Questions and prospects in quarkonium polarization measurements from proton-proton to nucleus-nucleus collisions

TL;DR

Quarkonium polarization measurements are vital for understanding quark-antiquark binding and the quark-gluon plasma, but current experimental and theoretical challenges hinder definitive conclusions, requiring refined analyses and new formalism.

Contribution

This paper discusses the challenges in quarkonium polarization measurements and introduces a frame-invariant formalism to improve analysis and interpretation.

Findings

Inconsistencies in existing experimental data.

Importance of considering azimuthal anisotropies and reference frame dependence.

Potential of polarization measurements to probe quark-gluon plasma properties.

Abstract

Polarization measurements are the best instrument to understand how quark and antiquark combine into the different quarkonium states, but no model has so far succeeded in explaining the measured J/psi and Upsilon polarizations. On the other hand, the experimental data in proton-antiproton and proton-nucleus collisions are inconsistent, incomplete and ambiguous. New analyses will have to properly address often underestimated issues: the existence of azimuthal anisotropies, the dependence on the reference frame, the influence of the experimental acceptance on the comparison with other measurements and with theory. Additionally, a recently developed frame-invariant formalism will provide an alternative and often more immediate physical viewpoint and, at the same time, will help probing systematic effects due to experimental biases. The role of feed-down decays from heavier states, a crucial missing piece in the current experimental knowledge, will have to be investigated. Ultimately, quarkonium polarization measurements will also offer new possibilities in the study of the properties of the quark-gluon plasma.