On the Singular Behaviour of Structure Functions at Low X

Abstract

We discuss the phenomenological extraction of the leading $j$-plane singularity from singlet structure functions $F_s$ measured at small $x.$ Using a saddle-point method we show that $\mathrm{dln}{F}_s/\mathrm{dln}\frac{1}{x}$ is a suitable observable for this purpose in the region $x\le 10^{-2}.$ As an application, we confront and distinguish in a model-independent way structure function parametrizations coming from two different QCD evolution equations, namely the Lipatov (or BFKL) equation and the Gribov-Lipatov -Altarelli-Parisi (or GLAP) equation. Recent results at HERA are discussed in this framework.

Comments: 14 pages with 3 figures, all in a compressed uuencoded postscript file, see instructions at the beginning of the file. If you cannot print postscript files, ask for a paper copy. SACLAY preprint T94/012