2,841 research outputs found
Searching for sgluons in multitop events at a center-of-mass energy of 8 TeV
Large classes of new physics theories predict the existence of new scalar
states, commonly dubbed sgluons, lying in the adjoint representation of the QCD
gauge group. Since these new fields are expected to decay into colored Standard
Model particles, and in particular into one or two top quarks, these theories
predict a possible enhancement of the hadroproduction rate associated with
multitop final states. We therefore investigate multitop events produced at the
Large Hadron Collider, running at a center-of-mass energy of 8 TeV, and employ
those events to probe the possible existence of color adjoint scalar particles.
We first construct a simplified effective field theory motivated by R-symmetric
supersymmetric models where sgluon fields decay dominantly into top quarks. We
then use this model to analyze the sensitivity of the Large Hadron Collider in
both a multilepton plus jets and a single lepton plus jets channel. After
having based our event selection strategy on the possible presence of two,
three and four top quarks in the final state, we find that sgluon-induced new
physics contributions to multitop cross sections as low as 10-100 fb can be
excluded at the 95% confidence level, assuming an integrated luminosity of 20
inverse fb. Equivalently, sgluon masses of about 500-700 GeV can be reached for
several classes of benchmark scenarios.Comment: 26 pages; 8 figures, 6 tables; version accepted by JHE
Invited - Spiking neuron circuits in ULSIC vs TFT technologies
Recent advances on computing systems have enabled increasing success of algorithms using artificial intelligence. Researchers are now exploring new computational paradigms and materials to enable computing at the level of the device, allowing increased privacy and also reduction in energy. One of the most promising techniques is to realize circuits that imitate how neurons in biological brains function. Spike-based neural networks have been shown to hold more computational power than other neuromorphic architectures and their integration into mainstream computing is projected to herald a new age of computational power. Integrating neuron circuits with the functionality of materials used in flexible electronics is likely to open up a large field of applications, most notably for sensors for continuous health monitoring. In traditional MOFET technologies, spiking neuron circuits are typically operated in the deep subthreshold in order to take advantage of the exponential dependence of Vg to achieve the spiking action and also to optimize energy consumption. Nevertheless, this gives rise to some challenging problems when implemented in flexible technologies where the desire for using low cost and low temperature processes leads to lower mobility and much greater variability in device processing.
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Techno-economic optimization of a scaled-up solar concentrator combined with CSPonD thermal energy storage
A molten salt direct absorption receiver, CSPonD, used to simultaneously collect and store thermal energy is being tested by Masdar Institute and MIT in Abu Dhabi, UAE. Whilst a research-scale prototype has been combined with a beam-down tower in Abu Dhabi, the original design coupled the receiver with a hillside heliostat field. With respect to a conventional power-tower setup, a hillside solar field presents the advantages of eliminating tower costs, heat tracing equipment, and high-pressure pumps. This analysis considers the industrial viability of the CSPonD concept by modeling
a 10 MWe up-scaled version of a molten salt direct absorption receiver combined with a hillside heliostat field. Five different slope angles are initially simulated to determine the optimum choice using a combination of lowest LCOE and highest IRR, and sensitivity analyses are carried out based on thermal energy storage duration, power output, and feed-in tariff price. Finally, multi-objective optimization is undertaken to determine a Pareto front representing optimum cases. The study indicates that a 40° slope and a combination of 14 h thermal energy storage with a 40-50 MW[subscript e] power output provide the best techno-economic results. By selecting one simulated result and using a feed-in tariff of 0.25 $/kWh, a competitive IRR of 15.01 % can be achieved
Transitional disks and their origins: an infrared spectroscopic survey of Orion A
Transitional disks are protoplanetary disks around young stars, with inner
holes or gaps which are surrounded by optically thick outer, and often inner,
disks. Here we present observations of 62 new transitional disks in the Orion A
star-forming region. These were identified using the \textit{Spitzer Space
Telescope}'s Infrared Spectrograph and followed up with determinations of
stellar and accretion parameters using the Infrared Telescope Facility's SpeX.
