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In the naïve quark model, the properties of the proton are determined by its three valence quarks. However, in the mid-1980s, polarized deep-inelastic scattering measurements by the European Muon Collaboration found that quark spins contribute very little to the spin of the proton. This proton “spin crisis†was the initial motivation to establish a spin physics program at the Brookhaven Relativistic Heavy Ion Collider (RHIC). During the past 20 years, the STAR and PHENIX experiments at RHIC have performed a sequence of measurements to determine the contributions of gluon and anti-quark spins to the spin of the proton. We have discovered that gluons carrying 5% or more of the proton momentum are polarized, and they make a larger contribution to the proton spin than quarks do. We have also discovered that there is an asymmetry between the ð‘¢Ì… and ð‘‘Ì… contributions to the proton spin that is opposite in sign to the famous ð‘‘Ì…/ð‘¢Ì… asymmetry in the unpolarized sector. This talk will describe how polarized proton collisions at RHIC elucidate the partonic origin of the proton spin, and what they have taught us. Host: Ming Liu (mliu@lanl.gov) |