4,598 research outputs found
Transfers to a gravitational saddle point: An extended mission design option for LISA Pathfinder
Any possible LISA Pathfinder extended mission will immediately follow the primary mission after completion of scientific observations and technical demonstrations in a Sun-Earth L1 libration point orbit. One extended mission concept with scientific appeal is a spacecraft path that includes multiple encounters with a gravitational equilibrium point. This point, also termed a saddle point, exists where the total gravitational acceleration sums to zero and is distinct from the five Lagrange points in the three-body problem. This investigation seeks a strategy to design such a path subject to a variety of constraints. Periodic, quasi-periodic, and manifold structures are explored to supply useful transit behavior as well as arcs that repeatedly encounter the saddle point. A selection of these structures from the Earth-Moon and Sun-Earth circular restricted three-body problems are linked together via Poincaré mapping techniques and corrected in a higher-fidelity Sun-Earth-Moon bicircular restricted four-body problem (BC4BP) and in an ephemeris environment. Additionally, natural motion in the BC4BP is leveraged to achieve the required encounters, and is similarly corrected to meet mission constraints. Results from both methods are detailed and compared to the mission requirements
Implications of diphoton searches for a Radion in the Bulk-Higgs Scenario
In this work we point out that the apparent diphoton excess initially
presented by the ATLAS and CMS collaborations could have originated from a
radion in the bulk Higgs scenario within a warped extra dimension. In this
scenario the couplings of the radion to massive gauge bosons are suppressed,
allowing it to evade existing searches. In the presence of mixing with the
Higgs, due to the strong constraints from diboson searches, only points near
what we denominate the alignment region were able to explain the diphoton
signal and evade other experimental constraints. In light of the new
measurements presented at ICHEP 2016 by both LHC collaborations, which do not
confirm the initial diphoton excess, we study the current and future collider
constraints on a radion within the bulk-Higgs scenario. We find that searches
in the diphoton channel provide the most powerful probe of this scenario and
already exclude large regions of parameter space, particularly for smaller warp
factors. The radion has a sizeable branching ratio into top pairs and this
channel may also give competitive constraints in the future. Finally, diHiggs
searches can provide a complementary probe in the case of non-zero radion-Higgs
mixing but strong alignment.Comment: 20 pages, 12 figures. Several changes including consequences from
ICHEP2016. Final version accepted by journa
Radion/Dilaton-Higgs Mixing Phenomenology in Light of the LHC
Motivated by the bulk mixing between a massive radion
and a bulk scalar Higgs in warped extra dimensions, we construct an effective
four dimensional action that---via the AdS/CFT correspondence---describes the
most general mixing between the only light states in the theory, the dilaton
and the Higgs. Due to conformal invariance, once the Higgs scalar is localized
in the bulk of the extra-dimension the coupling between the dilaton and the
Higgs kinetic term vanishes, implying a suppressed coupling between the dilaton
and massive gauge bosons. We comment on the implications of the mixing and
couplings to Standard Model particles. Identifying the recently discovered 125
GeV resonance with the lightest Higgs-like mixed state , we study the
phenomenology and constraints for the heaviest radion-like state . In
particular we find that in the small mixing scenario with a radion-like state
in the mass range [150,250] GeV, the diphoton channel can provide
the best chance of discovery at the LHC if the collaborations extend their
searches into this energy range.Comment: 28 pages, 6 figures; v2: version published in JHE
Novel Collider and Dark Matter Phenomenology of a Top-philic Z'
We consider extending the Standard Model by including an additional Abelian
gauge group broken at low energies under which the right-handed top quark is
the only effectively charged Standard Model fermion. The associated gauge boson
is then naturally top-philic and couples only to the rest of the SM
particle content at loop-level or via kinetic mixing with the hypercharge gauge
boson which is assumed to be small. Working at the effective theory level, we
demonstrate that such a minimal extension allows for an improved fitting of the
excess observed in searches at the LHC in a region
of parameter space that satisfies existing collider constraints. We also
present the reach of the LHC at 13 TeV in constraining the relevant region of
parameter space. Additionally we show that within the same framework a suitably
chosen fermion charged only under the exotic Abelian group can, in the region
of parameter space preferred by the measurements, simultaneously
explain the dark matter relic density and the -ray excess at the
galactic center observed by the Fermi-LAT experiment.Comment: 30 pages, 11 figures; v2: version published in JHE
Symmetric Molecular Dynamics
We derive a formulation of molecular dynamics that generates only symmetric
configurations. We implement it for all 2D planar and 3D space groups. An atlas
of 2D Lennard-Jones crystals under all planar groups is created with symmetric
molecular dynamics
Minnorm training: an algorithm for training over-parameterized deep neural networks
In this work, we propose a new training method for finding minimum weight
norm solutions in over-parameterized neural networks (NNs). This method seeks
to improve training speed and generalization performance by framing NN training
as a constrained optimization problem wherein the sum of the norm of the
weights in each layer of the network is minimized, under the constraint of
exactly fitting training data. It draws inspiration from support vector
machines (SVMs), which are able to generalize well, despite often having an
infinite number of free parameters in their primal form, and from recent
theoretical generalization bounds on NNs which suggest that lower norm
solutions generalize better. To solve this constrained optimization problem,
our method employs Lagrange multipliers that act as integrators of error over
training and identify `support vector'-like examples. The method can be
