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

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

The Linked Dipole Chain Monte Carlo

We present an implementation of the Linked Dipole Chain model for deeply inelastic ep scattering into the framework of the Ariadne event generator. Using this implementation we obtain results both for the inclusive structure function as well as for exclusive properties of the hadronic final state. (The original publication was based on results from an implementation containing an error. In this revised version this error has been corrected, some of the beyond leading-log assumptions have been revised and so have some of the results.)Comment: 26 page

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

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

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

Monte Carlo Generators and the CCFM Equation

We discuss three implementations of the CCFM evolution equations in event generator programs. We find that some of them are able to describe observables such as forward jet rates in DIS at HERA, but only if the so-called consistency constraint is removed. We also find that these results are sensitive to the treatment of non-singular terms in the gluon splitting function.Comment: 4 pages, 3 figures. Talk presented at the DIS 2000 workshop, Liverpool, England, April 200

Summary of activities in the Working Group on QCD Cascades of the HERA Monte Carlo workshop

We summarize the activities in working group 10 concerned with QCD cascades, and find that although much work still needs to be done, much progress was made during this workshop in understanding the merits and deficiencies of different programs.Comment: To be published in the proceedings of the HERA Monte Carlo workshop, Hamburg, Germany, 1998-199