LAUSR

We propose that rapidity-dependent momentum correlations can be used to extract the shear relaxation time τπ of the medium formed in highenergy nuclear collisions. Shear viscosity drives the initial fluctuations of the medium toward equilibrium at a rate characterized by the shear relaxation time. Momentum fluctuations, in excess of thermal fluctuations, that survive to freeze out, are remnants of the initial state, and influence the rapidity dependence of momentum correlations. We describe a method for calculating the rapidity dependence of two-particle momentum correlations with a second order, causal, diffusion equation that includes Langevin noise as a source of thermal fluctuations. In comparison to RHIC data, we find that the ratio τπ/ν ≈ 5–6, where ν = η/sT is the kinematic viscosity.