RHIC Serves as World’s First & Only Collider
of Polarized Protons for
As the world’s first and only collider of spin-polarized
protons, the Relativistic Heavy Ion Collider is being employed to
investigate a fundamental question about an important particle and a
universal property: What is responsible for the “spin,” or intrinsic
angular momentum, of the proton? While data from this spring’s run are
being analyzed, unexpected results from RHIC’s first spin-physics run are
generating great interest.
by Marsha Belford
FOR THE SECOND TIME SINCE ITS COMMISSIONING IN 2000 as the world’s highest energy, heavy-ion collider, the Relativistic Heavy Ion Collider (RHIC) took a break from colliding gold ions in the attempt to recreate the conditions of the early universe — to serve again as the world’s first and only collider of spin-polarized protons.
Physicists use RHIC in this fashion to investigate a fundamental question about an important particle and a universal property: What is responsible for proton “spin”? A magnetic property of particles as basic as mass and electrical charge, spin is a particle’s intrinsic angular momentum. In spin-polarized proton beams, most of the protons are spinning in the same direction. By colliding beams of polarized protons at RHIC , physicists can examine the structure underlying the proton’s spin.
To turn RHIC into the world’s first and only collider capable of accelerating and colliding high-energy, spin-polarized protons took many years, much hardware, a dedicated staff, and a partnership with RIKEN, the Japanese Institute of Physical and Chemical Research.
While the second RHIC run of polarized protons () was conducted over eight weeks this spring, unexpected results from the collisions of polarized protons during the first run in fall-winter 2001-02 are generating great interest within the international spin-physics community.
From two separate experiments, spin physicists have discovered that there are large asymmetries in the production of two different particles resulting from the inelastic collisions of unpolarized protons with vertically spinning, or transversely polarized, protons. When the polarized protons’ spin points up, many more of these particles emerge to the left side of the beam than to the right. When the beam spin is down, the direction of the asymmetrical particle production is also reversed, so more particles emerge to the right than the left.
“In addition to RHIC’s remarkable success in its first year as a polarized proton collider, it is exciting that the spin-physics experiments have already seen such large asymmetries,” comments Brookhaven senior physicists Gerry Bunce, who leads the RHIC spin collaboration. “This is the beginning of a new and unique program probing proton spin structure.”
Meanwhile, another RHIC experiment, called pp2pp, has also produced results from the first run, but from its analysis of data from elastic collisions. The purpose of the pp2pp experiment is to understand how spin affects how protons scatter elastically, like billiard balls, via the nuclear, or strong, force, which is one of the four fundamental forces in nature. Analysis of pp2pp’s first run data has produced the first measurement of what is called the nuclear slope parameter b at the highest proton-proton collision energy so far.