Origin of the Q^2-dependence of the DIS structure functions

TL;DR

This paper investigates the origin of the Q^2-dependence in deep inelastic scattering (DIS) structure functions, challenging the common attribution to the Q^2-dependence of the QCD coupling and proposing a more nuanced understanding of their asymptotic behavior.

Contribution

It demonstrates that the DGLAP parametrization of alpha_s is only valid at large x and introduces an effective coupling at small x, clarifying the true Q^2-dependence of structure functions.

Findings

DGLAP parametrization is valid only at large x

Effective coupling at small x is Q^2-independent

Intercepts of structure functions are Q^2-independent at small x

Abstract

We consider in detail the Q^2 -dependence of the DIS structure functions, with Q being the virtual photon momentum. Quite often this dependence is claimed to be originated by the Q^2-dependence of the QCD coupling. This leads to the small-x asymptotics of the structure functions with Q^2 -dependent intercepts. We demonstrate that the DGLAP parametrization alpha_s = alpha_s (Q^2) is an approximation valid in the region of large x (where 2pq can be approximated by Q^2) only, providing the factorization scale is also large. Outside this region, the DGLAP parametrization fails, so alpha_s should be replaced by an effective coupling which is independent of Q^2 at small x. As a consequence, intercepts of the structure functions are independent of Q^2. Nevertheless, the small-x asymptotics of the structure functions explicitly depend on Q^2, even when the coupling does not depend on it. We also consider the structure functions at small Q^2 and give a comment on power-Q^2 corrections to the structure functions at large and small Q^2