Unique new probe of proton spin structure at RHIC

Scientists hoping to unravel the mystery of proton spin at the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference particle accelerator at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, have a new tool at their disposal - the first to directly explore how quarks of different types, or ''flavors,'' contribute to the overall spin of the proton. The technique, described in papers just published by RHIC's STAR and PHENIX collaborations in Physical Review Letters, relies on the measurement of particles called W bosons, the mediators of the weak force responsible for the decay of radioactive nuclei.

''Exploring the mystery of proton spin has been one of the key scientific research goals at RHIC,'' said Steven Vigdor, Brookhaven's Associate Laboratory Director for Nuclear and Particle Physics. ''Like many scientific mysteries, this one turns out to be more complex the more we learn about it. The W boson measurements were enabled by new detection techniques at RHIC's STAR and PHENIX experiments and by extending RHIC's world-record energies for the acceleration of proton beams with a distinct spin orientation preference. The results will allow us to tease apart subtle details that were previously inaccessible, and should move the field closer to a quantitative understanding of proton spin structure and dynamics.''

Spin is a quantum property that describes a particle's intrinsic angular momentum. Like charge and mass, it's part of a particle's identity, whose magnitude is the same for all particles of a given type. 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 critically 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 experiments have confirmed that a directional preference among all these quark spins can account for only about 25 percent of the proton's total spin. RHIC was built with the ability to collide polarized protons - protons whose spins could be aligned in a controlled way - so scientists could probe other factors that might account for the ''missing'' spin. Much of the equipment needed to realize this unique capability was provided by the RIKEN Institute of Physical and Chemical Research of Japan, whose researchers form a critical part of the international collaborations carrying out this work.

After beginning polarized proton collisions at RHIC late in 2001, the first place the scientists looked for the missing spin was the gluons, the particles that hold a proton's quarks together via the strong force.

''The shock so far has been that we haven't found gluons carrying much of the spin,'' said PHENIX spokesperson Barbara Jacak, a physicist at Stony Brook University. Measurements from the STAR detector agree. After several polarized proton runs at various energies, RHIC data suggest with more and more certainty that gluons contribute much less than originally speculated to proton spin, so the source of the spin still remains a mystery.