Nucleon Spin Asymmetries: Are Quarks Spinning In the Nucleon as We Expected?

Do new data on the nucleon spin agree with what we expected in the high x_bj region? Left: Neutron spin asymmetry A1n; Right: Spin directions of quarks inside the nucleon.

Since the "proton spin crisis" in the late 1980s, it is now accepted that a nucleon's spin comes from not only the spin of its constituent quarks, but also from the quarks' orbital angular momentum (how they move inside the nucleon) and the gluons' angular momentum. However, how these three parts add up to form the nucleon's spin of 1/2 is unclear. Combining recent studies of neutron spin structure with existing proton data, it has been found that quarks may not be spinning in the way physicists expected.