QCD matrix elements plus parton showers. The NLO case

We present a process-independent technique to consistently combine next-to-leading order parton-level calculations of varying jet multiplicity and parton showers. Double counting is avoided by means of a modified truncated shower scheme. This method preserves both the fixed-order accuracy of the parton-level result and the logarithmic accuracy of the parton shower. We discuss the renormalisation and factorisation scale dependence of the approach and present results from an automated implementation in the SHERPA event generator using the test case of W-boson production at the Large Hadron Collider. We observe a dramatic reduction of theoretical uncertainties compared to existing methods which underlines the predictive power of our novel technique.