2,936 research outputs found
The Lambda_Q-Lambda_Q Potential
Lattice QCD simulations of the potential between two baryons, each containing
a heavy quark and two light quarks, such as the Lambda_Q-Lambda_Q potential,
will provide insight into the nucleon-nucleon interaction. As one-pion exchange
does not contribute to the Lambda_Q-Lambda_Q potential, the long-distance
behavior is dominated by physics that contributes to the intermediate-range
attraction between two nucleons. We compute the leading long-distance
contributions to the Lambda_Q-Lambda_Q potential in QCD and in
partially-quenched QCD in the low-energy effective field theory.Comment: 10 pages LaTeX, 3 eps figs, 3 ps fig
Narrow 0\u3csup\u3e+\u3c/sup\u3e state in \u3csup\u3e20\u3c/sup\u3eNe and 0\u3csub\u3e6\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e and 0\u3csub\u3e7\u3c/sub\u3e\u3csup\u3e+\u3c/sup\u3e rotational bands
A reanalysis of old data removes the (0+,2+) ambiguity for a very narrow state at Ex(20Ne)=11.55 MeV and gives a unique 0+ assignment. Such a 0+ state corresponds well to a predicted state at 11.494 MeV of unusually small reduced widths for decay to both the ground and first excited state of 16O. This new 0+ state is a better 06+ band head for the 8p-4h states at 15.159 MeV (6+) and 18.538 MeV (8+) than the currently accepted 0+ state at 12.44 MeV. Possible 2+ and 4+ members are considered. The higher 0+ level at Ex=12.44 starts a new 07+ band, and candidates for this band are critically discussed
Continuum and Emission-Line Properties of Broad Absorption Line Quasars
We investigate the continuum and emission-line properties of 224 broad
absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital
Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide
quasars. We find that low-ionization BALQSOs (LoBALs) are significantly
reddened as compared to normal quasars, in agreement with previous work.
High-ionization BALQSOs (HiBALs) are also more reddened than the average
nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount
of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is
E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We
find that there are differences in the emission-line properties between the
average HiBAL, LoBAL, and nonBAL quasar. These differences, along with
differences in the absorption line troughs, may be related to intrinsic quasar
properties such as the slope of the intrinsic (unreddened) continuum; more
extreme absorption properties are correlated with bluer intrinsic continua.
Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear
to be drawn from the same parent population as nonBALQSOs when both are
selected by their UV/optical properties. We find that the overall fraction of
traditionally defined BALQSOs, after correcting for color-dependent selection
effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and
shows no significant redshift dependence for 1.7<z<3.45. After a rough
completeness correction for the effects of dust extinction, we find that
approximately one in every six quasars is a BALQSO.Comment: 35 pages, 11 figures (1 color), 1 table; accepted by A
A LightâInduced DecarboxylativeâElimination of Substituted Maleimides as a Strategy Towards Triggered Photorelease
Herein we report on photodecarboxylations of various substituted maleimides, resulting in an elimination reaction. Furthermore, we establish facile wavelength tunability through modulation of the maleimide double bond substituents. We envisage that these versatile reagents, which are readily constructed and diversified by nucleophilic substitution reactions on bromomaleimides, will offer new opportunities for triggered photorelease
Integrated active and passive control design methodology for the LaRC CSI evolutionary model
A general design methodology to integrate active control with passive damping was demonstrated on the NASA LaRC CSI Evolutionary Model (CEM), a ground testbed for future large, flexible spacecraft. Vibration suppression controllers designed for Line-of Sight (LOS) minimization were successfully implemented on the CEM. A frequency-shaped H2 methodology was developed, allowing the designer to specify the roll-off of the MIMO compensator. A closed loop bandwidth of 4 Hz, including the six rigid body modes and the first three dominant elastic modes of the CEM was achieved. Good agreement was demonstrated between experimental data and analytical predictions for the closed loop frequency response and random tests. Using the Modal Strain Energy (MSE) method, a passive damping treatment consisting of 60 viscoelastically damped struts was designed, fabricated and implemented on the CEM. Damping levels for the targeted modes were more than an order of magnitude larger than for the undamped structure. Using measured loss and stiffness data for the individual damped struts, analytical predictions of the damping levels were very close to the experimental values in the (1-10) Hz frequency range where the open loop model matched the experimental data. An integrated active/passive controller was successfully implemented on the CEM and was evaluated against an active-only controller. A two-fold increase in the effective control bandwidth and further reductions of 30 percent to 50 percent in the LOS RMS outputs were achieved compared to an active-only controller. Superior performance was also obtained compared to a High-Authority/Low-Authority (HAC/LAC) controller
High prevalence of <i>Rickettsia africae</i> variants in <i>Amblyomma variegatum</i> ticks from domestic mammals in rural western Kenya: implications for human health
Tick-borne spotted fever group (SFG) rickettsioses are emerging human diseases caused by obligate intracellular Gram-negative bacteria of the genus Rickettsia. Despite being important causes of systemic febrile illnesses in travelers returning from sub-Saharan Africa, little is known about the reservoir hosts of these pathogens. We conducted surveys for rickettsiae in domestic animals and ticks in a rural setting in western Kenya. Of the 100 serum specimens tested from each species of domestic ruminant 43% of goats, 23% of sheep, and 1% of cattle had immunoglobulin G (IgG) antibodies to the SFG rickettsiae. None of these sera were positive for IgG against typhus group rickettsiae. We detected Rickettsia africaeâgenotype DNA in 92.6% of adult Amblyomma variegatum ticks collected from domestic ruminants, but found no evidence of the pathogen in blood specimens from cattle, goats, or sheep. Sequencing of a subset of 21 rickettsia-positive ticks revealed R. africae variants in 95.2% (20/21) of ticks tested. Our findings show a high prevalence of R. africae variants in A. variegatum ticks in western Kenya, which may represent a low disease risk for humans. This may provide a possible explanation for the lack of African tick-bite fever cases among febrile patients in Kenya
Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112
We report the discovery of a fifth image in the large separation lensed
quasar system SDSS J1004+4112. A faint point source located 0.2'' from the
center of the brightest galaxy in the lensing cluster is detected in images
taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera
and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux
ratio between the point source and the brightest lensed component in the ACS
image is similar to that in the NICMOS image. The location and brightness of
the point source are consistent with lens model predictions for a lensed image.
