/ 99 12 54 7 v 1 3 0 D ec 1 99 9 Unitarity and the QCD - improved dipole picture

As a consequence of QCD factorization theorems, a wide variety inclusive and exclusive cross sections may be formulated in terms of a universal colour dipole cross section at small-x. It is well known that for small transverse-size dipoles this cross section is related to the leading-log gluon density. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transverse-size region. We point out that the observed magnitude and small-x rise of the gluon density from conventional fits implies that unitarity corrections are required in, or close to, the HERA kinematic region, even for small ‘perturbative’ dipoles. This means that the usual perturbative leading twist description, for moderate virtualities, 1 < Q < 10 GeV, has rather large ‘higher-twist’ corrections at small x. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles (b ∼ > 0.4 fm) to F2, and also to FL. This also leads to deviations from the standard leading-twist DGLAP results, at small-x and moderate Q.