NLO BFKL Equation, Running Coupling and Renormalization Scales

TL;DR

This paper analyzes the NLO BFKL equation with a focus on the running coupling and renormalization scales, showing that the evolution in Q^2 can be predicted despite uncertainties in the input distribution at small x.

Contribution

It demonstrates the factorization of the BFKL solution into infrared dominated input and ultraviolet dominated evolution, clarifying the predictive power at small x.

Findings

Evolution in Q^2 is perturbatively calculable at all x.

Infrared dominated input distribution is contaminated by renormalons.

Resummed splitting functions successfully fit experimental data.

Abstract

I examine the solution of the BFKL equation with NLO corrections relevant for deep inelastic scattering. Particular emphasis is placed on the part played by the running of the coupling. It is shown that the solution factorizes into a part describing the evolution in Q^2, and a constant part describing the input distribution. The latter is infrared dominated, being described by a coupling which grows as x decreases, and thus being contaminated by infrared renormalons. Hence, for this part we agree with previous assertions that predictive power breaks down for small enough x at any Q^2. However, the former is ultraviolet dominated, being described by a coupling which falls like 1/(\ln(Q^2/\Lambda^2) + A(\bar\alpha_s(Q^2)\ln(1/x))^1/2)with decreasing x, and thus is perturbatively calculable at all x. Therefore, although the BFKL equation is unable to predict the input for a structure function for small x, it is able to predict its evolution in Q^2, as we would expect from the factorization theory. The evolution at small x has no true powerlike behaviour due to the fall of the coupling, but does have significant differences from that predicted from a standard NLO in alpha_s treatment. Application of the resummed splitting functions with the appropriate coupling constant to an analysis of data, i.e. a global fit, is very successful.