The PHENIX detector at Brookhaven National Laboratory. |
Grid tools are helping scientists solve a nuclear physics mystery—how the proton gets its spin. The origin of the proton's spin has been the subject of experiment and speculation for over 15 years, and preliminary results from the PHENIX experiment show that the proton is not as strange as some may have thought.
"We are striving to understand the fundamental structure of matter," said Abhay Deshpande, a physicist from the State University of New York at Stony Brook. "Protons and neutrons form 99% of the matter around us, and mass and spin are their two fundamental properties."
Spin is the direction a particle spins around an axis as it travels, just as the Earth spins on its axis as it travels around the sun. Until 1989, physicists assumed that the spin of the three quarks that make up a proton combine to create the total proton spin. That year, a European nuclear physics experiment using a method called Deep Inelastic Scattering reported that the three quarks only carry 20–30% of the proton's spin. This result, later confirmed by DIS experiments around the world and labeled the "spin crisis," caused physicists to suspect that the gluons that bind quarks together may be the carriers of the missing spin.
Gluon spin is difficult to measure in DIS fixed-target experiments. Initial measurements resulted in large gluon spin values with larger errors, suggesting that the gluons could be fully polarized, contributing a huge amount of positive or negative spin that would have to be balanced by yet another factor; they could make up the missing 80% of the proton's spin; or they could contribute no spin at all.
The PHENIX and STAR experiments at Brookhaven National Laboratory's Relativistic Heavy Ion Collider are the next generation of experiments studying the mystery of proton and neutron spin. The spin physics program of the PHENIX experiment, in which Deshpande participates, measures what happens when protons moving almost at the speed of light collide in the center of the PHENIX detector.
Since protons are composed of quarks and gluons, proton collisions are really quark and gluon collisions. Physicists study the spin of the gluons by changing the spins of the colliding protons and measuring the number of certain particles produced in the collisions. The experiment produced vast amounts of data that needed to be analyzed, but the computing facilities at BNL were busy processing data from the detectors' other physics programs. The PHENIX collaboration used grid computing tools to transfer the spin data to the PHENIX regional computing center at the RIKEN Radiation Laboratory in Wako, Japan, where it was analyzed. GridFTP and grid security tools helped transfer the data securely, making the process painless for the PHENIX shift crews.
"We sent the raw data to Japan about 10 hours after it was collected in the detector," said Martin Purschke, a PHENIX physicist from BNL. "All 270 terabytes were transferred in about ninety days; a real breakthrough for us."
The grid-assisted preliminary results from the PHENIX spin physics program were presented at the Particles and Nuclei International Conference, October 24-28 in Santa Fe, New Mexico. "The maximal gluon scenarios—really huge gluon spins that had been predicted by some scientists—are essentially ruled out, so we're not looking at an extremely weird proton structure," said Deshpande. Although large gluon spins are eliminated as a possibility, PHENIX and STAR scientists continue to analyze data to determine the exact gluon spin contribution to the proton spin.
Learn more at the PHENIX Web site.
—Katie Yurkewicz
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