14,533 research outputs found
The exact solution of the Riemann problem with non-zero tangential velocities in relativistic hydrodynamics
We have generalised the exact solution of the Riemann problem in special
relativistic hydrodynamics for arbitrary tangential flow velocities. The
solution is obtained by solving the jump conditions across shocks plus an
ordinary differential equation arising from the self-similarity condition along
rarefaction waves, in a similar way as in purely normal flow. The dependence of
the solution on the tangential velocities is analysed, and the impact of this
result on the development of multidimensional relativistic hydrodynamic codes
(of Godunov type) is discussed.Comment: 26 pages, 4 figures. Accepted for publication in Journal of Fluid
Mechanic
Mammal population densities at a global scale are higher in human‐modified areas
Global landscapes are changing due to human activities with consequences for both biodiversity and ecosystems. For single species, terrestrial mammal population densities have shown mixed responses to human pressure, with both increasing and decreasing densities reported in the literature. How the impacts of human activities on mammal populations translates into altered global density patterns remains unclear. Here we aim to disentangle the effect of human impacts on large‐scale patterns of mammal population densities using a global dataset of 6729 population density estimates for 468 mammal species (representing 59% and 44% of mammalian orders and families). We fitted a mixed effect model to explain the variation in density based on a 1‐degree resolution as a function of the human footprint index (HFI), a global proxy of direct and indirect human disturbances, while accounting for body mass, trophic level and primary productivity (normalized vegetation index; NDVI). We found a significant positive relationship between population density and HFI, where population densities were higher in areas with a higher HFI (e.g. agricultural or suburban areas – no populations were located in very high HFI urban areas) compared to areas with a low HFI (e.g. wilderness areas). We also tested the effect of the individual components of the HFI and still found a consistent positive effect. The relationships remained positive even across populations of the same species, although variability among species was high. Our results indicate shifts in mammal population densities in human modified landscapes, which is due to the combined effect of species filtering, increased resources and a possible reduction in competition and predation. Our study provides further evidence that macroecological patterns are being altered by human activities, where some species will benefit from these activities, while others will be negatively impacted or even extirpated
Factorization of Large-x Quark Distributions in a Hadron
We present a factorization formula for valence quark distributions in a
hadron in x-->1 limit. For the example of pion, we arrive at the form of
factorization by analyzing momentum flow in the leading and high-order Feynman
diagrams. The result confirms the well-known 1-x scaling rule to all orders in
perturbation theory, providing the non-perturbative matrix elements for the
infrared-divergence factors. We comment on re-summation of perturbative single
and double logarithms in 1-x.Comment: 7 pages, 2 figures include
Serum levels of matrix metalloproteinases-2 and-9 and their tissue inhibitors in inflammatory neuromuscular disorders
We monitored serum levels of matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) before and during intravenously applied immunoglobulin (IVIG) therapy in 33 patients with chronic immune-mediated neuropathies and myopathies and 15 controls. Baseline MMP-2 and TIMP-2 serum levels were lower and MMP-9 and TIMP-1 serum levels higher in all patients compared to age-matched controls. Eight days after IVIG treatment, MMP-2, TIMP-2, and TIMP-1 serum levels increased, while MMP-9 serum levels decreased, indicating tissue repair. After 60 days, MMP-9 levels increased, MMP-2 approached normal levels, while TIMP-1 and TIMP-2 serum levels were below day 8 levels, indicating relapsing tissue damage. Comparing the MMP/TIMP results with the clinical courses, IVIG treatment tended to change MMP/TIMP levels in a way that paralleled clinical improvement and relapse. In sum, during a distinct time period, IVIG therapy seems to be able to modulate VIMP-mediated tissue repair. Copyright (c) 2006 S. Karger AG, Basel
On heavy-quarkonia suppression by final-state multiple scatterings in most central Au+Au collisions at RHIC
We study heavy-quarkonia suppression under final-state multiple scatterings
in most central Au~+~Au collisions at RHIC energy. We first calculate the
survival probability of a heavy quarkonium under multiple scattering in
Bjorken's expanding QGP at large . Then, we calculate the rapidity
dependence of the nuclear modification factor for heavy-quarkonia
production by considering final-state multiple scatterings in most central
Au~+~Au collisions in a simplified model. In our formula a constant is
also introduced to estimate the possible cold nuclear effects. By fitting the
data for production in most central Au + Au collisions at
~GeV at RHIC, we find that the transportation coefficient
\hat{q}_0\simeq(0.33-0.95)~\mbox{GeV}^2/fm, and, accordingly, the energy
density at is \epsilon_0\simeq(1.39-5.62)~\mbox{GeV}/\mbox{fm}^3 in
perturbative thermal QCD. A better understanding of cold nuclear effects is
essential for us to get a more accurate analysis. The small values of the
transportation coefficient in our estimate are in sharp contrast
with those obtained by the analysis of high- hadron spectra in Ref. [31].Comment: 20 pages, 8 figures, final version accepted for publication in
Nuclear Physics
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