The Colour-Dipole model and the Ariadne program at high Q^2

I present a modification of the Colour Dipole Model for DIS as implemented in Ariadne to better describe events at high Q^2.Comment: To be published in the proceedings of the HERA Monte Carlo workshop, Hamburg, Germany, 1998-199

Small-x Physics and Forward Jet Production at THERA

We discuss some aspects of forward jet production as a signature for small $x$ physics at THERA energies.Comment: to appear in THERA Boo

Fooling Around with the Sudakov Veto Algorithm

The Sudakov veto algorithm for generating emission and no-emission probabilities in parton showers is revisited and some reweighting techniques are suggested to improve statistics by oversampling in specific cases

Unitarising Matrix Element + Parton Shower merging

We revisit the CKKW-L method for merging tree-level matrix elements with parton showers, and amend it with an add/subtract scheme to minimise dependencies on the merging scale. The scheme is constructed to, as far as possible, recover the unitary nature of the underlying parton shower, so that the inclusive cross section is retained for each jet multiplicity separately.Comment: 30 pages, 13 figures, updated references, submitted to JHE

Multi-hadron Final States

This summary aims to highlight major results and insights gained from recent studies of hadronic final states in ep, ppbar, e+e-, as well as relevant theoretical developments, presented in the Multi-hadron final states parallel sessions of the DIS2002 workshop held in Krakow, Poland in May 2002.Comment: Contributed to the X International Workshop on Deep Inelastic Scattering (DIS2002), Krakow, Poland, 30 April - 4 May, 200

Hard Colour Singlet Exchange at the Tevatron

We have performed a detailed phenomenological investigation of the hard colour singlet exchange process which is observed at the Tevatron in events which have a large rapidity gap between outgoing jets. We include the effects of multiple interactions to obtain a prediction for the gap survival factor. Comparing the data on the fraction of gap events with the prediction from BFKL pomeron exchange we find agreement provided that a constant value of alpha_s is used in the BFKL calculation. Moreover, the value of alpha_s is in line with that extracted from measurements made at HERA.Comment: 22 pages, 19 figure

Merging Multi-leg NLO Matrix Elements with Parton Showers

We discuss extensions the CKKW-L and UMEPS tree-level matrix element and parton shower merging approaches to next-to-leading order accuracy. The generalisation of CKKW-L is based on the NL3 scheme previously developed for e+e- -annihilation, which is extended to also handle hadronic collisions by a careful treatment of parton densities. NL3 is further augmented to allow for more readily accessible NLO input. To allow for a more careful handling of merging scale dependencies we introduce an extension of the UMEPS method. This approach, dubbed UNLOPS, does not inherit problematic features of CKKW-L, and thus allows for a theoretically more appealing definition of NLO merging. We have implemented both schemes in Pythia8, and present results for the merging of W- and Higgs-production events, where the zero- and one-jet contribution are corrected to next-to-leading order simultaneously, and higher jet multiplicities are described by tree-level matrix elements. The implementation of the procedure is completely general and can be used for higher jet multiplicities and other processes, subject to the availability of programs able to correctly generate the corresponding partonic states to leading and next-to-leading order accuracy.Comment: updated references, submitted to JHE