A general algorithm to generate unweighted events for next-to-leading order calculations in electron-positron annihilation

Given a next-to-leading order calculation, we show how to set up a computer program, which generates a sequence of unweighted momentum configurations, each configuration containing either n or n+1 four-vectors, such that for any infrared safe observable the average over these configurations coincides with the NLO calculation up to errors of order y_res. The core of the algorithm is a method to combine real emission and virtual corrections on a point-by-point basis in hard phase space. The algorithm can be implemented on top of existing NLO calculations.Comment: 19 pages, published versio

Automated computation of spin- and colour-correlated Born matrix elements

I report on an implementation of an algorithm for the automated numerical calculation of spin- and colour-correlated Born matrix elements in QCD. These spin- and colour-correlated matrix elements are needed for NLO calculations in combination with the subtraction method. Both massless and massive quarks are considered. There are no restrictions on the number of external particles. As a trivial sub-case, the algorithm also applies to Born matrix elements without any correlations. These are sufficient for leading order calculations.Comment: 28 page