On the maximal use of Monte Carlo samples: re-weighting events at NLO accuracy

Accurate Monte Carlo simulations for high-energy events at CERN's Large Hadron Collider, are very expensive, both from the computing and storage points of view. We describe a method that allows to consistently re-use parton-level samples accurate up to NLO in QCD under different theoretical hypotheses. We implement it in MadGraph5_aMC rate at NLO and show its validation by applying it to several cases of practical interest for the search of new physics at the LHC. (orig.)