Colour and logarithmic accuracy in final-state parton showers

Abstract

Standard dipole parton showers are known to yield incorrect subleading-colour contributions to the leading (double) logarithmic terms for a variety of observables. In this work, concentrating on final-state showers, we present two simple, computationally efficient prescriptions to correct this problem, exploiting a Lund-diagram type classification of emission regions. We study the resulting effective multiple-emission matrix elements generated by the shower, and discuss their impact on subleading colour contributions to leading and next-to-leading logarithms (NLL) for a range of observables. In particular we show that the new schemes give the correct full colour NLL terms for global observables and multiplicities. Subleading colour issues remain at NLL (single logarithms) for non-global observables, though one of our two schemes reproduces the correct full-colour matrix-element for any number of energy-ordered commensurate-angle pairs of emissions. While we carry out our tests within the PanScales shower framework, the schemes are sufficiently simple that it should be straightforward to implement them also in other shower frameworks.

Four-lepton production in gluon fusion at NLO matched to parton showers

“…Thus, the default scheme yields a better description of the logarithmically enhanced region, while it also overpopulates the hard region of the spectrum. A detailed discussion of the logarithmic accuracy of the parton shower goes beyond the purposes of this article and the choice of the recoil scheme has important implications at higher logarithmic orders [74][75][76][77][78][79]. However, since the choice of the recoil scheme only affects our predictions beyond the claimed accuracy, a comparison of the two options available in PYTHIA 8 should help assess the size of the total theoretical uncertainty.…”

Dynamical Grooming meets LHC data

“…Regarding the Sudakov safe observables, we would like to understand the feasibility of the dedicated matching scheme proposed in this paper and, specifically, quantify its dependence on the adhoc cut off. Finally, it would be insightful to compare these analytic results for dynamically groomed observables to the newly developed parton showers that aim at achieving perturbative control beyond leading double logarithmic accuracy and leading color [35,36,61]. From an experimental perspective, these theoretical efforts would highly benefit from a high-p t measurement where non-perturbative corrections are deemed to be milder.…”

“…where the c nm coefficients have to be determined. Then, we adopt the following convention [35,36]: the logarithmic accuracy of Σ(κ (b,c) ) is said to be N p DL if the c nm coefficients are known for all n and 2n − p ≤ m ≤ 2n. Notice that in many other jet substructure calculations it is customary to define the logarithmic accuracy at the level of ln Σ instead of on the cumulative distribution itself.…”

Hadronic decays of Higgs boson at NNLO matched with parton shower

“…However, they can certainly help with evaluations of the physics performances of the detector designs which are under investigation. On another hand, with recent progresses on developments of parton shower MCs [101][102][103][104][105][106][107] and given the available matrix elements at higher orders [37], we expect further improvements on modeling of hadronic decays of the Higgs boson in the near future.…”