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Exploring the Mysteries of Spin

Illustration of a proton-proton collision

Illustration of a proton-proton collision

At RHIC, scientists are also hoping to unravel the mystery of proton spin. Spin, a quantum property that describes a particle's intrinsic angular momentum, is part of a particle’s identity like charge and mass. But unlike charge and mass, spin has a direction that can be oriented differently for individual particles of a given species. The interactions among particles inside atoms, nuclei, and protons depend on their relative spin orientations, with influence on a wide range of electrical, magnetic, optical, and other properties of matter. Yet, despite the fact that proton spin is used in everyday applications like magnetic resonance imaging (MRI), exactly how, and how much the individual particles that make up protons, contribute to spin remains a mystery.

Scientists know that the quarks inside a proton each have their own intrinsic spin. But numerous experiment’s have confirmed that a directional preference among all these quark spins can account for only about 25 percent of the proton’s total spin. Scientists at RHIC are trying to discover other factors that account for the total spin. New detection techniques and the ability to collide polarized (i.e. spin-aligned) proton beams at very high energies at RHIC will allow for directly probing the polarization contributions from different types of quarks inside protons as well as from the gluons, which carry the forces binding the quarks together and also have intrinsic spins. Extending RHIC’s world-record energies for the accelerations of proton beams should move scientists closer to a quantitative understanding of proton spin and dynamics.

2011-2737  |  INT/EXT  |  Media & Communications Office

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