The Brookhaven National Laboratory’s Relativistic Heavy-Ion Collider (RHIC) collided its first polarized protons last November. Since its start-up in 2000, the RHIC research programme has concentrated on the physics of heavy-ion collisions. Polarized protons open up a new avenue of research into the spin structure of nucleons.
In 1988, CERN’s European Muon Collaboration announced that quarks alone could not account for the spin of the nucleon. Since then, experiments at CERN, Hamburg’s DESY laboratory and SLAC in California have progressively pinned down this phenomenon, attributing the missing spin partly to the gluons that bind quarks into nucleons and partly to the intrinsic angular momentum of nucleons. RHIC’s polarized proton beams provide an ideal tool for studying the gluon contribution.
For RHIC’s first proton run, which finished at the end of January, four Siberian snakes, a spin-flipper and polarimeters were installed into RHIC and a new high-intensity polarized proton source was commissioned.
The Siberian snakes were built at Brookhaven and funded by the Japanese Institute of Physical and Chemical Research (RIKEN) as part of the RIKEN-Brookhaven Research Center initiative. This is the first time that Siberian snakes have been used in a high-energy machine. They quickly proved their worth by maintaining beam polarization up to RHIC’s full collision energy.
By the end of the run, 25% beam polarization was being maintained at a centre-of-mass collision energy of 200 GeV. Substantial improvements are expected when the Alternating Gradient Synchrotron’s Siberian snake is upgraded. The big PHENIX and STAR detectors were also upgraded to make the most of polarized proton collisions. They, along with the smaller PP2PP proton elastic scattering experiment, have reported that useful data were recorded during RHIC’s first polarized proton run.
