The proton spin puzzle: where are we today ?

Understand

The proton spin puzzle has challenged our understanding of QCD for the last 20 years.

• New measurements of polarized glue, valence and sea quark polarization, including strange quark polarization, are available.
• What is new and exciting in the data, and what might this tell us about the structure of the proton ? The proton spin puzzle seems to be telling us about the interplay of valence quarks with the complex vacuum structure of QCD.