Unlocking the Mystery of Proton Spin

The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has just released new preliminary results for single spin asymmetries in 200GeV polarized proton collisions. What's a single spin asymmetry? Simply put, this is the difference in the probability for a particle produced in the collision of a spin-polarized proton to go to the left of the spin as opposed to the right of the spin.  These asymmetries give us an opportunity to probe the internal dynamics of the proton as a QCD bound state object.  The new PHENIX results are the work of Prof. John Lajoie and graduate student Feng Wei of the ISU Experimental Nuclear Physics group.

In these new measurements, the ISU researchers took advantage of a new trigger designed for the PHENIX experiment by the ISU group to select events with charged particles in the PHENIX forward sepctrometers. These particles originate from scattered partons (quarks and gluons) in a kinematic region that has been previously observed to be related to the spin the transversely polarized protons. Because the PHENIX forward spectrometers were originally designed to measure muons, and not charged hadrons, new analysis techniques needed to be developed to analyze the data.

The new results, shown at a recent joint meeting of the Japanese and American Physical Societies, show that the asymmetries become significant at smaller xF and larger transverse momentum than was measured by previous experiments. They are suggestive that that source of the asymmetries is a final state effect, namely how a spin polarized quark fragments into hadrons (known as the Collins effect), as opposed to a correlation between the spin of the proton and the transverse momentum of the partons in the proton (known as the Sivers effect).