7,964 research outputs found
Reconsidering the Work Disincentive Effects of Social Security
This paper shows that, contrary to commonly held views, the provisions of the social security law actually provide strong work incentives for older men. The reason is that, for most workers, higher current earnings lead to higher future social security benefits. These incentives have been particularly strong for workers under 65 years of age and, although they will be reduced somewhat when the 1977 amendments to the social security law become fully effective, they will remain substantial. The findings raise serious questions about recent econometric work attributing the decline in labor force participation rates of older men to the social security system.
Modulus Stabilization with Bulk Fields
We propose a mechanism for stabilizing the size of the extra dimension in the
Randall-Sundrum scenario. The potential for the modulus field that sets the
size of the fifth dimension is generated by a bulk scalar with quartic
interactions localized on the two 3-branes. The minimum of this potential
yields a compactification scale that solves the hierarchy problem without fine
tuning of parameters.Comment: 8 pages, LaTeX; minor typo correcte
Criteria for the experimental observation of multi-dimensional optical solitons in saturable media
Criteria for experimental observation of multi-dimensional optical solitons
in media with saturable refractive nonlinearities are developed. The criteria
are applied to actual material parameters (characterizing the cubic
self-focusing and quintic self-defocusing nonlinearities, two-photon loss, and
optical-damage threshold) for various glasses. This way, we identify operation
windows for soliton formation in these glasses. It is found that two-photon
absorption sets stringent limits on the windows. We conclude that, while a
well-defined window of parameters exists for two-dimensional solitons (spatial
or spatiotemporal), for their three-dimensional spatiotemporal counterparts
such a window \emph{does not} exist, due to the nonlinear loss in glasses.Comment: 8 pages, to appear in Phys. Rev.
Heavy Quark Fragmentation to Baryons Containing Two Heavy Quarks
We discuss the fragmentation of a heavy quark to a baryon containing two
heavy quarks of mass . In this limit the heavy quarks
first combine perturbatively into a compact diquark with a radius small
compared to , which interacts with the light hadronic
degrees of freedom exactly as does a heavy antiquark. The subsequent evolution
of this diquark to a baryon is identical to the fragmentation of a
heavy antiquark to a meson. We apply this analysis to the production of baryons
of the form , , and .Comment: 9 pages, 1 figure included, uses harvmac.tex and epsf.tex, UCSD/PTH
93-11, CALT-68-1868, SLAC-PUB-622
The X-ray luminosity function of AGN at z~3
We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra
X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN.
Applying careful corrections for both the optical and X-ray selection
functions, the data allow us to make the most accurate determination to date of
the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology
recovers a number density of X-ray sources at this redshift which is at least
as high as previous surveys, demonstrating that it is an effective way of
selecting high z AGN. Comparing to results at z=1, we find no evidence that the
faint slope of the XLF flattens at high z, but we do find significant (factor
~3.6) negative evolution of the space density of low luminosity AGN. Combining
with bright end data from very wide surveys we also see marginal evidence for
continued positive evolution of the characteristic break luminosity L*. Our
data therefore support models of luminosity-dependent density evolution between
z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest
luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of
pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA
Pseudospectral time-domain (PSTD) methods for the wave equation: Realising boundary conditions with discrete sine and cosine transforms
Pseudospectral time domain (PSTD) methods are widely used in many branches of
acoustics for the numerical solution of the wave equation, including biomedical
ultrasound and seismology. The use of the Fourier collocation spectral method
in particular has many computational advantages. However, the use of a discrete
Fourier basis is also inherently restricted to solving problems with periodic
boundary conditions. Here, a family of spectral collocation methods based on
the use of a sine or cosine basis is described. These retain the computational
advantages of the Fourier collocation method but instead allow homogeneous
Dirichlet (sound-soft) and Neumann (sound-hard) boundary conditions to be
imposed. The basis function weights are computed numerically using the discrete
sine and cosine transforms, which can be implemented using O(N log N)
operations analogous to the fast Fourier transform. Practical details of how to
implement spectral methods using discrete sine and cosine transforms are
provided. The technique is then illustrated through the solution of the wave
equation in a rectangular domain subject to different combinations of boundary
conditions. The extension to boundaries with arbitrary real reflection
coefficients or boundaries that are non-reflecting is also demonstrated using
the weighted summation of the solutions with Dirichlet and Neumann boundary
conditions.Comment: 21 pages, 10 figure
Chiral Estimates of Strong CP Violation Revisited
The effects of the CP violating term in the QCD Lagrangian upon low
energy hadronic phenomenology are reconsidered. Strong CP violating
interactions among Goldstone bosons and octet baryons are incorporated into an
effective chiral Lagrangian framework. The term's impact upon the
decays and is then investigated but
found to be extremely small. A refined model independent estimate of
nonanalytic contributions to the neutron electric dipole moment is also
determined using velocity dependent Baryon Chiral Perturbation Theory. We
obtain the approximate upper bound .Comment: 11 pages with 3 figures not included but available upon request,
CALT-68-184
Chiral Perturbation Theory for , , and
We use heavy vector meson chiral perturbation theory
to predict differential decay distributions for and in the kinematic region where
(here or ) is much smaller than the
chiral symmetry breaking scale. Using the large number of colors limit we also
predict the rate for in this region (now
). Comparing our prediction with experimental data, we determine
one of the coupling constants in the heavy vector meson chiral Lagrangian.Comment: 14 pages, latex 2e. We include the decay of the tau into the omega,
pi minus and the tau neutrino, and extract a value for the coupling constant
g2, using experimental dat
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