2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 16, 641-656
DOI 10.1007/s100520000448

Unitarity and the QCD-improved dipole picture

M. McDermott1 - L. Frankfurt2 - V. Guzey3,4 - M. Strikman3

1 Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
2 Nuclear Physics Department, School of Physics and Astronomy, Tel Aviv University, 69978 Tel Aviv, Israel
3 Department of Physics, Penn State University, University Park, PA 16802-6300, USA
4 Special Research Centre for the Subatomic Structure of Matter (CSSM), University of Adelaide, 5005, Australia

Received: 22 March 2000 / Revised version: 16 May 2000 /
Published online: 6 July 2000 - © Springer-Verlag 2000

Abstract
As a consequence of QCD factorization theorems, a wide variety of 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 the DGLAP approximation 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 impact parameters. This means that the usual perturbative leading twist description, for moderate virtualities, 1 < Q2 < 10 GeV2, has rather large `higher twist' corrections at small x. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles ( $b \gtrsim0.4$ fm) to F2, and also to FL. This also leads to deviations from the standard leading twist DGLAP results, at small x and moderate Q2. Our model also describes the low Q2 data very well without any further tuning. We generalize the Gribov unitarity limit for the structure functions of a hadron target to account for the blackening of the interaction at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.


Copyright Società Italiana di Fisica, Springer-Verlag 2000