Abstract
We discuss and illustrate the properties of several parton-shower algorithms available in Pythia and Vincia, in the context of Higgs production via vector boson fusion (VBF). In particular, the distinctive colour topology of VBF processes allows to define observables sensitive to the coherent radiation pattern of additional jets. We study a set of such observables, using the Vincia sector-antenna shower as our main reference, and contrast it to Pythia's transverse-momentum-ordered DGLAP shower as well as Pythia's dipole-improved shower. We then investigate the robustness of these predictions as successive levels of higher-order perturbative matrix elements are incorporated, including next-to-leading-order matched and tree-level merged calculations, using Powheg Box and Sherpa respectively to generate the hard events.
Highlights
Higgs boson production via Vector Boson Fusion (VBF) — fig. 1 — is among the most important channels for Higgs studies at the Large Hadron Collider (LHC)
We have here studied the effect of QCD radiation in VBF Higgs production, focusing in particular on how the coherent emission patterns exhibited by this process are modelled by various parton-shower approaches that are available in the PYTHIA event generator, and how significant the corrections to that modelling are, from higher fixed-order matrix elements
From a QCD point of view, the main hallmark of VBF is that gluon emission in the central region originates from intrinsically coherent interference between initial- and final-state radiation
Summary
Higgs boson production via Vector Boson Fusion (VBF) — fig. 1 — is among the most important channels for Higgs studies at the Large Hadron Collider (LHC). Since we are primarily interested in exploring the coherence properties of the perturbative stages of the event simulation, most of the results will be at the so-called “parton level”, i.e. without accounting for non-perturbative or non-factorisable effects such as hadronisation, primordial kT, or multi-parton interactions (MPI) This is not directly comparable to physical measurements (nor is the definition universal since different shower algorithms define the cutoff differently), the factorised nature of the infrared and collinear safe observables we consider imply that, while non-perturbative effects may act to smear out the perturbative differences and uncertainties, they would not in general be able to obviate them, 2We do note that a technical (but due to the use of incoherent IF kinematics unphysical) fix was introduced in PYTHIA 8.242 and is planned to be re-implemented in a future version of PYTHIA 8.3. With jet pT values going down to 25 GeV and HT being sensitive to the overall amount of energy scattered into the central region, we include further comparisons illustrating the effect of non-perturbative corrections at the end of section 3
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