Department of Physics & Astronomy

Nucleon Spin Physics

Participating Faculty

Prof. K. Barish

Where does the spin of the proton come from?

By some mysterious mechanism, having to do with the complex behavior of QCD, the gluons which have one unit of spin and quarks which have 1/2 a unit of spin, bind together to form something that has exactly 1/2 of a unit of "spin".

Spin is a property of particles as fundamental as charge and mass. The spin of the proton was first determined in 1927, yet we still do not know what makes up the spin of the proton. It was believed that the spin was carried by the quarks that make up the proton. However, experiments in the 1980’s led to the startling discovery that quarks contribute very little to the proton spin, setting off the “proton spin crisis”. It is now theorized that the spin is carried by gluons, which hold the proton together. Spin measurements have historically yielded surprising results and are a stringent test to theories as spin is an intrinsically relativistic and quantum mechanical aspect of particle interactions.

The spin physics program at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory offers the first chance to test this hypothesis experimentally. RHIC can collide polarized protons in addition to heavy-ions (see the seperate UC Riverside heavy-ion physics page). The UC Riverside spin and heavy-ion physics groups work together closely.

Our group is part of the PHENIX collaboration, which is one of the two major detectors at RHIC. This detector is originally optimized for heavy-ion collisions, but the UC Riverside group has helped improve its capabilities for he spin physics program by developing the triggers necessary to take advantage of the high luminosities available in p+p collisions at RHIC. Ken Barish is a convenor of the PHENIX spin physics working group.

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