MC@NLO event generation by reweighting unweighted Born events

Abstract

We propose a computational strategy for NLO+PS simulations in the MC@NLO framework that starts from Born-accurate (LO) events and reweights them to the full MC@NLO S-event weight, while generating H-events separately. We validate the approach on two representative LHC processes and compare to direct NLO event generation for both standard MC@NLO and MC@NLO-Delta matching. Employing large folding values in the radiative variables stabilizes the S-event integral, reduces weight variance, and significantly lowers the fraction of negative weights compared to S-event generation without folding. At fixed precision, this pipeline has comparable wall-clock times relative to standard S-event generation and unweighting, with room for further optimisation.