implemented as a wrapper around gradient based methods and uses standard
back-propagation of gradients from the NN for both regression and
classification versions of the algorithm. We provide theoretical justifications
for the effectiveness of this algorithm in comparison to early stopping and
-regularization using simple, analytically tractable settings. In
particular, we show faster convergence to the max-margin hyperplane in a
shallow network (compared to vanilla gradient descent); faster convergence to
the minimum-norm solution in a linear chain (compared to -regularization);
and initialization-independent generalization performance in a deep linear
network. Finally, using the MNIST dataset, we demonstrate that this algorithm
can boost test accuracy and identify difficult examples in real-world datasets
PaperQA: Retrieval-Augmented Generative Agent for Scientific Research
Large Language Models (LLMs) generalize well across language tasks, but
suffer from hallucinations and uninterpretability, making it difficult to
assess their accuracy without ground-truth. Retrieval-Augmented Generation
(RAG) models have been proposed to reduce hallucinations and provide provenance
for how an answer was generated. Applying such models to the scientific
literature may enable large-scale, systematic processing of scientific
knowledge. We present PaperQA, a RAG agent for answering questions over the
scientific literature. PaperQA is an agent that performs information retrieval
across full-text scientific articles, assesses the relevance of sources and
passages, and uses RAG to provide answers. Viewing this agent as a question
answering model, we find it exceeds performance of existing LLMs and LLM agents
on current science QA benchmarks. To push the field closer to how humans
perform research on scientific literature, we also introduce LitQA, a more
complex benchmark that requires retrieval and synthesis of information from
full-text scientific papers across the literature. Finally, we demonstrate
PaperQA's matches expert human researchers on LitQA
Truncation of the lipopolysaccharide outer core affects susceptibility to antimicrobial peptides and virulence of Actinobacillus pleuropneumoniae serotype 1.
Abstract We reported previously that the core oligosaccharide region of the lipopolysaccharide (LPS) is essential for optimal adhesion of Actinobacillus pleuropneumoniae, an important swine pathogen, to respiratory tract cells. Rough LPS and core LPS mutants of A. pleuropneumoniae serotype 1 were generated by using a mini-Tn10 transposon mutagenesis system. Here we performed a structural analysis of the oligosaccharide region of three core LPS mutants that still produce the same O-antigen by using methylation analyses and mass spectrometry. We also performed a kinetic study of proinflammatory cytokines production such as interleukin (IL)-6, tumor necrosis factor-α, IL1-ÎČ, MCP-1, and IL8 by LPS-stimulated porcine alveolar macrophages, which showed that purified LPS of the parent strain, the rough LPS and core LPS mutants, had the same ability to stimulate the production of cytokines. Most interestingly, an in vitro susceptibility test of these LPS mutants to antimicrobial peptides showed that the three core LPS mutants were more susceptible to cationic peptides than both the rough LPS mutant and the wild type parent strain. Furthermore, experimental pig infections with these mutants revealed that the galactose (Gal I) and d,d-heptose (Hep IV) residues present in the outer core of A. pleuropneumoniae serotype 1 LPS are important for adhesion and overall virulence in the natural host, whereas deletion of the terminal GalNAc-Gal II disaccharide had no effect. Our data suggest that an intact core-lipid A region is required for optimal protection of A. pleuropneumoniae against cationic peptides and that deletion of specific residues in the outer LPS core results in the attenuation of the virulence of A. pleuropneumoniae serotype 1
A compact acoustic spanner to rotate macroscopic objects
Waves can carry both linear and angular momentum. When the wave is transverse (e.g. light), the angular momentum can be characterised by the âspinâ angular momentum associated with circular polarisation, and the âorbitalâ angular momentum (OAM) arising from the phase cross-section of the beam. When the wave is longitudinal (e.g. sound) there is no polarization and hence no spin angular momentum. However, a suitably phase-structured sound beam can still carry OAM. Observing the transfer of OAM from sound to a macroscopic object provides an excellent opportunity to study the exchange of energy between waves and matter. In this paper we show how to build a compact free-space acoustic spanner based on a 3D-printed sound-guiding structure and common electronic components. We first characterise the sound fields by measuring both phase and amplitude maps, and then show a video of our free-space acoustic spanner in action, in which macroscopic objects spin in a circular motion and change direction of rotation according to the handedness of the OAM acoustic field
Binding of herpes simplex virus-1 US11 to specific RNA sequences
Herpes simplex virus-1 US11 is a RNA-binding protein with a novel RNA-binding domain. US11 has been reported to exhibit sequence- and conformation-specific RNA-binding, but the sequences and conformations important for binding are not known. US11 has also been described as a double-stranded RNA (dsRNA)-binding protein. To investigate the US11âRNA interaction, we performed in vitro selection of RNA aptamers that bind US11 from a RNA library consisting of >10(14) 80 base sequences which differ in a 30 base randomized region. US11 bound specifically to selected aptamers with an affinity of 70 nM. Analysis of 23 selected sequences revealed a strong consensus sequence. The US11 RNA-binding domain and â€46 bases of selected RNA containing the consensus sequence were each sufficient for binding. US11 binding protected the consensus motif from hydroxyl radical cleavage. RNase digestions of a selected aptamer revealed regions of both single-stranded RNA and dsRNA. We observed that US11 bound two different dsRNAs in a sequence non-specific manner, but with lower affinity than it bound selected aptamers. The results define a relatively short specific sequence that binds US11 with high affinity and indicate that dsRNA alone does not confer high-affinity binding
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