Gribov L.V., Levin E.M., Ryskin M.G. Semihard Processes in QCD

Physics Reports (Review Section of Physics Letters) 100, Nos. 1 & 2 (1983) 1-
150. North-Holland Publishing Company.Abstract
Deep inelastic scattering, large p_t, particle production in hadron collisions and e⁺e^- annihilation into hadrons are studied in perturbative QCD in the semihard region of very small x and large Q² (p²). At small x the parton densities increase rapidly, so that the effects of parton-parton interactions (screenings), that stop the increase of the cross sections near their unitarity limit, and the coherent emission of soft gluons by the global colour charge of whole groups of harder particles, become essential. These effects are studied in detail.
It is shown that at very small x~x_b(Q²) (ln(1/x_b)~a_s^-2(Q²)) semihard processes acquire some features essential of soft processes. Their cross sections reach values comparable to the geometrical dimensions of hadrons. At very high energies gluon jets with large p_t.
(x~x_b) are emitted in abundance and become the main source of secondary hadrons.
In e⁺e^- annihilation the coherence of soft gluon emission leads to a dip in the inclusive spectra at small x. The colour correlations in jets and the preconfinement effects are discussed.
A convenient technique for calculating and summing leading log diagrams with two logarithmically large parameters (ln1/x and lnQ²) is presented. A reggeon-type diagram technique is developed to take into account parton-parton interactions.
A special chapter is devoted to the phenomenology of semihard processes, in particular to large p_t production at the SPS Collider energy and to large E_t processes.
The possible applications of perturbative QCD to soft processes are sketched.