Reweighting QCD matrix-element and parton-shower calculations
  We present the implementation and validation of the techniques used to efficiently evaluate parametric and perturbative theoretical uncertainties in matrix-element plus parton-shower simulations within the Sherpa event-generator framework. By tracing the full $\alpha_s$ and PDF dependences, including the parton-shower component, as well as the fixed-order scale uncertainties, we compute variational event weights on-the-fly, thereby greatly reducing the computational costs to obtain theoretical-uncertainty estimates. 
Enrico Bothmann, Marek Sch\"onherr, Steffen Schumann
hep-ph hep-ex