Quarkonium production and polarization: where do we stand?

TL;DR

This paper reviews the current understanding of heavy quarkonium production and polarization, highlighting the variability in proposed mechanisms and discussing potential observables to distinguish among them.

Contribution

It provides a comprehensive overview of quarkonium production models, emphasizing the role of polarization and introducing new observables for future discrimination.

Findings

Production mechanisms vary significantly with data choices.

Polarization helps discriminate between different models.

Additional observables like $ ext{η}_c$ production are promising.

Abstract

We review the current status of heavy quarkonium production phenomenology based on nonrelativistic effective field theories, focusing on spin-triplet $S$-wave states such as $J/\psi$, $\psi(2S)$, and $\Upsilon$. We present some representative examples for heavy quarkonium production mechanisms proposed in the literature, which vary significantly depending on the choice of data employed in analyses. We then discuss the r\^{o}le of polarization in discriminating between the different possible scenarios for quarkonium production. Other observables that may be useful in pinpointing the production mechanism are also introduced, such as the $\eta_c$ production, associated production of $J/\psi$ plus a gauge boson, and $J/\psi$ production at the Electron-Ion Collider.