125 research outputs found
Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem
We explore a nuclear physics resolution to the discrepancy between the
predicted standard big-bang nucleosynthesis (BBN) abundance of 7Li and its
observational determination in metal-poor stars. The theoretical 7Li abundance
is 3-4 times greater than the observational values, assuming the
baryon-to-photon ratio, eta_wmap, determined by WMAP. The 7Li problem could be
resolved within the standard BBN picture if additional destruction of A=7
isotopes occurs due to new nuclear reaction channels or upward corrections to
existing channels. This could be achieved via missed resonant nuclear
reactions, which is the possibility we consider here. We find some potential
candidate resonances which can solve the lithium problem and specify their
required resonant energies and widths. For example, a 1^- or 2^- excited state
of 10C sitting at approximately 15.0 MeV above its ground state with an
effective width of order 10 keV could resolve the 7Li problem; the existence of
this excited state needs experimental verification. Other examples using known
states include 7Be+t \rightarrow 10B(18.80 MeV), and 7Be+d \rightarrow 9B(16.71
MeV). For all of these states, a large channel radius (a > 10 fm) is needed to
give sufficiently large widths. Experimental determination of these reaction
strengths is needed to rule out or confirm these nuclear physics solutions to
the lithium problem.Comment: 37 pages, 9 figures. Additional discussion of channel widths and
radii. Matches published versio
A Decentralized Federated Learning using Reputation
Nowadays Federated learning (FL) is established as one of the best techniques for collaborative machine learning. It allows a set of clients to train a common model without disclosing their sensitive and private
dataset to a coordination server. The latter is in charge of the model aggregation. However, FL faces some problems, regarding the security of updates, integrity of computation and the availability of a server.
In this paper, we combine some new ideas like clientsâ reputation with techniques like secure aggregation using Homomorphic Encryption and verifiable secret sharing using Multi-Party Computation techniques to design a decentralized FL system that addresses the issues of incentives, security and availability amongst others. One of the original contributions of this work is the new leader election protocol which uses a secure shuffling and is based on a proof of reputation. Indeed, we propose to select an aggregator among the clients participating to
the FL training using their reputations. That is, we estimate the reputation of each client at every FL iteration and then we select the next round aggregator from the set of clients with the best reputations. As such, we remove misbehaving clients (e.g., byzantines) from the list of clients eligible for the role of aggregation server
Practical Multi-Key Homomorphic Encryption for More Flexible and Efficient Secure Federated Aggregation (preliminary work)
In this work, we introduce a lightweight communication-efficient multi-key approach suitable for the Federated Averaging rule. By combining secret-key RLWE-based HE, additive secret sharing and PRFs, we reduce approximately by a half the communication cost per party when compared to the usual public-key instantiations, while keeping practical homomorphic aggregation performances. Additionally, for LWE-based instantiations, our approach reduces the communication cost per party from quadratic to linear in terms of the lattice dimension
CFPKM : A Key Encapsulation Mechanism based on Solving System of non-linear multivariate Polynomials
Practical Multi-Key Homomorphic Encryption for More Flexible and Efficient Secure Federated Aggregation (preliminary work)
In this work, we introduce a lightweight communication-efficient multi-key approach suitable for the Federated Averaging rule. By combining secret-key RLWE-based HE, additive secret sharing and PRFs, we reduce approximately by a half the communication cost per party when compared to the usual public-key instantiations, while keeping practical homomorphic aggregation performances. Additionally, for LWE-based instantiations, our approach reduces the communication cost per party from quadratic to linear in terms of the lattice dimension
Higher D or Li: Probes of Physics beyond the Standard Model
Standard Big Bang Nucleosynthesis at the baryon density determined by the
microwave anisotropy spectrum predicts an excess of \li7 compared to
observations by a factor of 4-5. In contrast, BBN predictions for D/H are
somewhat below (but within ~2 \sigma) of the weighted mean of observationally
determined values from quasar absorption systems. Solutions to the \li7 problem
which alter the nuclear processes during or subsequent to BBN, often lead to a
significant increase in the deuterium abundance consistent with the highest
values of D/H seen in absorption systems. Furthermore, the observed D/H
abundances show considerable dispersion. Here, we argue that those systems with
D/H \simeq 4 \times 10^{-5} may be more representative of the primordial
abundance and as a consequence, those systems with lower D/H would necessarily
have been subject to local processes of deuterium destruction. This can be
accounted for by models of cosmic chemical evolution able to destroy in situ
Deuterium due to the fragility of this isotope.Comment: 22 pages, 8 figure
