Multi-parton cross sections at hadron colliders

Abstract

We present an alternative method to calculate cross sections for multi-parton scattering processes in the Standard Model at leading order. The helicity amplitudes are computed using recursion relations in the number of particles, based on Dyson-Schwinger equations whereas the summation over colour and helicity configurations is performed by Monte Carlo methods. The computational cost of our algorithm grows asymptotically as 3 n , where n is the number of particles involved in the process, as opposed to the n!-growth of the Feynman diagram approach. Typical results for the total cross section, differential distributions of invariant masses and transverse momenta of partons are presented and cross checked by explicit summation over colours.

Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

“…• HELAC: The parton-level matrix elements were generated with HELAC [35,36] and the phase space generation is performed by PHEGAS [49]. The subsequent evolution used the default virtuality-ordered shower in PYTHIA 6.4 [34] according to the MLM procedure.…”

Helac - A Monte Carlo Generator for Multi-Jet Processes

“…For the phase space generation we used the algorithm described in Ref. [7], whereas in all cases results were cross checked with PHEGAS [12], HAAG [13] and RAMBO [14]. In Tab.1 the results for the total cross section for processes with gluons are presented.…”

Recursive generation of one-loop amplitudes in the Standard Model

“…The tree-level recursive algorithm is inspired by the Dyson-Schwinger equations [63][64][65] and follows closely the strategy of Helac [66][67][68], using off-shell currents as basic building blocks.…”

“…[66], where off-shell currents would carry explicit colour indices and would have to be computed for each index separately, slowing down the calculation considerably. Although the number of colour-dressed amplitudes to be computed can be decreased by a Monte Carlo sampling over colour configurations [67], the number of operations at intermediate steps remains large. In order to optimise the colour treatment further, we developed an alternative approach based on "structure-dressed" amplitudes, where each current gets an explicit colour structure.…”