Jefferson Lab scientist puts intriguing view on spin of the proton

Newport News, Va. - A new result from the HERMES experiment at The German Electron Synchrotron DESY indicates that the gluons which bind quarks into the proton show only slight polarization. The result will be presented by Elke-Caroline Aschenauer, a former HERMES spokesperson and current 12 GeV Upgrade Hall D Leader at the Department of Energy's Thomas Jefferson National Accelerator Facility.

In the late 1980s, the first measurement of the contribution of quarks to the spin of the proton at CERN revealed that the quarks' spin can't account for all of the proton's spin. This so-called spin crisis spurred a number of new experiments to identify the proton's silent spin contributors. Aschenauer will present the newest result from one of the longest-running experiments for determining proton structure at 10:30 a.m. on Sunday, April 15, in Session J2.00001: The Spin of the Nucleon - Highlights from HERMES.

"HERMES is the first experiment which aims to address all the contributions to the helicity spin structure of the proton. The latest measurements confirm, with very high precision, that quarks are not the only pieces in the spin puzzle," Aschenauer says. The HERMES experiment is currently running at the HERA facility at The German Electron Synchrotron DESY. HERMES is a fixed-target experiment using longitudinally polarized electrons or positrons stored in HERA's lepton ring. The data presented here were taken on longitudinal/transversely polarized hydrogen and deuterium gas targets.

"We measured the inclusive structure function g1. These data allow us, with some assumption, to get the total contribution from the quarks. The contribution is small, about 33%; but it is higher than what was seen over the last few years," she notes. The full result is DS=0.330±0.011±0.025±0.028 in the range 0.021<x<0.9. Previous results ranged from 15-25%.

Aschenauer says the contribution of the orbital angular momenta of quarks and gluons are the other pieces in the puzzle we know nearly nothing about. The framework of generalized parton distributions (GPDs) offers the promise of access to these contributions. The GPDs can be constrained in exclusive reactions, for which HERMES has new measurements on deeply virtual Compton scattering (DVCS) and exclusive rho-production.

New results disentangling the role of the gluons in spin structure will also be presented. "This result covers only a small range in x, which hints at a relatively small gluon contribution," Aschenauer explains. "The gluons have only a very slight tendency to be polarized in the same direction as the proton." The result obtained is Dg/g=0.071±0.034±0.01+0.127-0.100, at <x> ~0.22.

"The novel thing is, indeed, that we get hints on all the individual contributions to the spin of the proton. But there is also some long way to go to pin down all the contributions. And to disentangle these, you need more measurements," Aschenauer says. "Some of these measurements can be performed with unprecedented precision at Jefferson Lab after its 12 GeV Upgrade."

"At 12 GeV, there will be, for example, the possibility to determine g1 at very high-x. Also, by combining the HERMES results with future results in exclusive reactions, like DVCS at 12 GeV, we can learn something about the three-dimensional structure of the proton," Aschenauer says.

The 12 GeV Upgrade project at Jefferson Lab will double the maximum electron energy of the Continuous Electron Beam Accelerator Facility (CEBAF) from about 6 billion electron-volts (GeV) to 12 billion opening up a new realm of scientific capabilities. The three existing experimental halls (Halls A, B, and C) will be upgraded, and a fourth experimental hall (Hall D) will be built to study the confinement of quarks. The project received Critical Decision One (CD-1) approval from DOE in February 2006, allowing the Project Engineering and Design (PED) phase to begin, and CD-2 approval is anticipated in September 2007.

For more information, contact:
Kandice Carter
kcarter@jlab.org
757-269-7263

Thomas Jefferson National Accelerator Facility’s (Jefferson Lab’s) basic mission is to provide forefront scientific facilities, opportunities and leadership essential for discovering the fundamental structure of nuclear matter; to partner in industry to apply its advanced technology; and to serve the nation and its communities through education and public outreach. Jefferson Lab, located at 12000 Jefferson Avenue, is a Department of Energy Office of Science research facility managed by the Jefferson Science Associates, LLC.