Spin Physics
RHIC has been the world's only machine capable of colliding high-energy beams of polarized protons, making it a unique tool for exploring the puzzle of the proton's “missing” spin.
In addition to colliding heavy ions, RHIC could collide single protons. These collisions were interesting to physicists for reasons other than the study of the quark-gluon plasma. Scientists wanted to know more about a property of particles called "spin."
Spin is a quantum property that is somewhat like the direction a particle is spinning around an axis as it travels — similar to the way Earth spins on its axis as it travels around the sun. Each proton has a specific spin, which helps give it a characteristic magnetic property.
In this picture of a proton-proton collision, the spins of the colliding particles are shown as arrows circling the spheres. The red and green particles emerging from the collision were “seen” and analyzed by RHIC detectors.
Proton beams in RHIC were “spin polarized,” meaning that the protons in a given beam were aligned to spin in the same direction. RHIC was the first machine in the world with the capability to collide such beams head-on.
The Importance of Spin
Why is proton spin important to understand? Astronomers studying the universe use proton spin and magnetism as important measuring properties. Spin is also what allows doctors to use an MRI (magnetic resonance imaging) machine to see inside the human body to diagnose disease.
But physicists don’t fully understand how different factors influence a proton’s spin. Experiments elsewhere have shown that the spins of the proton’s constituent quarks (and antiquarks), in some cases account for only about 30% of its total spin.
RHIC spin experiments provided key information on how much the spin of gluons contributes to the proton’s spin. These studies revealed that gluons contribute about as much as the quarks.
But the quark and gluon spins together still do not account for the proton’s total spin. The only remaining source available to “balance the books” is the movement of quarks and gluons relative to one another. RHIC’s measurements of the spin substructure of the proton suggest that understanding how quarks move inside protons and other particles is essential to solving the proton spin mystery. This will be a major research focus at the Electron-Ion Collider (EIC).
'Snake' Magnets
Keeping particle beams polarized as they made their way through the chain of five accelerators that made up the RHIC complex was extremely challenging. Specialized sets of magnets called “Siberian snakes” were used to minimize polarization loss.
The magnets’ corkscrew-like design caused the direction of the magnetic field to spiral along the direction of the beam. As the beam moved through two snakes in each of RHIC’s 2.4-mile-circumference rings, the magnetic field flipped the polarization, or direction of spin, and averaged out many smaller effects of RHIC’s magnets that could have depolarize the proton beams.
Over the course of RHIC operations, partial snake magnets were added inside the Alternating Gradient Synchrotron (AGS), one component of the chain of accelerators that fed beams into RHIC. This resulted in dramatic improvements in maintaining polarization and important discoveries about proton spin. Snakes will also be used to help maintain polarization in the EIC.
