Comix, a new matrix element generator

Abstract: We present a new tree-level matrix element generator, based on the recently introduced colour dressed Berends-Giele recursive relations. We discuss two new algorithms for phase space integration, dedicated to be used with large multiplicities and colour sampling.

Local Impurity Phase Pinning and Pinning Force in Charge Density Waves

Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional incommensurate charge density wave (CDW) in quasi one-dimensional conductors a solvable model for local phase pinning by impurities is defined and studied. We find that average CDW energy and average pinning force show critical behaviour with respect to the pinning parameter $h$. Specifically the pinning force exhibits a threshold at $h=1$ with exponent $\beta=2$. Our model examplifies a general concept of local impurity pinning in which the force exerted by the impurity on the periodic CDW structure becomes multivalued and metastable states appear beyond a threshold. It is found that local impurity pinning becomes less effective at low temperatures and may eventually cease completely. These results are independent of spatial dimensionality as expected for local impurity pinning. Comparison with Larkin's model is also made.Comment: Latex, 16 pages, 3 figure

Event Generator for Particle Production in High-Energy Collisions

Event generators are an indispensable tool for the preparation and analysis of particle-physics experiments. In this contribution, physics principles underlying the construction of such computer programs are discussed. Results, within and beyond the Standard Model of particle physics, obtained with a new event generator are presented. This generator is capable to describe signal processes for exotic physics and their backgrounds at electron-positron and proton-(anti)proton colliders.Comment: 10 pages, 13 figures. Talk given by G. Soff at the 25th Course of the International School on Nuclear Physic