450 research outputs found
BL Lac Objects in the Synchrotron Proton Blazar Model
We calculate the spectral energy distribution (SED) of electromagnetic
radiation and the spectrum of high energy neutrinos from BL Lac objects in the
context of the Synchrotron Proton Blazar Model. In this model, the high energy
hump of the SED is due to accelerated protons, while most of the low energy
hump is due to synchrotron radiation by co-accelerated electrons. To accelerate
protons to sufficiently high energies to produce the high energy hump, rather
high magnetic fields are required. Assuming reasonable emission region volumes
and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs),
which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there
is a significant contribution to the high frequency hump of the SED from pion
photoproduction and subsequent cascading, including synchrotron radiation by
muons. In contrast, in HBLs we find that the high frequency hump of the SED is
dominated by proton synchrotron radiation. We are able to model the SED of
typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We
also calculate the expected neutrino output of typical BL Lac objects, and
estimate the diffuse neutrino intensity due to all BL Lacs. Because pion
photoproduction is inefficient in HBLs, as protons lose energy predominantly by
synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino
intensity. We suggest that nearby LBLs may well be observable with future
high-sensitivity TeV gamma-ray telescopes.Comment: 33 pages, 20 Figures. Astropart. Phys., accepte
Neutrino Background Flux from Sources of Ultrahigh-Energy Cosmic-Ray Nuclei
Motivated by Pierre Auger Observatory results favoring a heavy nuclear
composition for ultrahigh-energy (UHE) cosmic rays, we investigate implications
for the cumulative neutrino background. The requirement that nuclei not be
photodisintegrated constrains their interactions in sources, therefore limiting
neutrino production via photomeson interactions. Assuming a injection spectrum and
photodisintegration via the giant dipole resonance, the background flux of
neutrinos is lower than if UHE nuclei ubiquitously survive in
their sources. This is smaller than the analogous Waxman-Bahcall flux for UHE
protons by about one order of magnitude, and is below the projected IceCube
sensitivity. If IceCube detects a neutrino background, it could be due to other
sources, e.g., hadronuclear interactions of lower-energy cosmic rays; if it
does not, this supports our strong restrictions on the properties of sources of
UHE nuclei.Comment: 7 pages, 3 figure
Advanced process design for re-contouring using a time-domain dynamic material removal simulation
The repair of components often requires the removal of excess weld material. This removal is considered as re-contouring. Re-contouring processes have to be designed individually for each case of damage to fulfil the high quality requirements. Therefore, a prognosis of the machined surface topography is crucial. The material removal simulation introduced in this paper allows the prediction of process stability and surface topography for 5-axis ball end milling including dynamic effects. Different process strategies for re-contouring of Ti-6Al-4V welds are examined. It is shown, that selecting suitable process parameters can lead to high surface quality while maintaining productivity. © 2019 The Author(s)
Markov chain generative adversarial neural networks for solving Bayesian inverse problems in physics applications
In the context of solving inverse problems for physics applications within a Bayesian framework, we present a new approach, the Markov Chain Generative Adversarial Neural Network (MCGAN), to alleviate the computational costs associated with solving the Bayesian inference problem. GANs pose a very suitable framework to aid in the solution of Bayesian inference problems, as they are designed to generate samples from complicated high-dimensional distributions. By training a GAN to sample from a low-dimensional latent space and then embedding it in a Markov Chain Monte Carlo method, we can highly efficiently sample from the posterior, by replacing both the high-dimensional prior and the expensive forward map. This comes at the cost of a potentially expensive offline stage in which training data must be simulated or gathered and the GAN has to be trained. We prove that the proposed methodology converges to the true posterior in the Wasserstein-1 distance and that sampling from the latent space is equivalent to sampling in the high-dimensional space in a weak sense. The method is showcased in two test cases where we perform both state and parameter estimation simultaneously and it is compared with two conventional approaches, polynomial chaos expansion and ensemble Kalman filter, and a deep learning-based approach, deep Bayesian inversion. The method is shown to be more accurate than alternative approaches while also being computationally faster, in multiple test cases, including the important engineering setting of detecting leaks in pipelines
Do CBCT scans alter surgical treatment plans? Comparison of preoperative surgical diagnosis using panoramic versus cone-beam CT images
Cone beam CT and/or panoramic images are often required for a successful diagnosis in oral and maxillofacial surgery. The aim of this study was to evaluate if 3D diagnostic imaging information had a significant impact on the decision process in six different classes of surgical indications. Material and methods: Records of all patients who had undergone both panoramic X-ray and CBCT imaging due to surgical indications between January 2008 and December 2012 were examined retrospectively. In February 2013, all surgically relevant diagnoses of both conventional panoramic radiographs and CBCT scans were retrieved from the patient's charts. It was recorded whether (1) 3D imaging presented additional surgically relevant information and (2) if the final decision of surgical therapy had been based on 2D or 3D imaging. Results: A total of 253 consecutive patients with both panoramic radiographs and CBCT analysis were eligible for the study. 3D imaging provided significantly more surgically relevant information in cases of implant dentistry, maxillary sinus diagnosis and in oral and maxillofacial traumatology. However, surgical strategies had not been influenced to any significant extent by 3D imaging. Conclusion: Within the limitations of this study it may be concluded that CBCT imaging results in significantly more surgically relevant information in implant dentistry, maxillary sinus diagnosis and in cases of oral and maxillofacial trauma. However, 3D imaging information did not alter significantly the surgical plan that was based on 2D panoramic radiography. Further studies are necessary to define indications for CBCT in detail
Role of the Carrier-Envelope Offset Phase of Few-Cycle Pulses in Nonperturbative Resonant Nonlinear Optics
Photon-Photon Entanglement with a Single Trapped Atom
An experiment is performed where a single rubidium atom trapped within a
high-finesse optical cavity emits two independently triggered entangled
photons. The entanglement is mediated by the atom and is characterized both by
a Bell inequality violation of S=2.5, as well as full quantum-state tomography,
resulting in a fidelity exceeding F=90%. The combination of cavity-QED and
trapped atom techniques makes our protocol inherently deterministic - an
essential step for the generation of scalable entanglement between the nodes of
a distributed quantum network.Comment: 5 pages, 4 figure
Evidence for Third-Harmonic Generation in Disguise of Second-Harmonic Generation in Extreme Nonlinear Optics
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