Can sea quark asymmetry shed light on the orbital angular momentum of the proton?

TL;DR

This paper explores the potential link between sea quark asymmetry and proton orbital angular momentum, analyzing experimental data and model predictions to assess their proportionality and implications for understanding proton spin structure.

Contribution

It systematically compares model expectations with experimental data, highlighting the uncertainties and proposing methods to better constrain the relationship between sea asymmetry and angular momentum.

Findings

Data allows proportionality between proton angular momentum and sea asymmetry but with large uncertainties.

Current data cannot discriminate between different theoretical models.

Unquenched quark model predicts a larger quark spin contribution than conventional expectations.

Abstract

A striking prediction of several extensions of the constituent quark model, including the unquenched quark model, the pion cloud model and the chiral quark model, is a proportionality relationship between the quark sea asymmetry and the orbital angular momentum of the proton. We investigate to which extent a relationship of this kind is corroborated by the experiment, through a systematic comparison between expectations based on models and predictions obtained from a global analysis of hard-scattering data in perturbative Quantum Chromodynamics. We find that the data allows the angular momentum of the proton to be proportional to its sea asymmetry, though with a rather large range of the optimal values of the proportionality coefficient. Typical values do not enable us to discriminate among expectations based on different models. In order to make our comparison conclusive, the extrapolation uncertainties on the proportionality coefficient should be reduced, hopefully by means of accurate measurements in the region of small proton momentum fractions, where the data is currently lacking. Nevertheless, the unquenched quark model predicts that quarks account for a proton spin fraction much larger than that accepted by the conventional wisdom. We explicitly demonstrate that such a discrepancy can be reabsorbed in the unknown extrapolation region, without affecting the description of current data, by imposing the unquenched quark model expectation as a boundary condition in the analysis of the data itself. We delineate how the experimental programs at current and future facilities may shed light on the region of small momentum fractions.