18,337 research outputs found
Transport Coefficients in Large Gauge Theory: Testing Hard Thermal Loops
We compute shear viscosity and flavor diffusion coefficients for
ultra-relativistic gauge theory with many fermionic species, Nf >> 1, to
leading order in 1/Nf. The calculation is performed both at leading order in
the effective coupling strength g^2 Nf, using the Hard Thermal Loop (HTL)
approximation, and completely to all orders in g^2 Nf. This constitutes a
nontrivial test of how well the HTL approximation works. We find that in this
context, the HTL approximation works well wherever the renormalization point
sensitivity of the leading order HTL result is small.Comment: 31 pages, including 9 figures. Error in vacuum self-energy, arising
from trusting Arthur Weldon, fixed, thank you Tony Rebhan. Results and
conclusions slightly but not significantly change
Transport Coefficients in Hot QCD
I give a physical explanation of what shear viscosity is, and what physics
determines its value. Then I explain why determining the shear viscosity of the
Quark-Gluon Plasma is interesting. I outline the leading-order calculation of
the QGP shear viscosity (and baryon number diffusion constant), explaining why
the quite complicated physics of parton splitting and Landau-Pomeranchuk-Migdal
interference effects are required for its calculation. Then I briefly explore
the range of applicability, emphasizing the importance of plasma instabilities.Comment: 10 pages, invited talk given at the conference "Strong and
Electroweak Matter," Helsinki, Finland 16-19 June 200
Problems with lattice methods for electroweak preheating
Recently Garcia Bellido et. al. have proposed that electroweak baryogenesis
may occur at the end of inflation, in a scenario where the reheat temperature
is too low for electroweak symmetry restoration. I show why the scenario is
difficult to test reliably by classical field techniques on the lattice.Comment: 10 pages with 3 figure
Electroweak Bubble Wall Friction: Analytic Results
We present an entirely analytic, leading log order determination of the
friction an electroweak bubble wall feels during a first order electroweak
phase transition. The friction is dominated by W bosons, and gives a wall
velocity parametrically ~ alpha_w, and numerically small, ~ .01 -- 0.1
depending on the Higgs mass.Comment: 8 pages, no figures. Slight revision of introduction: published
version (JHEP
Transport Coefficients at Leading Order: Kinetic Theory versus Diagrams
I review what is required to compute transport coefficients in
ultra-relativistic, weakly coupled gauge theories, at leading order in ,
using kinetic theory. Then I discuss how the calculation would look in
alternative approaches: the 2PI method, and direct diagrammatic analysis. I
argue that the 2PI method may be a good way to derive the kinetic theory, but
is not very useful directly (in a gauge theory). The diagrammatic approach is
almost hopeless.Comment: 10 pages, 8 figures, to appear in the proceedings of SEWM200
Climate change: carbon losses in the Alps
The response of the terrestrial carbon cycle to global change is one of the main uncertainties in current climate change predictions1. Most terrestrial carbon is held in soils as organic matter derived from the decay of plant material (Fig. 1). Soil organic matter accounts for roughly three times more carbon than living vegetation, and for more carbon than vegetation and the atmosphere combined. Because elevated atmospheric CO2 concentrations have a fertilizing effect on plant growth, anthropogenic CO2 emissions have triggered increases in the land carbon sink2. However, models predict that other factors — such as water and nutrients — will eventually become limiting to plant growth, and hence to the land carbon sink. In contrast, the turnover of soil organic matter producing CO2 is expected to increase as the Earth warms. As a result, simulations using coupled carbon–climate models predict that the land surface will become a net source of CO2 before the end of the century, leading to a feedback loop between climate and soil carbon losses: increased emissions of CO2 from soil organic matter will lead to enhanced warming, which may then feedback to cause further soil organic matter losses. Prietzel and colleagues3, writing in Nature Geoscience, now provide evidence that warming has already caused a decline in soil organic matter in the German Alps
- …