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Analysis of 83mKr prompt scintillation signals in the PIXeY detector
Prompt scintillation signals from 83mKr calibration sources are a useful metric to calibrate the spatial variation of light collection efficiency and electric field magnitude of a two phase liquid-gas xenon time projection chamber. Because 83mKr decays in two steps, there are two prompt scintillation pulses for each calibration event, denoted S1a and S1b. We study the ratio of S1b to S1a signal sizes in the Particle Identification in Xenon at Yale (PIXeY) experiment and its dependence on the time separation between the two signals (Δ t), notably its increase at low Δ t. In PIXeY data, the Δ t dependence of S1b/S1a is observed to exhibit two exponential components: one with a time constant of 0.05 ± 0.02 μ s, which can be attributed to processing effects and pulse overlap and one with a time constant of 10.2 ± 2.2 μs that increases in amplitude with electric drift field, the origin of which is not yet understood
Peeking inside the Black Box: Interpreting Deep-learning Models for Exoplanet Atmospheric Retrievals
Deep-learning algorithms are growing in popularity in the field of exoplanetary science due to their ability to model highly nonlinear relations and solve interesting problems in a data-driven manner. Several works have attempted to perform fast retrievals of atmospheric parameters with the use of machine-learning algorithms like deep neural networks (DNNs). Yet, despite their high predictive power, DNNs are also infamous for being "black boxes." It is their apparent lack of explainability that makes the astrophysics community reluctant to adopt them. What are their predictions based on? How confident should we be in them? When are they wrong, and how wrong can they be? In this work, we present a number of general evaluation methodologies that can be applied to any trained model and answer questions like these. In particular, we train three different popular DNN architectures to retrieve atmospheric parameters from exoplanet spectra and show that all three achieve good predictive performance. We then present an extensive analysis of the predictions of DNNs, which can inform us–among other things–of the credibility limits for atmospheric parameters for a given instrument and model. Finally, we perform a perturbation-based sensitivity analysis to identify to which features of the spectrum the outcome of the retrieval is most sensitive. We conclude that, for different molecules, the wavelength ranges to which the DNNs predictions are most sensitive do indeed coincide with their characteristic absorption regions. The methodologies presented in this work help to improve the evaluation of DNNs and to grant interpretability to their predictions
Peeking inside the Black Box: Interpreting Deep-learning Models for Exoplanet Atmospheric Retrievals
Deep-learning algorithms are growing in popularity in the field of exoplanetary science due to their ability to model highly nonlinear relations and solve interesting problems in a data-driven manner. Several works have attempted to perform fast retrievals of atmospheric parameters with the use of machine-learning algorithms like deep neural networks (DNNs). Yet, despite their high predictive power, DNNs are also infamous for being "black boxes." It is their apparent lack of explainability that makes the astrophysics community reluctant to adopt them. What are their predictions based on? How confident should we be in them? When are they wrong, and how wrong can they be? In this work, we present a number of general evaluation methodologies that can be applied to any trained model and answer questions like these. In particular, we train three different popular DNN architectures to retrieve atmospheric parameters from exoplanet spectra and show that all three achieve good predictive performance. We then present an extensive analysis of the predictions of DNNs, which can inform us–among other things–of the credibility limits for atmospheric parameters for a given instrument and model. Finally, we perform a perturbation-based sensitivity analysis to identify to which features of the spectrum the outcome of the retrieval is most sensitive. We conclude that, for different molecules, the wavelength ranges to which the DNNs predictions are most sensitive do indeed coincide with their characteristic absorption regions. The methodologies presented in this work help to improve the evaluation of DNNs and to grant interpretability to their predictions
On the Strength of the Carbon Nanotube-Based Space Elevator Cable: From Nano- to Mega-Mechanics
In this paper different deterministic and statistical models, based on new
quantized theories proposed by the author, are presented to estimate the
strength of a real, thus defective, space elevator cable. The cable, of ~100
megameters in length, is composed by carbon nanotubes, ~100 nanometers long:
thus, its design involves from the nano- to the mega-mechanics. The predicted
strengths are extensively compared with the experiments and the atomistic
simulations on carbon nanotubes available in the literature. All these
approaches unequivocally suggest that the megacable strength will be reduced by
a factor at least of ~70% with respect to the theoretical nanotube strength,
today (erroneously) assumed in the cable design. The reason is the unavoidable
presence of defects in a so huge cable. Preliminary in silicon tensile
experiments confirm the same finding. The deduced strength reduction is
sufficient to pose in doubt the effective realization of the space elevator,
that if built as today designed will surely break (according to the s opinion).
