String Formation Beyond Leading Colour

TL;DR

This paper introduces a new hadronisation model that incorporates subleading colour effects and baryonic configurations, improving agreement with experimental data in both $e^+e^-$ and $pp$ collisions.

Contribution

The model extends string formation beyond leading colour by including multiplet structures and energy minimisation, implemented in PYTHIA 8.

Findings

Improves description of charged particle $p_ op$ distributions.

Accurately reproduces kaon and hyperon production rates.

Shows small modifications in $e^+e^-$ but significant effects in $pp$ collisions.

Abstract

We present a new model for the hadronisation of multi-parton systems, in which colour correlations beyond leading $N_C$ are allowed to influence the formation of confining potentials (strings). The multiplet structure of $SU(3)$ is combined with a minimisation of the string potential energy, to decide between which partons strings should form, allowing also for "baryonic" configurations (e.g., two colours can combine coherently to form an anticolour). In $e^+e^-$collisions, modifications to the leading-colour picture are small, suppressed by both colour and kinematics factors. But in $pp$ collisions, multi-parton interactions increase the number of possible subleading connections, counteracting their naive $1/N_C^2$ suppression. Moreover, those that reduce the overall string lengths are kinematically favoured. The model, which we have implemented in the PYTHIA 8 generator, is capable of reaching agreement not only with the important $\left<p_\perp\right>(n_\mathrm{charged})$ distribution but also with measured rates (and ratios) of kaons and hyperons, in both $ee$ and $pp$ collisions. Nonetheless, the shape of their $p_\perp$ spectra remains challenging to explain.