Part 3: Improving parton showers with fixed-order calculations a) Recap b) Dedicated improvements: Matrix element corrections and NLO matching c) Iterative improvements: Multi-jet merging

Recap of last lecture • QCD scattering cross sections factorise. • The factorisation can be cast into a probabilistic form suitable for a numerical implementation. • Parton showers tell us how the inclusive cross section is sliced up into exclusive objects, where exclusive means a fixed number of resolved jets. • Exclusive cross sections are defined through no-emission probabilities. • All cross sections can be writen as a polynomial of logarithms. • This log-structure can be illustrated on figures. • Systematic improvements of modern showers are possible due to local energy-momentum conservation. = ⇒ This lecture