Unitarity and the QCD-improved dipole picture

As a consequence of QCD factorization theorems, a wide variety of inclusive and exclusive cross sections may be formulated in terms of a universal col our dipole cross section at small x. It is well known that for small transv erse size dipoles this cross section is related to the leading-log gluon de nsity. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transve rse sire region. We point out that the observed magnitude and small x rise of the gluon density from conventional fits implies that the DGLAP approxim ation has a restricted region of applicability. We found that 'higher twist ' or unitarity corrections are required in, or close to, the HERA kinematic region, even for small 'perturbative' dipoles for scattering at central im pact parameters. This means that the usual perturbative leading twist descr iption, for moderate virtualities, 1 < Q(2) < 10 GeV2 has rather large 'hig her twist' corrections at small x. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles (b gre ater than or similar to 0.4 fm) to F-2, and also to F-L. This also leads to deviations from the standard leading twist DGLAP results, at small x and m oderate Q(2). Our model also describes the low Q(2) data very well without any further tuning; We generalize the Gribov unitarity limit for the struct ure functions of a hadron target to account for the blackening of the inter action at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.