Modelling of the vector boson scattering process $pp\rightarrow W^\pm W^\pm jj$ in Monte Carlo generators in ATLAS

Predicted cross sections for electroweak scattering of two same-sign $W$ bosons in association with two jets ($pp\rightarrow W^\pm W^\pm jj$) are compared for a number of Monte Carlo configurations. Fiducial cross sections and kinematic distributions calculated using Madgraph5_aMC@NLO, Powheg and Sherpa are presented. For Madgraph5_aMC@NLO and POWHEG, results are provided both with the parton shower modelled using Pythia 8 and using Herwig 7. Fiducial cross sections predicted using a variety of Madgraph5_aMC@NLO and Powheg configurations agree to better than 10%. For NLO configurations, these variations are larger than those obtained by changing the QCD renormalization and factorization scale and by using the eigenvariations provided with the Parton Distribution Function, indicating that such variations underestimate the systematic uncertainty. Fiducial cross sections predicted by Sherpa using a CKKW matched calculation with up to one additional parton are significantly lower than for the other generators. A comparison of the shapes of the distribution of the invariant mass and rapidity difference of the two leading jets shows that Sherpa and Powheg bracket the prediction obtained using Madgraph5_aMC@NLO. Differences less than 20% are observed. Such variations can be explained by differences in the parton shower model, which are studied in detail as well.