Measurements of Delta G/G

TL;DR

This paper reviews current measurements of the gluon polarization Delta g/g from fixed-target and collider experiments, highlighting that recent data suggest a smaller gluon contribution to nucleon spin than previously thought.

Contribution

It provides a comprehensive overview of experimental results on Delta g/g from various scattering processes and collider data, updating the understanding of gluon polarization.

Findings

Gluon polarization Delta g/g appears smaller than earlier estimates.

Recent collider data from RHIC provide the most precise measurements.

Gluons may still significantly contribute to nucleon spin despite small polarization.

Abstract

Our present information on the gluon polarisation Delta g/g is reviewed. The data from fixed-target lepton-nucleon experiments are in context with the recent data from the RHIC polarised pp collider. The main tools to study Delta g/g in lepton-nucleon scattering are scaling violations of the g_1 structure functions and longitudinal spin asymmetries in hadron production. Results from high-p_T hadron pairs, inclusive hadrons as well as open-charm production are discussed. At RHIC the most precise data presently came from inclusive pi^0 and jet production. All data indicate that the gluon polarisation is small compared to earlier expectations, but still can make a major contribution to the nucleon spin.