5,384 research outputs found
M-Theory on the Orbifold C^2/Z_N
We construct M-theory on the orbifold C^2/Z_N by coupling 11-dimensional
supergravity to a seven-dimensional Yang-Mills theory located on the orbifold
fixed plane. It is shown that the resulting action is supersymmetric to leading
non-trivial order in the 11-dimensional Newton constant. This action provides
the starting point for a reduction of M-theory on G_2 spaces with co-dimension
four singularities.Comment: 33 pages, Late
An Ultra Fast Image Generator (UFig) for wide-field astronomy
Simulated wide-field images are becoming an important part of observational
astronomy, either to prepare for new surveys or to test measurement methods. In
order to efficiently explore vast parameter spaces, the computational speed of
simulation codes is a central requirement to their implementation. We introduce
the Ultra Fast Image Generator (UFig) which aims to bring wide-field imaging
simulations to the current limits of computational capabilities. We achieve
this goal through: (1) models of galaxies, stars and observational conditions,
which, while simple, capture the key features necessary for realistic
simulations, and (2) state-of-the-art computational and implementation
optimizations. We present the performances of UFig and show that it is faster
than existing public simulation codes by several orders of magnitude. It allows
us to produce images more quickly than SExtractor needs to analyze them. For
instance, it can simulate a typical 0.25 deg^2 Subaru SuprimeCam image (10k x
8k pixels) with a 5-sigma limiting magnitude of R=26 in 30 seconds on a laptop,
yielding an average simulation time for a galaxy of 30 microseconds. This code
is complementary to end-to-end simulation codes and can be used as a fast,
central component of observational methods relying on simulations.Comment: Submitted to Astronomy and Computing. 13 pages, 9 figure
Eye velocity gain fields for visuo- motor coordinate transformations
’Gain-field-like’ tuning behavior is characterized by a modulation of the neuronal response depending on a certain variable, without changing the actual receptive field characteristics in relation to another variable. Eye position gain fields were first observed in area 7a of the posterior parietal cortex (PPC), where visually responsive neurons are modulated by
ocular position. Analysis of artificial neural networks has shown that this type of tuning
function might comprise the neuronal substrate for coordinate transformations.
In this work, neuronal activity in the dorsal medial superior temporal area (MSTd) has
been analyzed with an focus on it’s involvement in oculomotor control. MSTd is part of
the extrastriate visual cortex and located in the PPC. Lesion studies suggested a participation
of this cortical area in the control of eye movements. Inactivation of MSTd severely
impairs the optokinetic response (OKR), which is an reflex-like kind of eye movement that
compensates for motion of the whole visual scene.
Using a novel, information-theory based approach for neuronal data analysis, we were able
to identify those visual and eye movement related signals which were most correlated to the mean rate of spiking activity in MSTd neurons during optokinetic stimulation. In a majority of neurons firing rate was non-linearly related to a combination of retinal image velocity and eye velocity. The observed neuronal latency relative to these signals is in line with a system-level model of OKR, where an efference copy of the motor command signal is used to generate an internal estimate of the head-centered stimulus velocity signal. Tuning functions were obtained by using a probabilistic approach. In most MSTd neurons these functions exhibited gain-field-like shapes, with eye velocity modulating the visual
response in a multiplicative manner. Population analysis revealed a large diversity of
tuning forms including asymmetric and non-separable functions. The distribution of gain
fields was almost identical to the predictions from a neural network model trained to perform
the summation of image and eye velocity. These findings therefore strongly support the hypothesis of MSTd’s participation in the OKR control system by implementing the transformation from retinal image velocity to an estimate of stimulus velocity. In this sense, eye velocity gain fields constitute an intermediate step in transforming the eye-centered to a head-centered visual motion signal.Another aspect that was addressed in this work was the comparison of the irregularity of MSTd spiking activity during optokinetic response with the behavior during pure visual stimulation. The goal of this study was an evaluation of potential neuronal mechanisms
underlying the observed gain field behavior. We found that both inter- and intra-trial
variability were decreased with increasing retinal image velocity, but increased with eye
velocity. This observation argues against a symmetrical integration of driving and modulating
inputs. Instead, we propose an architecture where multiplicative gain modulation is achieved by simultaneous increase of excitatory and inhibitory background synaptic input. A conductance-based single-compartment model neuron was able to reproduce realistic gain modulation and the observed stimulus-dependence of neural variability, at the same time.