Exploiting inflammation for therapeutic gain in pancreatic cancer
Pancreatic ductal adenocarcinoma (PDAC) is an aggressive malignancy associated with <5% 5-year survival, in which standard chemotherapeutics have limited benefit. The disease is associated with significant intra- and peritumoral inflammation and failure of protective immunosurveillance. Indeed, inflammatory signals are implicated in both tumour initiation and tumour progression. The major pathways regulating PDAC-associated inflammation are now being explored. Activation of leukocytes, and upregulation of cytokine and chemokine signalling pathways, both have been shown to modulate PDAC progression. Therefore, targeting inflammatory pathways may be of benefit as part of a multi-target approach to PDAC therapy. This review explores the pathways known to modulate inflammation at different stages of tumour development, drawing conclusions on their potential as therapeutic targets in PDAC
The Star-Forming Galaxy Contribution to the Cosmic MeV and GeV Gamma-Ray Background
While star-forming galaxies could be major contributors to the cosmic GeV
-ray background, they are expected to be MeV-dim because of the "pion
bump" falling off below ~100 MeV. However, there are very few observations of
galaxies in the MeV range, and other emission processes could be present. We
investigate the MeV background from star-forming galaxies by running one-zone
models of cosmic ray populations, including Inverse Compton and bremsstrahlung,
as well as nuclear lines (including Al), emission from core-collapse
supernovae, and positron annihilation emission, in addition to the pionic
emission. We use the Milky Way and M82 as templates of normal and starburst
galaxies, and compare our models to radio and GeV--TeV -ray data. We
find that (1) higher gas densities in high-z normal galaxies lead to a strong
pion bump, (2) starbursts may have significant MeV emission if their magnetic
field strengths are low, and (3) cascades can contribute to the MeV emission of
starbursts if they emit mainly hadronic -rays. Our fiducial model
predicts that most of the unresolved GeV background is from star-forming
galaxies, but this prediction is uncertain by an order of magnitude. About ~2%
of the claimed 1 MeV background is diffuse emission from star-forming galaxies;
we place a firm upper limit of <~10% based on the spectral shape of the
background. The star-formation contribution is constrained to be small, because
its spectrum is peaked, while the observed background is steeply falling with
energy through the MeV-GeV range.Comment: Published in ApJ, 27 pages, emulateapj format. Readers may be
interested in the concurrent paper by Chakraborty and Fields
(arXiv:1206.0770), a calculation of the Inverse Compton background from
star-forming galaxie
Strong interface-induced spin-orbit coupling in graphene on WS2
Interfacial interactions allow the electronic properties of graphene to be
modified, as recently demonstrated by the appearance of satellite Dirac cones
in the band structure of graphene on hexagonal boron nitride (hBN) substrates.
Ongoing research strives to explore interfacial interactions in a broader class
of materials in order to engineer targeted electronic properties. Here we show
that at an interface with a tungsten disulfide (WS2) substrate, the strength of
the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The
induced SOI leads to a pronounced low-temperature weak anti-localization (WAL)
effect, from which we determine the spin-relaxation time. We find that
spin-relaxation time in graphene is two-to-three orders of magnitude smaller on
WS2 than on SiO2 or hBN, and that it is comparable to the intervalley
scattering time. To interpret our findings we have performed first-principle
electronic structure calculations, which both confirm that carriers in
graphene-on-WS2 experience a strong SOI and allow us to extract a
spin-dependent low-energy effective Hamiltonian. Our analysis further shows
that the use of WS2 substrates opens a possible new route to access topological
states of matter in graphene-based systems.Comment: Originally submitted version in compliance with editorial guidelines.
Final version with expanded discussion of the relation between theory and
experiments to be published in Nature Communication
Review of particle physics
The Review summarizes much of particle physics and cosmology. Using data from previous editions, plus 3,283 new measurements from 899 papers, we list, evaluate, and average measured properties of gauge bosons and the recently discovered Higgs boson, leptons, quarks, mesons, and baryons. We summarize searches for hypothetical particles such as heavy neutrinos, supersymmetric and technicolor particles, axions, dark photons, etc. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as Supersymmetry, Extra Dimensions, Particle Detectors, Probability, and Statistics. Among the 112 reviews are many that are new or heavily revised including those on: Dark Energy, Higgs Boson Physics, Electroweak Model, Neutrino Cross Section Measurements, Monte Carlo Neutrino Generators, Top Quark, Dark Matter, Dynamical Electroweak Symmetry Breaking, Accelerator Physics of Colliders, High-Energy Collider Parameters, Big Bang Nucleosynthesis, Astrophysical Constants and Cosmological Parameters. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.Ibi.gov
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