Positive gluon polarization indicated by direct photons.
A new publication by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC) provides definitive evidence that gluon "spins" are aligned in the same direction as the spin of the proton they're in. The result provides theorists with new input for calculating how much gluons contribute to a proton's spin. The new PHENIX result is one of the "golden" measurements proposed as a key motivator for the RHIC spin physics program. It's a comparison of the number of "direct photons" (particles of light) emitted when RHIC collides protons with their spins pointing in opposite directions with the number of direct photons produced when the protons in the two beams are pointing in the same direction.