Soft spin correlations in final-state parton showers

TL;DR

This paper presents a method to incorporate spin-correlation effects in parton showers across both collinear and soft wide-angle limits, validated against fixed-order calculations and introducing a new observable for testing these effects.

Contribution

It introduces a simple procedure for including single-logarithmic spin correlations in final-state parton showers beyond the collinear limit, completing the PanScales framework.

Findings

Validated approach up to third order in strong coupling

Demonstrated small residual subleading-colour effects

Introduced a new spin-sensitive non-global observable

Abstract

We introduce a simple procedure that resolves the long-standing question of how to account for single-logarithmic spin-correlation effects in parton showers not just in the collinear limit, but also in the soft wide-angle limit, at leading colour. We discuss its implementation in the context of the PanScales family of parton showers, where it complements our earlier treatment of the purely collinear spin correlations. Comparisons to fixed-order matrix elements help validate our approach up to third order in the strong coupling, and an appendix demonstrates the small size of residual subleading-colour effects. To help probe wide-angle soft spin correlation effects, we introduce a new declustering-based non-global spin-sensitive observable, the first of its kind. Our showers provide a reference for its single-logarithmic resummation. The work in this paper represents the last step required for final-state massless showers to satisfy the broad PanScales next-to-leading logarithmic accuracy goals.