Scientists hoping to unravel the mystery of proton spin at the Relativistic Heavy Ion Collider (RHIC) 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.
Illustration of a new measurement using W boson production in polarized proton collisions at RHIC. Collisions of polarized protons (beam entering from left) and unpolarized protons (right) result in the production of W bosons (in this case, W-). RHIC's detectors identify the particles emitted as the W bosons decay (in this case, electrons, e-) and the angles at which they emerge. The colored arrows represent different possible directions, which probe how different quark flavors (e.g., “anti-up,” ū; and “down,” d) contribute to the proton spin.
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.
Spin is used by a wide range of people, from astronomers studying the contents of the universe to doctors using an MRI (magnetic resonance imaging) machine to see inside the human body. But where spin comes from is still a mystery.
Physicists have long thought that the spin of a proton was simply the sum of the spins of its three component quarks. But experiments have shown that the quarks account for only about 25 percent of the proton’s spin. What accounts for the missing 75 percent? RHIC is the world’s only machine capable of colliding high-energy beams of polarized protons — a useful approach for investigating this question.
After beginning polarized proton collisions at RHIC late in 2001, scientists first looked for the missing spin in the gluons, the particles that hold a proton’s quarks together via the strong force. But so far, gluons have been found to contribute much less than originally speculated to proton spin.
Now, RHIC scientists have a new tool to guide their search. Thanks to new detection techniques and the ability to run polarized proton collisions at very high energies — 500 GeV, or 500 billion electron volts — scientists at both PHENIX and STAR are able to directly probe the polarization contributions from different flavored quarks (known by the names “up” and “down”) inside protons for the first time.
Read more about this new technique here.
-Karen McNulty Walsh, BNL Media & Communications
