QCD Splitting/Joining Functions at Finite Temperature in the Deep LPM Regime

Abstract: There exist full leading-order-in-alpha_s numerical calculations of the rates for massless quarks and gluons to split and join in the background of a quark-gluon plasma through hard, nearly collinear bremsstrahlung and inverse bremsstrahlung. In the limit of partons with very high energy E, where the physics is dominated by the Landau-Pomeranchuk-Migdal (LPM) effect, there are also analytic leading-log calculations of these rates, where the logarithm is ln(E/T). We extend those analytic calculations to next-to-leading-log order. We find agreement with the full result to within roughly 20% for E(less) >~ 10 T, where E(less) is the energy of the least energetic parton in the splitting/joining process. We also discuss how to account for the running of the coupling constant in the case that E/T is very large. Our results are also applicable to isotropic non-equilibrium plasmas if the plasma does not change significantly over the formation time associated with particle splitting.