We combine these new observations with our previous results on transitional
disks in Taurus, Chamaeleon I, Ophiuchus and Perseus, and with archival X-ray
observations. This produces a sample of 105 transitional disks of "cluster" age
3 Myr or less, by far the largest hitherto assembled. We use this sample to
search for trends between the radial structure in the disks and many other
system properties, in order to place constraints on the possible origins of
transitional disks. We see a clear progression of host star accretion rate and
the different disk morphologies. We confirm that transitional disks with
complete central clearings have median accretion rates an order of magnitude
smaller than radially continuous disks of the same population. Pre-transitional
disks --- those objects with gaps that separate inner and outer disks --- have
median accretion rates intermediate between the two. Our results from the
search for statistically significant trends, especially related to ,
strongly support that in both cases the gaps are far more likely to be due to
the gravitational influence of Jovian planets or brown dwarfs orbiting within
the gaps, than to any of the photoevaporative, turbulent or grain-growth
processes that can lead to disk dissipation. We also find that the fraction of
Class II YSOs which are transitional disks is large, 0.1-0.2, especially in the
youngest associations.Comment: 96 pages, 25 figures, resubmitted to Ap
Validation of an optical model applied to the beam down CSP facility at the Masdar Institute Solar Platform
In the framework of the CSPonD Demo project, the optical characterization of the Beam Down Optical Experiment (BDOE) heliostats field is an important step to certify the required power is provided. To achieve this goal, an experiment involving a single heliostat is carried out. The results of the experiment and the comparison with simulated results are presented in this paper. Only the reflection on the heliostat is observed in order to have a better assessment of its optical performance. The heliostat reflectance is modified and the experimental and simulated concentration distribution are confronted. Results indicate that the shapes of the concentration distributions are quite similar, hence validating the optical model respects the geometry of the BDOE. Moreover these results lead to an increase of the optimized heliostat reflectance when the incident angle on the heliostat decreases. Further investigation is required to validate this method with all the individual heliostats of the BDOE solar field.MIT & Masdar Institute Cooperative Progra
Correlation between the spatial distribution of circumstellar disks and massive stars in the young open cluster NGC 6611. II: Cluster members selected with Spitzer/IRAC
Context: the observations of the proplyds in the Orion Nebula Cluster,
showing clear evidence of ongoing photoevaporation, have provided a clear proof
about the role of the externally induced photoevaporation in the evolution of
circumstellar disks. NGC 6611 is an open cluster suitable to study disk
photoevaporation, thanks to its large population of massive members and of
stars with disk. In a previous work, we obtained evidence of the influence of
the strong UV field generated by the massive cluster members on the evolution
of disks around low-mass Pre-Main Sequence members. That work was based on a
multi-band BVIJHK and X-ray catalog purposely compiled to select the cluster
members with and without disk. Aims: in this paper we complete the list of
candidate cluster members, using data at longer wavelengths obtained with
Spitzer/IRAC, and we revisit the issue of the effects of UV radiation on the
evolution of disks in NGC 6611. Methods: we select the candidate members with
disks of NGC 6611, in a field of view of 33'x34' centered on the cluster, using
IRAC color-color diagrams and suitable reddening-free color indices. Besides,
using the X-ray data to select Class III cluster members, we estimate the disks
frequency vs. the intensity of the incident radiation emitted by massive
members. Results: we identify 458 candidate members with circumstellar disks,
among which 146 had not been revealed in our previous work. Comparing of the
various color indices we used to select the cluster members with disk, we claim
that they detect the excesses due to the emission of the same physical region
of the disk: the inner rim at the dust sublimation radius. Our new results
confirm that UV radiation from massive stars affects the evolution of nearby
circumstellar disks.Comment: Accepted for publication at Astronomy & Astrophysic
Design of a 100 kW Concentrated Solar Power on Demand Volumetric Receiver With Integral Thermal Energy Storage Prototype
A new concept of Thermal Energy Storage (TES) system based on current available technologies is being developed under the framework of the Masdar Institute (MI) and Massachusetts Institute of Technology (MIT) collaborative Flagship Program. The key feature of this concept lies on concentrating sun light directly on the molten salt storage tank, avoiding the necessity of pumping the salts to the top of a tower thereby avoiding thermal losses and pumping and electric tracing needs inherent in most conventional CSP plants.
This Concentrated Solar Power on Demand (CSPonD) volumetric receiver/TES unit prototype will be tested in the existing MI heliostat field and beam down tower in Abu Dhabi (UAE) which will collect and redirect solar energy to an upwards-facing final optical element (FOE). These energy will be concentrated on the aperture of the prototype designed to store 400 kWh of energy allowing 16 hours of continuous production after sunset using Solar Salt (60%NaNO3 + 40%KNO3) as storage material.
The tank is divided in two volumes: one cold in the bottom region, where Solar Salt is at 250 °C and another hot on the upper region, at 550 °C. A moving divider plate with active control separates both volumes. The plate includes mixing enhancement features to help with convection on the hot volume of salts.
It’s expected that results will demonstrate the technical feasibility and economic viability of this concept allowing its scale up at commercial size
Probing the Inner Disk Emission of the Herbig Ae Stars HD 163296 and HD 190073
The physical processes occurring within the inner few astronomical units of
proto-planetary disks surrounding Herbig Ae stars are crucial to setting the
environment in which the outer planet-forming disk evolves and put critical
constraints on the processes of accretion and planet migration. We present the
most complete published sample of high angular resolution H- and K-band
observations of the stars HD 163296 and HD 190073, including 30 previously
unpublished nights of observations of the former and 45 nights of the latter
with the CHARA long-baseline interferometer, in addition to archival VLTI data.
We confirm previous observations suggesting significant near-infrared emission
originates within the putative dust evaporation front of HD 163296 and show
this is the case for HD 190073 as well. The H- and K-band sizes are the same
within for HD 163296 and within for HD 190073. The
radial surface brightness profiles for both disks are remarkably Gaussian-like
with little or no sign of the sharp edge expected for a dust evaporation front.
Coupled with spectral energy distribution analysis, our direct measurements of
the stellar flux component at H and K bands suggest that HD 190073 is much
younger (<400 kyr) and more massive (~5.6 M) than previously thought,
mainly as a consequence of the new Gaia distance (891 pc).Comment: 19 pages, 6 figure
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