Quarkonium Parton Shower in Herwig 7

TL;DR

This paper introduces a new automated quarkonium parton shower in Herwig 7, integrating NRQCD factorisation with spin-colour projections to improve simulation accuracy of heavy-quark bound states.

Contribution

It presents the first fully automated implementation of a quarkonium shower in Herwig 7, incorporating colour mechanisms, spin correlations, and feed-down effects based on NRQCD.

Findings

Improved agreement with LHC data.

Explicit derivation of splitting functions for various states.

Preservation of spin correlations and polarisation effects.

Abstract

We present the implementation of a fully automated quarkonium parton shower in Herwig 7, based on non-relativistic QCD (NRQCD) factorisation with spin-colour projections. The framework systematically incorporates colour-singlet and colour-octet production mechanisms, gluon fragmentation, and diquark production processes. Perturbative short-distance coefficients are combined with non-perturbative NRQCD matrix elements to simulate heavy-quark bound state formation. Splitting functions for $S$-, $P$- and $D$-wave states are explicitly derived and integrated into the angular-ordered shower evolution. The implementation preserves spin correlations and polarisation effects while accurately accounting for feed-down contributions. Results demonstrate improved agreement with the existing LHC data. This quarkonium parton shower will become publicly available with the release of Herwig-7.4.0.