Image: Courtesy BNL

When it comes to a proton's spin, the whole is more than the sum of its quarks. Each proton contains three quarks, the spins of which can only account for about 20 percent of the proton's spin. But new experiments using Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) may help untangle the problem of the proton's missing spin by further investigating the properties of gluons, the particles that hold quarks together and carry the so-called strong force. According to team member Naohito Saito, the scientists "hope to gain a better understanding of the structure of matter and the strong force that holds the components of protons together."

Using newly built magnetic coils called Siberian snakes (see image), Saito and colleagues created two beams of polarized protons and moved them through RHIC's five accelerators. "These will be the first-ever experiments where the protons in two colliding beams are all spinning in a controlled direction," says Thomas Roser, head of the Brookhaven accelerator group.

In the next phase of research, scheduled to begin this week, the investigators will smash the two beams into one another. By analyzing the collision products, the scientists can investigate impacts between a quark in one beam and a gluon in the other. "Unlike previous experiments," physicist Gerry Bunce of Brookhaven National Laboratory explains, "collisions of spin-aligned protons at RHIC will allow us to tease apart the individual contributions of both the quarks and the gluons." And that may reveal the source of the mysterious spin.