We therefore conclude that the point source is likely to be a fifth image of
the source quasar. In addition, the NICMOS image reveals the lensed host galaxy
of the source quasar, which can strongly constrain the structure of the lensing
critical curves and thereby the mass distribution of the lensing cluster.Comment: 5 pages, 5 figures, accepted for publication in PAS
The Sloan Digital Sky Survey Reverberation Mapping Project: Ensemble Spectroscopic Variability of Quasar Broad Emission Lines
We explore the variability of quasars in the MgII and Hbeta broad emission
lines and UV/optical continuum emission using the Sloan Digital Sky Survey
Reverberation Mapping project (SDSS-RM). This is the largest spectroscopic
study of quasar variability to date: our study includes 29 spectroscopic epochs
from SDSS-RM over months, containing 357 quasars with MgII and 41 quasars
with Hbeta . On longer timescales, the study is also supplemented with
two-epoch data from SDSS-I/II. The SDSS-I/II data include an additional
quasars with MgII and 572 quasars with Hbeta. The MgII emission line is
significantly variable ( 10% on 100-day timescales), a necessary
prerequisite for its use for reverberation mapping studies. The data also
confirm that continuum variability increases with timescale and decreases with
luminosity, and the continuum light curves are consistent with a damped
random-walk model on rest-frame timescales of days. We compare the
emission-line and continuum variability to investigate the structure of the
broad-line region. Broad-line variability shows a shallower increase with
timescale compared to the continuum emission, demonstrating that the broad-line
transfer function is not a -function. Hbeta is more variable than MgII
(roughly by a factor of ), suggesting different excitation mechanisms,
optical depths and/or geometrical configuration for each emission line. The
ensemble spectroscopic variability measurements enabled by the SDSS-RM project
have important consequences for future studies of reverberation mapping and
black hole mass estimation of quasars.Comment: 20 pages, 25 figures. ApJ accepted: minor revisions following referee
repor
A Spectroscopic Survey of Faint Quasars in the SDSS Deep Stripe: I. Preliminary Results from the Co-added Catalog
In this paper we present the first results of a deep spectroscopic survey of
faint quasars in the Sloan Digital Sky Survey (SDSS) Southern Survey, a deep
survey carried out by repeatedly imaging a 270 deg^2 area. Quasar candidates
were selected from the deep data with good completeness over 0<z<5, and 2 to 3
magnitudes fainter than the SDSS main survey. Spectroscopic follow-up was
carried out on the 6.5m MMT with Hectospec. The preliminary sample of this SDSS
faint quasar survey (hereafter SFQS) covers ~ 3.9 deg^2, contains 414 quasars,
and reaches g=22.5. The overall selection efficiency is ~ 66% (~ 80% at
g<21.5); the efficiency in the most difficult redshift range (2<z<3) is better
than 40%. We use the 1/V_{a} method to derive a binned estimate of the quasar
luminosity function (QLF) and model the QLF using maximum likelihood analysis.
The best model fits confirm previous results showing that the QLF has steep
slopes at the bright end and much flatter slopes (-1.25 at z<2.0 and -1.55 at
z>2.0) at the faint end, indicating a break in the QLF slope. Using a
luminosity-dependent density evolution model, we find that the quasar density
at M_{g}<-22.5 peaks at z~2, which is later in cosmic time than the peak of
z~2.5 found from surveys of more luminous objects. The SFQS QLF is consistent
with the results of the 2dF QSO Redshift Survey, the SDSS, and the 2dF-SDSS LRG
and QSO Survey, but probes fainter quasars. We plan to obtain more quasars from
future observations and establish a complete faint quasar sample with more than
1000 objects over 10 deg^2.Comment: 25 pages, 13 figures, accepted for publication in A
Modelling of optical traps for aerosols
Experimental observations suggest that there are differences between the
behavior of particles optically trapped in air and trapped in a liquid phase.
We present a modified version of Mie Debye Spherical Aberration theory to
numerically simulate such optical system in attempt to explain and predict
these effects. The model incorporates Mie scattering and focussing of the
trapping beam through media of stratified refractive index. Our results show a
geometrical optics approach cannot correctly describe our system and that
spherical aberration must be included. We successfully qualitatively explain
the observed phenomena and those of other authors, before discussing the limits
of our experimental techniques and methods to improve it. We draw the important
conclusion that when optically trapping aerosols the system does not behave as
a true `optical tweezers', varying between levitation and single beam gradient
force trapping depending on particle and beam parameters
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