Pade-Summation Approach to QCD Beta-Function Infrared Properties

Abstract

We address whether Padé-summations of the $\overline{MS}$ QCD $\beta$-function for a given number of flavours exhibit an infrared-stable fixed point, or alternatively, an infrared attractor of a double valued couplant as noted by Kogan and Shifman for the case of supersymmetric gluodynamics. Below an approximant-dependent flavour threshold $(6\le n_f\le 8)$, we find that Padé-summation $\beta$-functions incorporating $[2|1],[1|2],[2|2],[1|3]$, and $[3|1]$ approximants always exhibit a positive pole prior to the occurrence of their first positive zero, precluding any identification of this first positive zero as an infrared-stable fixed point of the $\beta$- function. This result is shown to be true regardless of the magnitude of the presently-unknown five-loop $\beta$-function contribution. Moreover, the pole in question suggests the occurrence of dynamics in which both a strong and an asymptotically-free phase share a common infrared attractor. We briefly discuss the possible relevance of infrared-attractor dynamics to the success of recent calculations of the glueball mass spectra in QCD with $N_c\to \infty$ via supergravity. As $n_f$ increases above an approximant-dependent flavour threshold, Padé-summation $\beta$-functions incorporating $[2|2],[1|3]$, and $[3|1]$ approximants exhibit dynamics controlled by an infrared-stable fixed point over a widening domain of the five-loop $\overline{MS}$ $\beta$-function parameter $({\beta }_4/{\beta }_0)$. Above this threshold, all approximants considered exhibit infrared-stable fixed points that decrease in magnitude with increasing flavour number.

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