Parton distributions and small X physics

Abstract

We determine the parton distributions of the pion from a consistent next-to-leading order analysis of several high statistics π±N experiments including both Drell-Yan and prompt photon production. The results are compared with earlier distributions and with the predictions of lattice QCD. We study the general behaviour of parton distributions at small x and, in particular, the predictions of the Lipatov equation. The very-small-a; behaviour of the gluon distribution in a proton is obtained by solving this equation with a suitable nonlinear shadowing term incorporated. We find, with decreasing x, the emergence of an x(^- λ) behaviour one? the eventual taming of this singular behaviour by the shadowing term. We compare our dynamically generated gluon distribution with (i) the results of a recent next-to-leading order QCD structure function analysis which incorporated both a singular behaviour and shadowing corrections, (ii) the double-leading-leading logarithm approximation (DLLA) and (iii) the semiclassical approximation of the DLLA. Finally, we examine the proposal that deep-inelastic scattering events which contain an identified jet, with transverse momentum squared k(^2) ~Q(^2) allow an ideal determination of the QCD behaviour at very small x. We solve the relevant Lipatov equation to predict the shape of the jet spectrum in such events and discuss whether it will be possible to observe such jets at the HERA collider.