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RHIC Serves as World’s First & Only Collider
of Polarized Protons for ‘Spin’ Physics

Spin ‘crisis’

The spin “crisis” in high-energy physics began in 1988, when it was first discovered that, contrary to expectation, the spin of the quarks inside the proton do not account for all or even very much of the proton’s spin. As it has been learned since from accelerator-based experiments with polarized protons at lower energies, quark spin only accounts for one quarter of the proton’s spin.

In this diagram representing the head-on collision of a quark (red ball) from one proton (orange ball) with a gluon (green ball) from another proton with opposite spin, spin is represented by the blue arrows circling the protons and the quark. The blue question marks circling the gluon represent the question: Are gluons polarized? Ejected from the collision are a shower of quarks and a photon of light (purple ball).

The remaining three-quarters of a proton’s spin may be carried by particles called gluons, which bind quarks inside the proton, or by something called orbital angular momentum, or by both. If it is discovered that gluons do carry proton spin, then this knowledge may lead to an understanding of why quarks and gluons are confined within the proton and the neutron, which are the main constituents of the atomic nucleus and are collectively known as nucleons.

As RIKEN BNL Research Center Director and Nobel laureate T.D. Lee explains: “The shape of a nucleon is defined by the spreading of an infinite number of ‘soft’ gluons. Each of these gluons possesses an integral spin, which can be readily excited because of the confinement property of Quantum Chromodynamics. Hence, spin physics offers us the unique opportunity to measure the proton’s shape.”

"The research performed at the RIKEN BNL Research Center exemplifies the international nature of science, while advancing our understanding of the universe's most basic matter."
- T.D. Lee

To uncover what fraction of proton spin is carried by gluons, physicists at RHIC just completed experiments during the second run using two beams of longitudinally polarized protons, which allow quarks in one proton beam to probe the spin of gluons in the opposing beam (see diagram).

When quarks and gluons in longitudinally polarized beams collide, what is observed is the creation of photons and other particles, which emerge in groups called jets. The rates at which jets and photons are produced depend upon the spin of the incident beams. If both beams have the same spin direction, then the rates of particle production are expected to be higher than when the beams have the opposite spin. But whether or not that expectation is met will be determined by the now ongoing analysis of the second run’s data.

Measurable & large asymmetries

During the first run, experimenters on the PHENIX and STAR experiments used especially built detectors to look for spin asymmetries as a way to monitor spin direction when transversely polarized beams collided. After analyzing the first p≠p≠ run’s data, the scientists were surprised to find is that not only were the asymmetries measurable, but they are also large.

With the polarized proton beam’s spin up, PHENIX physicists detected 20 percent more neutrons produced to the left than the right following glancing collisions, in which the opposing proton beams barely hit. This asymmetry had not been seen before by accelerator-based experiments collecting data at lower energies and is not yet understood.

Meanwhile, STAR observed twice as many particles called neutral pions emerging to the left than to the right as a result of hard, head-on collisions of an unpolarized beam with a spin-up polarized beam. Although this asymmetry is just as large as what has been seen from much lower energy accelerator experiments, it is nonetheless a surprise that has also not yet been explained.

With the results of the first run to be explained and those of the second run under analysis, Bunce foresees another five or more years work ahead of the program: “Along the way, we will look for violations of mirror symmetry from the weak interaction, to measure the spin direction of the anti-quarks within the proton. And, in ten years or so, to learn more about the spin structure of the proton and the nature of the strong interaction governing quarks and gluons, we hope to be colliding polarized protons with high-energy polarized electrons, in an alternative configuration of RHIC that we call e-RHIC.”

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