The 16\half-N_f Expansion and the Infrared Fixed Point in Perturbative QCD

Abstract

In QCD with 16\half-epsilon massless quark flavours there is an infrared fixed point with alpha_s/pi = (8/321)epsilon in the limit epsilon -> 0+. I develop the idea of Banks and Zaks to expand about N_f=16\half. This expansion is certainly useful for N_f=16, 15, 14, ..., and arguably it can reach the phenomenologically interesting case N_f=2, where it suggests that alpha_s/pi "freezes" to a value of order of magnitude 0.4 in the infrared.

Comments: 10 pages, LaTeX. 3 figures (see top of file). Revised version; some extra discussion. (To appear in Phys. Lett. B.) DE-FG05-92ER40717-12