In summary, this work leads to improved knowledge about MSTd’s role in visuomotor transformation by analyzing various functional and mechanistic aspects of eye velocity gain fields on a systems-, network-, and neuronal level
Calibrated Ultra Fast Image Simulations for the Dark Energy Survey
Weak lensing by large-scale structure is a powerful technique to probe the
dark components of the universe. To understand the measurement process of weak
lensing and the associated systematic effects, image simulations are becoming
increasingly important. For this purpose we present a first implementation of
the (; Refregier & Amara
2014), a coherent framework for studying systematic effects in weak lensing. It
allows us to model and calibrate the shear measurement process using image
simulations from the Ultra Fast Image Generator (UFig; Berge et al. 2013). We
apply this framework to a subset of the data taken during the Science
Verification period (SV) of the Dark Energy Survey (DES). We calibrate the UFig
simulations to be statistically consistent with DES images. We then perform
tolerance analyses by perturbing the simulation parameters and study their
impact on the shear measurement at the one-point level. This allows us to
determine the relative importance of different input parameters to the
simulations. For spatially constant systematic errors and six simulation
parameters, the calibration of the simulation reaches the weak lensing
precision needed for the DES SV survey area. Furthermore, we find a sensitivity
of the shear measurement to the intrinsic ellipticity distribution, and an
interplay between the magnitude-size and the pixel value diagnostics in
constraining the noise model. This work is the first application of the
framework to data and shows how it can be used to methodically
study the impact of systematics on the cosmic shear measurement.Comment: 14 pages, 9 Figures, submitted to Ap
Quantum non-Gaussianity of multi-phonon states of a single atom
Quantum non-Gaussian mechanical states from inherently nonlinear quantum
processes are already required in a range of applications spanning from quantum
sensing up to quantum computing with continuous variables. The discrete
building blocks of such states are the energy eigenstates - Fock states.
Despite the progress in their preparation, the remaining imperfections can
still invisibly cause loss of the critical quantum non-Gaussian aspects of the
phonon distribution relevant in the applications. We derive the most
challenging hierarchy of quantum non-Gaussian criteria for the individual
mechanical Fock states and demonstrate its implementation on the
characterization of single trapped-ion oscillator states up to 10~phonons. We
analyze the depth of quantum non-Gaussian features under mechanical heating and
predict their application in quantum sensing. These results uncover that the
crucial quantum non-Gaussian features are demanded to reach quantum advantage
in the applications
Planning for cycling in the dispersed city: Establishing a hierarchy of effectiveness of municipal cycling policies
Urban utility cycling is being promoted widely due to various health, social, economic and environmental benefits. This study seeks to identify and rank which municipal-level policies and other factors are most influential in increasing cycling as a means of everyday transport and improving the real and perceived cycling safety in car-oriented urban centres. This is achieved by identifying the key factors thought to influence cycle use and by establishing a hierarchy of effectiveness of municipal cycling policies. Data was collected through interviews with a panel of experts who also completed a Delphi study, a technique rarely used in cycling policy research, to collect and compare expert opinions to predict the outcomes of policies and external factors. Policies and external factors were scored in a theoretical policy framework according to their perceived relative influence on cycling levels and cycling safety. The results reinforce previous findings in the literature but allow for generalisation in car-oriented urban centres due to the breadth of factors evaluated. It was found that providing cycling infrastructure is perceived to be a prerequisite for inducing utility cycling mode share. External factors such as urban form, the relative attractiveness of cycling to travel by car and wider governmental policy were perceived to have a strong influence. The generation and maintenance of political and public support is also suggested to be critical success factor
Untersuchungen zum Vorkommen von PRRSV in einem definierten Gebiet in Bayern sowie Bewertung von Einflussfaktoren auf den PRRSV-Infektionsstatus
Untersuchungen zum Vorkommen von PRRSV in einem definierten Gebiet in Bayern sowie Bewertung von Einflussfaktoren auf den PRRSV-Infektionsstatus
Pay and the Pandemic:The decoupling of Dutch boardroom pay from performance during the Covid-19 outbreak
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