In this letter we report the direct perturbative QCD evaluation of twist-4 effects in diffractive DIS. They are large and have a strong impact on the Q 2 dependence of diffractive structure functions at large β. Based on the AGK rules, we comment on the possible contribution from diffractive higher twists to ∝ 1 Q 2 corrections to proton structure function at small x. These corrections to the longitudinal structure function F L may be particularly large.

This document describes the first proof-of-concept version of the Pythia7 program.Pythia7 is a complete re-write of the Pythia program in C++. It is mainly intended to be a replacement for the 'Lund' family of event generators, but is also a toolkit with a structure suitable for implementing any event generator model.In this document, the structure of the program is presented both from the user and the developer point of view. It is not intended to be a complete manual, but together with the documentation provided in the distribution, it should be sufficient to start working with the program.

We show that one single Pomeron compatible with the Froissart limit, can account for all the present HERA data.

We discuss some properties of the Pomeron in high energy elastic hadron-hadron and deep inelastic lepton-hadron scattering. A number of issues concerning the nature and the origin of the Pomeron are briefly recalled here. The novelty in this paper resides essentially in its presentation; we strive at discussing all these various issues in the following unifying perspective : it is our contention that the Pomeron is one and the same in all reactions. Various examples will be provided illustrating why we do not believe that one should invoke additional tools to describe the data.