Can the QCD running coupling have a causal analyticity structure?

Solving the QCD renormalization group equation at the 2-loop and 3-loop orders we obtain explicit expressions for the coupling as a function of the scale in terms of the Lambert W function. We study the nature of the "Landau singularities" in the complex Q² plane and show that perturbative freezing can lead, in certain cases, to an analyticity structure that is consistent with causality. We analyze the Analytic Perturbation Theory (APT) approach which is intended to remove the "Landau singularities", and show that at 2-loops it is uniquely defined in terms of the Lambert W function, and that, depending on the value of the first two β function coefficients β₀ and β₁, it is either consistent with perturbative freezing (for β₁ < -β₀²) with an infrared limit of -β₀/β₁ or leads to a non-perturbative infrared coupling with a limit of 1/β₀ (for β₁ > -β₀²). The possibility of a causal perturbative coupling is in accordance with the idea that a purely perturbative Banks-Zaks phase with an infrared fixed-point exists in QCD if the number of flavours (N_f) is increased. The causality condition implies that the perturbative phase is realized for N_f ≥ 10.