Credit: Brookhaven National Laboratory
At RHIC, collisions of protons polarized transverse to their direction of travel (say, with the polarization axis facing up) with an unpolarized proton beam (where the proton spins can be pointing in any direction) result in an imbalance in the probability for a particle created in the collision to go to the left versus to the right. Run 15 will help physicists explore the source of this phenomenon, which could be connected to the transverse momentum of the quarks and gluons inside the proton.