Infrared and pinching singularities in out of equilibrium QCD plasmas

We examine collinear, infrared and pinching singularities in the production of weakly interacting particles from out of equilibrium relativistic plasmas. We show that collinear singularities cancel out in a scalar theory, exactly as at equilibrium. The same result holds in a quark-gluon plasma, provided the matter degrees of freedom are quantized in a physical gauge. However the cancellation does not hold in a covariant gauge; we comment on a possible explanation of this result. Finally we show that pinching singularities give contributions of order g ² δn, where δn is the deviation from equilibrium of the distribution function and g the QCD coupling constant.