9,579 research outputs found
Model Independent Results for Heavy Quarkonium
We review a number of results for the spectrum and inclusive decays of heavy
quarkonium systems which can be derived from QCD under well controlled
approximations. They essentially follow from the hierarchy of scales in these
systems, which can be efficiently exploited using non-relativistic effective
field theories. In particular, we discuss under which conditions
non-relativistic potential models emerge as effective theories of QCD.Comment: 15 pages. Invited brief revie
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Optical realization of nonlinear quantum dynamics
In a cavity filled with a Kerr medium it is possible to generate the
superposition of coherent states, i.e. Schroodinger cat states may be realized
in this system. We show that such a medium may be mimicked by the propagation
of a conveniently shaped Gaussian beam in a GRIN device. This is attained by
introducing a second order correction to the paraxial propagation of the beam.
An additional result is that a Gaussian beam propagating in GRIN media, may
split into two Gaussian beams
Do State Economics or Individual Characteristics Determine Whether Older Men Work?
The difference in labor force participation rates of men aged 55-64 across the United States is astounding. For example, West Virginia has a participation rate below 60 percent, while South Dakota has a participation rate approaching 90 percent (see Figure 1). This fact in itself has significant implications for the pressures that states will face as the baby boom starts to retire in the face of a contracting retirement income system, declining housing prices, and a lackluster stock market. Despite these marked differences, little is known about the reasons for such variations in work patterns. An earlier brief, using the Current Population Survey for the period 1977-2007, demonstrated that the differences in the labor force participation of older men were related to labor market conditions, the nature of employment, and the employee characteristics in each state as well as to a “pseudo replacement rate.” These variables explained more than one-third of the total variation...
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