The mechanics of the cable is also revised and possibly damage sources
discussed
Fast detector of the ellipticity of infrared and terahertz radiation based on HgTe quantum well structures
We report a fast, room temperature detection scheme for the polarization
ellipticity of laser radiation, with a bandwidth that stretches from the
infrared to the terahertz range. The device consists of two elements, one in
front of the other, that detect the polarization ellipticity and the azimuthal
angle of the ellipse. The elements respectively utilise the circular
photogalvanic effect in a narrow gap semiconductor and the linear photogalvanic
effect in a bulk piezoelectric semiconductor. For the former we characterized
both a HgTe quantum well and bulk Te, and for the latter, bulk GaAs. In
contrast with optical methods our device is an easy to handle all-electric
approach, which we demonstrated by applying a large number of different lasers
from low power, continuous wave systems to high power, pulsed sources.Comment: 7 pages, 5 figure
Mesoscopic Effects in the Quantum Hall Regime
We report results of a study of (integer) quantum Hall transitions in a
single or multiple Landau levels for non-interacting electrons in disordered
two-dimensional systems, obtained by projecting a tight-binding Hamiltonian to
corresponding magnetic subbands. In finite-size systems, we find that
mesoscopic effects often dominate, leading to apparent non-universal scaling
behaviour in higher Landau levels. This is because localization length, which
grows exponentially with Landau level index, exceeds the system sizes amenable
to numerical study at present. When band mixing between multiple Landau levels
is present, mesoscopic effects cause a crossover from a sequence of quantum
Hall transitions for weak disorder to classical behaviour for strong disorder.
This behaviour may be of relevance to experimentally observed transitions
between quantum Hall states and the insulating phase at low magnetic fields.Comment: 13 pages, 6 figures, Proceedings of the International Meeting on
Mesoscopic and Disordered Systems, Bangalore December 2000, to appear in
Pramana, February 200
Systems analysis of bioenergetics and growth of the extreme halophile Halobacterium salinarum
Halobacterium salinarum is a bioenergetically flexible, halophilic microorganism that can generate energy by respiration, photosynthesis, and the fermentation of arginine. In a previous study, using a genome-scale metabolic model, we have shown that the archaeon unexpectedly degrades essential amino acids under aerobic conditions, a behavior that can lead to the termination of growth earlier than necessary. Here, we further integratively investigate energy generation, nutrient utilization, and biomass production using an extended methodology that accounts for dynamically changing transport patterns, including those that arise from interactions among the supplied metabolites. Moreover, we widen the scope of our analysis to include phototrophic conditions to explore the interplay between different bioenergetic modes. Surprisingly, we found that cells also degrade essential amino acids even during phototropy, when energy should already be abundant. We also found that under both conditions considerable amounts of nutrients that were taken up were neither incorporated into the biomass nor used as respiratory substrates, implying the considerable production and accumulation of several metabolites in the medium. Some of these are likely the products of forms of overflow metabolism. In addition, our results also show that arginine fermentation, contrary to what is typically assumed, occurs simultaneously with respiration and photosynthesis and can contribute energy in levels that are comparable to the primary bioenergetic modes, if not more. These findings portray a picture that the organism takes an approach toward growth that favors the here and now, even at the cost of longer-term concerns. We believe that the seemingly "greedy" behavior exhibited actually consists of adaptations by the organism to its natural environments, where nutrients are not only irregularly available but may altogether be absent for extended periods that may span several years. Such a setting probably predisposed the cells to grow as much as possible when the conditions become favorable
Neutrino Parameters, Abelian Flavor Symmetries, and Charged Lepton Flavor Violation
Neutrino masses and mixings have important implications for models of fermion
masses, and, most directly, for the charged lepton sector. We consider
supersymmetric Abelian flavor models, where neutrino mass parameters are
related to those of charged leptons and sleptons. We show that processes such
as \tau to \mu\gamma, \mu to e\gamma and \mu-e conversion provide interesting
probes. In particular, some existing models are excluded by current bounds,
while many others predict rates within reach of proposed near future
experiments. We also construct models in which the predicted rates for charged
lepton flavor violation are below even the proposed experimental sensitivities,
but argue that such models necessarily involve loss of predictive power.Comment: 27 pages, refs added, published versio
Counterion adsorption on flexible polyelectrolytes: comparison of theories
Counterion adsorption on a flexible polyelectrolyte chain in a spherical
cavity is considered by taking a "permuted" charge distribution on the chain so
that the "adsorbed" counterions are allowed to move along the backbone. We
compute the degree of ionization by using self-consistent field theory (SCFT)
and compare with the previously developed variational theory. Analysis of
various contributions to the free energy in both theories reveals that the
equilibrium degree of ionization is attained mainly as an interplay of the
adsorption energy of counterions on the backbone, the translational entropy of
the small ions, and their correlated density fluctuations. Degree of ionization
computed from SCFT is significantly lower than that from the variational
formalism. The difference is entirely due to the density fluctuations of the
small ions in the system, which are accounted for in the variational procedure.
When these fluctuations are deliberately suppressed in the truncated
variational procedure, there emerges a remarkable quantitative agreement in the
various contributing factors to the equilibrium degree of ionization, in spite
of the fundamental differences in the approximations and computational
procedures used in these two schemes. Nevertheless, since the significant
effects from density fluctuations of small ions are not captured by the SCFT,
and due to the close agreement between SCFT and the other contributing factors
in the more transparent variational procedure, the latter is a better
computational tool for obtaining the degree of ionization
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