Shear Viscosity of a Hot Pion Gas

The shear viscosity of an interacting pion gas is studied using the Kubo formalism as a microscopic description of thermal systems close to global equilibrium. We implement the skeleton expansion in order to approximate the retarded correlator of the viscous part of the energy-momentum tensor. After exploring this in $g\phi^4$ theory we show how the skeleton expansion can be consistently applied to pions in chiral perturbation theory. The shear viscosity $\eta$ is determined by the spectral width, or equivalently, the mean free path of pions in the heat bath. We derive a new analytical result for the mean free path which is well-conditioned for numerical evaluation and discuss the temperature and pion-mass dependence of the mean free path and the shear viscosity. The ratio $\eta/s$ of the interacting pion gas exceeds the lower bound $1/4\pi$ from AdS/CFT correspondence.Comment: 12 pages, 7 figures. Revision includes additional Appendix B. Matches published versio

Specifying Real Numbers in CASL

We present a weak theory BasicReal of the real numbers in the first order specification language Casl. The aim is to provide a datatype for practical purposes, including the central notions and results of basic analysis. BasicReal captures for instance e and ; as well as the trigonometric and other standard functions. Concepts such as continuity, differentiation and integration are shown to be definable and tractable in this setting; Newton's Method is presented as an example of a numerical application. Finally, we provide a proper connection between the specified datatype BasicReal and specifications of the real numbers in higher order logic and various set theories