Unitarising Matrix Element + Parton Shower merging

TL;DR

The paper revisits LO merging of tree-level matrix elements with parton showers and identifies unitarity gaps in CKKW-L due to merging-scale dependencies. It introduces UMEPS, an add/subtract scheme that preserves inclusive cross sections while using weighted multi-jet MEs to induce resummation for lower-multiplicity states. Results for W-boson and dijet production show reduced merging-scale sensitivity and competitive data agreement, albeit with negative-weight events that increase statistical fluctuations. The approach builds on CKKW-L infrastructure (Pythia8) and opens a path toward NLO/NNLO extensions, potentially yielding more robust multi-jet predictions at the LHC.

Abstract

We revisit the CKKW-L method for merging tree-level matrix elements with parton showers, and amend it with an add/subtract scheme to minimise dependencies on the merging scale. The scheme is constructed to, as far as possible, recover the unitary nature of the underlying parton shower, so that the inclusive cross section is retained for each jet multiplicity separately.