beijing, and Tianjin
In a typical driving restriction, vehicle use is restricted based on the vehicleâs license plate; one cannot drive vehicles with certain license plate numbers on certain days. Driving restrictions have been used as a method to reduce urban air pollution or traffic congestion because they are easy and inexpensive to implement. We investigate whether driving restrictions introduced in SĂŁo Paulo, BogotĂĄ, Beijing and Tianjin have improved air quality. Across different versions of the driving restrictions there is no evidence that the overall air quality at different places has been improved. However, several important results show up in this extensive analysis. Temporal shifting of driving is likely to appear when the restrictions are only effective during certain hours of weekdays. Driving restrictions could potentially reduce the extreme concentrations of air pollutants. Driving restrictions can only be expected to alleviate air pollution when implemented with an extended schedule or in an extended region. The effects of the driving restrictions are primarily on the concentrations of CO and PM10.driving restriction, air quality, Environmental Economics and Policy,
Vacuum polarization for neutral particles in 2+1 dimensions
In 2+1 dimensions there exists a duality between a charged Dirac particle
coupled minimally to a background vector potential and a neutral one coupled
nonminimally to a background electromagnetic field strength. A constant uniform
background electric current induces in the vacuum of the neutral particle a
fermion current which is proportional to the background one. A background
electromagnetic plane wave induces no current in the vacuum. For constant but
nonuniform background electric charge, known results for charged particles can
be translated to give the induced fermion number. Some new examples with
infinite background electric charge are presented. The induced spin and total
angular momentum are also discussed.Comment: REVTeX, 7 pages, no figur
Toward a Deterministic Model of Planetary Formation IV: Effects of Type-I Migration
In a further development of a deterministic planet-formation model (Ida & Lin
2004), we consider the effect of type-I migration of protoplanetary embryos due
to their tidal interaction with their nascent disks. During the early embedded
phase of protostellar disks, although embryos rapidly emerge in regions
interior to the ice line, uninhibited type-I migration leads to their efficient
self-clearing. But, embryos continue to form from residual planetesimals at
increasingly large radii, repeatedly migrate inward, and provide a main channel
of heavy element accretion onto their host stars. During the advanced stages of
disk evolution (a few Myr), the gas surface density declines to values
comparable to or smaller than that of the minimum mass nebula model and type-I
migration is no longer an effective disruption mechanism for mars-mass embryos.
Over wide ranges of initial disk surface densities and type-I migration
efficiency, the surviving population of embryos interior to the ice line has a
total mass several times that of the Earth. With this reservoir, there is an
adequate inventory of residual embryos to subsequently assemble into rocky
planets similar to those around the Sun. But, the onset of efficient gas
accretion requires the emergence and retention of cores, more massive than a
few M_earth, prior to the severe depletion of the disk gas. The formation
probability of gas giant planets and hence the predicted mass and semimajor
axis distributions of extrasolar gas giants are sensitively determined by the
strength of type-I migration. We suggest that the observed fraction of
solar-type stars with gas giant planets can be reproduced only if the actual
type-I migration time scale is an order of magnitude longer than that deduced
from linear theories.Comment: 32 pages, 8 figures, 1 table, accepted for publication in Ap
Exact Scaling Functions for Self-Avoiding Loops and Branched Polymers
It is shown that a recently conjectured form for the critical scaling
function for planar self-avoiding polygons weighted by their perimeter and area
also follows from an exact renormalization group flow into the branched polymer
problem, combined with the dimensional reduction arguments of Parisi and
Sourlas. The result is generalized to higher-order multicritical points,
yielding exact values for all their critical exponents and exact forms for the
associated scaling functions.Comment: 5 pages; v2: factors of 2 corrected; v.3: relation with existing
theta-point results clarified, some references added/update
Configuration-Space Location of the Entanglement between Two Subsystems
In this paper we address the question: where in configuration space is the
entanglement between two particles located? We present a thought-experiment,
equally applicable to discrete or continuous-variable systems, in which one or
both parties makes a preliminary measurement of the state with only enough
resolution to determine whether or not the particle resides in a chosen region,
before attempting to make use of the entanglement. We argue that this provides
an operational answer to the question of how much entanglement was originally
located within the chosen region. We illustrate the approach in a spin system,
and also in a pair of coupled harmonic oscillators. Our approach is
particularly simple to implement for pure states, since in this case the
sub-ensemble in which the system is definitely located in the restricted region
after the measurement is also pure, and hence its entanglement can be simply
characterised by the entropy of the reduced density operators. For our spin
example we present results showing how the entanglement varies as a function of
the parameters of the initial state; for the continuous case, we find also how
it depends on the location and size of the chosen regions. Hence we show that
the distribution of entanglement is very different from the distribution of the
classical correlations.Comment: RevTex, 12 pages, 9 figures (28 files). Modifications in response to
journal referee
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