We compute the gluon distribution in deep inelastic scattering at small x by solving numerically the angular ordering evolution equation. The leading order contribution, obtained by neglecting angular ordering, satisfies the BFKL equation. Our aim is the analysis of the subleading corrections. Although not completethe exact next-to-leading contribution is not yet available -these corrections are important since they come from the physical property of coherence of QCD radiation. In particular we discuss the subleading correction to the BFKL characteristic function and the gluon distribution's dependence on the maximum available angle. Conformal invariance of the BFKL equation is lost, however this is not enough to bring the small-x gluon distribution into the perturbative regime: although large momentum regions are enhanced by angular ordering, the small momentum regions are not fully suppressed. As a consequence, the gluon anomalous dimension is finite and tends to the BFKL value γ = 1/2 for α S → 0. The main physical differences with respect to the BFKL case are that angular ordering leads to 1) a larger gluon anomalous dimension, 2) less singular behaviour for x → 0 and 3) reduced diffusion in transverse momentum.

This article presents the results of a quantitative study of the small-x data at HERA, using the CCFM equation. The first step consists of choosing the version of the CCFM equation to be used, corresponding to selecting a particular subset of next-to-leading-logarithmic corrections -the choice is constrained by requiring a phenomenologically reasonable small-x growth. For the time being, the parts of the splitting functions that are finite at z = 0 have been left out. We then examine results for F c 2 , R, the transverse energy flow, the charged-particle transverse-momentum spectrum and the forward-jet cross section and compare to data. While some of the data is reproduced better than with DGLAP-based calculations, the agreement is not entirely satisfactory, suggesting that the approach developed here is not yet suitable for detailed phenomenology. We discuss why, and suggest directions for future work.

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We examine the effect of using an angular ordered evolution equation for the unintegrated gluon distribution at small x.