Spinning protons and gluons in the $\eta'$

Abstract

The proton spin puzzle has inspired a vast programme of experiments and theoretical work challenging our understanding of QCD and its role in the structure of hadrons.The proton's internal spin structure is connected to chiral symmetry and, through gluon degrees of freedom in the flavour singlet channel, to the physics of the $\eta'$ meson. Why do quarks contribute just about one third of the proton's spin? Why are $\eta'$ mesons and their interactions so sensitive to gluonic degrees of freedom? We review the status of these topics and some key observables for forthcoming experiments.