Cosmic-Ray Signatures of Dark Matter Decay

In light of recent observations of an anomalous excess of high-energy positrons and electrons by the PAMELA and Fermi LAT experiments, we investigate exotic cosmic-ray signatures in scenarios with unstable dark matter that decays with an extremely long lifetime. We identify decay modes capable of explaining the observed anomalies and mention constraints arising from measurements of antiprotons and gamma rays. We also discuss complementary tests by measurements of anisotropies in diffuse gamma rays which should be accessible to Fermi.Comment: 4 pages, 1 figure. To appear in the proceedings of SUSY 2009, June 5 - 10, Northeastern University, Boston, M

Application of screened hybrid functionals to the bulk transition metals Rh, Pd, and Pt

We present the results of calculations on bulk transition metals Rh, Pd, and Pt using the screened hybrid functional YS-PBE0 [F. Tran and P. Blaha, Phys. Rev. B \textbf{83}, 235118 (2011)]. The results for the equilibrium geometry are compared with those obtained from (semi)local functionals, namely, the local density approximation and the generalized gradient approximation PBE of Perdew \textit{et al}. [J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. \textbf{77}, 3865 (1996)]. It is shown that the screened hybrid functional yields more accurate equilibrium geometry than PBE, but, overall, it is not more accurate than LDA. However, in contradiction with experiment, we find that the screened hybrid functional favors a ferromagnetic state as the ground state for all three transition metals. Therefore, the use of hybrid functionals for, e.g., the study of catalytically active systems with correlated oxides on a metal support is questionable

Impeded inverse energy transfer in the Charney--Hasegawa--Mima model of quasi-geostrophic flows

The behaviour of turbulent flows within the single-layer quasi-geostrophic (Charney--Hasegawa--Mima) model is shown to be strongly dependent on the Rossby deformation wavenumber $\lambda$ (or free-surface elasticity). Herein, we derive a bound on the inverse energy transfer, specifically on the growth rate \d\ell/\dt of the characteristic length scale $\ell$ representing the energy centroid. It is found that \d\ell/\dt\le2\norm q_\infty/(\ell_s\lambda^2), where \norm q_\infty is the supremum of the potential vorticity and $\ell_s$ represents the potential enstrophy centroid of the reservoir, both invariant. This result implies that in the potential energy dominated regime ($\ell\ge\ell_s\gg\lambda^{-1}$), the inverse energy transfer is strongly impeded, in the sense that under the usual time scale no significant transfer of energy to larger scales occurs. The physical implication is that the elasticity of the free surface impedes turbulent energy transfer in wavenumber space, effectively rendering large-scale vortices long-lived and inactive. Results from numerical simulations of forced-dissipative turbulence confirm this prediction.Comment: 8 pages, 2 figures, accepted for publication in JF

Personalised Search Time Prediction using Markov Chains

For improving the effectiveness of Interactive Information Retrieval (IIR), a system should minimise the search time by guiding the user appropriately. As a prerequisite, in any search situation, the system must be able to estimate the time the user will need for finding the next relevant document. In this paper, we show how Markov models derived from search logs can be used for predicting search times, and describe a method for evaluating these predictions. For personalising the predictions based upon a few user events observed, we devise appropriate parameter estimation methods. Our experimental results show that by observing users for only 100 seconds, the personalised predictions are already significantly better than global predictions

Controlled cortical impact traumatic brain injury in 3xTg-AD mice causes acute intra-axonal amyloid-β accumulation and independently accelerates the development of tau abnormalities

Alzheimer\u27s disease (AD) is a neurodegenerative disorder characterized pathologically by progressive neuronal loss, extracellular plaques containing the amyloid-β (Aβ) peptides, and neurofibrillary tangles composed of hyperphosphorylated tau proteins. Aβ is thought to act upstream of tau, affecting its phosphorylation and therefore aggregation state. One of the major risk factors for AD is traumatic brain injury (TBI). Acute intra-axonal Aβ and diffuse extracellular plaques occur in ∼30% of human subjects after severe TBI. Intra-axonal accumulations of tau but not tangle-like pathologies have also been found in these patients. Whether and how these acute accumulations contribute to subsequent AD development is not known, and the interaction between Aβ and tau in the setting of TBI has not been investigated. Here, we report that controlled cortical impact TBI in 3xTg-AD mice resulted in intra-axonal Aβ accumulations and increased phospho-tau immunoreactivity at 24 h and up to 7 d after TBI. Given these findings, we investigated the relationship between Aβ and tau pathologies after trauma in this model by systemic treatment of Compound E to inhibit γ-secretase activity, a proteolytic process required for Aβ production. Compound E treatment successfully blocked posttraumatic Aβ accumulation in these injured mice at both time points. However, tau pathology was not affected. Our data support a causal role for TBI in acceleration of AD-related pathologies and suggest that TBI may independently affect Aβ and tau abnormalities. Future studies will be required to assess the behavioral and long-term neurodegenerative consequences of these pathologies