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.
Highlights
Parton showers are ubiquitous tools in high-energy collider physics
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
The two schemes that we develop, which are computationally efficient, will achieve NDL-FC and NLL-FC accuracy for multiplicities and global event shapes respectively, going beyond the accuracy of the scheme proposed in ref
Summary
It has become clear that differences between parton showers are among the limiting systematics in many collider physics applications. The majority of today’s most commonly used showers, in particular those of the dipole family [1], make use of the idea of approximating QCD as if it had a large number of colours (Nc). Within this approximation one can view each event as a collection of independent colour dipoles: each gluon in an event functions as the colour-triplet end of one dipole and the colour anti-triplet end of another, while each (anti-)quark is the colour (anti-)triplet end of a single dipole. This makes it relatively straightforward, at each stage of the showering, to generate a radiation pattern that is correct across small and large angles in the large-Nc limit
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