Transport Coefficients in Large NfN_f 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 $g$, 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