QCD coherence in the structure function and associated distributions at small x^{★}
Abstract
We recall the origin of angular ordering in soft parton emission and show that at small x this coherent structure is masked in the structure function while it can be detected in the associated distributions. This is due to the fact that collinear singularities cancel completely in the structure function at fixed transverse momentum for x → 0. In this limit the dependence on the hard scale is lost, the angular ordered region becomes equivalent to the multiRegge region in which all transverse momenta are of the same order, and one derives the BFKL equation. For the associated distributions at small x such a complete cancellation of collinear singularities does not hold in general, thus large singular contributions are neglected if angular ordering is replaced by multiRegge phase space. The deduction of these features requires an analysis without any collinear approximations which is done by extending to small x the soft gluon factorization techniques typically used in the region of large x. Since the coherent structure of parton emission is the same in the small and large x regions, one can formulate a unified evolution equation for the structure function, a unified coherent branching and jet algorithm which allows the calculation of associated distributions in all x regions. Such a unified formulation, valid for all x, is presented and compared with usual treatments.
 Publication:

Nuclear Physics B
 Pub Date:
 February 1995
 DOI:
 10.1016/05503213(95)00149M
 arXiv:
 arXiv:hepph/9412327
 Bibcode:
 1995NuPhB.445...49M
 Keywords:

 High Energy Physics  Phenomenology
 EPrint:
 LaTeX file of 27 pages