Homing in on spinning gluons: New study nearly nixes the negative

NEWPORT NEWS, VA – Researchers have been working for decades to understand the architecture of the subatomic world. One of the knottier questions has been where the proton gets its intrinsic angular momentum, otherwise referred to as its spin.

Nuclear physicists surmise that the proton’s spin most likely comes from its constituents: quarks bound together by gluons carrying the strong force. But the details of the quark and gluon contributions have remained elusive.

Now, a new investigation from an international collaboration of physicists compiles evidence from observational results and analysis using lattice quantum chromodynamics (QCD) to present a compelling argument regarding how much of the proton’s spin comes from its gluons.

A new paper featuring the results of the investigation, “New Data-Driven Constraints on the Sign of Gluon Polarization in the Proton,” was recently published in the journal Physical Review Letters by members of the Jefferson Lab Angular Momentum (JAM) collaboration. The collaboration includes theorists, experimentalists and computer scientists who investigate subatomic particles’ internal structure through QCD. QCD is a theory that describes how quarks and gluons interact via the strong force.

The authors, members of the JAM collaboration’s Spin PDF Analysis Group, are Wally Melnitchouk and Nobuo Sato of the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility; Nicholas Hunt-Smith, Anthony Thomas and Martin White, of the Centre for the Subatomic Structure of Matter (CSSM) and the ARC Centre of Excellence for Dark Matter Particle Physics in the Department of Physics at the University of Adelaide, Australia; and Christopher Cocuzza of the William & Mary Department of Physics.

Thomas noted that Jefferson Lab’s fruitful connection with the University of Adelaide goes back decades, with the physicists working together in spite of a time difference of 14.5 hours.

“Many years ago, Wally held a joint position between Adelaide and Jefferson Lab when Nathan Isgur was chief scientist,” Thomas said. “And I was at Jefferson Lab for six years as chief scientist, from 2004 to 2009. The physics collaboration has been ongoing.”

The group’s latest work addresses a knotty question that must first be resolved in order to make progress on the understanding of the origin of the proton’s spin: What’s the sign of ∆g? In other words, is the gluons’ spin negative or positive?

The proton’s intrinsic spin

When physicists discuss “spin” of a proton — or any particle — they’re referring to its intrinsic angular momentum. As physicists’ concept of subatomic spin is a quantum mechanical idea, explanations from our visible world only go so far; it carries this angular momentum even when it is at rest. Spin can be negative or positive — think clockwise and counterclockwise.

“Spin is a peculiar characteristic of the quantum world,” Sato explained, “and the proton has spin of ½. It's not point-51, it's not point-501. It's not point-50001. It’s point-5. And that number is made of the components inside the proton, each of which has its own spin.”

Melnitchouk said the value of the spin of the proton has been known since the 1920s. He added that it makes sense that the spin of the proton comes from the aggregate spins of the constituents making up the protons.

“What we don't know is: What portion of the proton’s spin is carried by the three valence quarks in the proton? What portion is carried by gluons?” he said. “What portion is maybe due to the rotation, the orbital angular momentum, of the quarks and gluons in the proton?”

Nailing down the sign of ∆g is vital to figuring out how the proton gets its spin, and years of observational work as well as theory have not delivered a conclusive answer.

Most physicists thought a positive ∆g seemed more likely, but “These negative ∆g solutions have always been sticking around,” Hunt-Smith said. “There's no reason why we can't have a negative ∆g, just from a purely physics perspective.”

A new analysis with world-first data

The JAM collaborators built their findings from previous analytical work as well as older and newer observational data from a number of experiments, including programs at Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF) and at the Relativistic Heavy Ion Collider (RHIC) at DOE’s Brookhaven National Lab. Both of these particle accelerators are DOE Office of Science user facilities, which are used by nuclear physicists from around the world to conduct their research.

“Our analysis was different from other groups, in that we really tried as much as possible to remove theoretical assumptions about how things should behave,” Melnitchouk said.

One such assumption was that unpolarized parton distributions have to have a probabilistic interpretation. Melnitchouk acknowledged that this assumption regarding partons (the collective term for gluons, quarks and other constituent particles inside the proton) is a “technicality.”

“But it’s something that has been assumed in previous analyses by other groups around the world,” he added. “And we now believe that’s not really something that is fundamental.”

Melnitchouk said that originally, observational data from RHIC created a global consensus leading in the direction of a positive ∆g.

“But further analysis made things less clear,” he noted, “because there was a corner of parameter space that allowed for both positive and negative gluon spin.”

He said the JAM collaborators published a paper a couple years ago that pointed out that negative ∆g was still a viable possibility. That paper was controversial, as it bucked the common wisdom of the global nuclear physics community at the time, which leaned heavily in the direction of a positive gluon spin.

Meanwhile, the HadStruc collaboration had been addressing the same question in a different way. They were using supercomputers and the lattice QCD formulation to calculate the underlying QCD theory that describes the interactions among quarks and gluons in the proton.

“You basically discretize space-time,” Hunt-Smith explained. “You cut space-time into a series of slices, and it makes it possible to calculate a lot of physical properties.” 

A subsequent joint JAM and HadStruc collaboration analysis, led by Joe Karpie, the Nathan Isgur Fellow in the Theory Center, combined the new lattice QCD results with the experimental data, noting that while this narrowed the room, it still admitted a negative Δg.

“There was a slight preference for the positive solution, but still not statistically significant,” Hunt-Smith said. “This is where our new research comes in.”

Energetic particles add new data for analysis

He went on to explain how the collaborators included data from deep-inelastic scattering experiments, including those using CEBAF at Jefferson Lab.

“You take an electron and fire it at a proton, and the idea is that you're able to probe what's inside the proton — the various quarks and gluons that are inside it — and you can understand more of the structure of the proton in that way,” he said.

Hunt-Smith said the JAM group particularly focused on the high-x data from the deep-inelastic scattering experiments, the particularly significant bits of the results. This data refers to particles that have been detected with particularly high momentum – or very high energy – resulting from the electron’s collision with the constituents of the proton. He noted that incorporation of the high-x data required some additional theoretical considerations.

“So, we added that theory in,” he said. “We added more parameters into the analysis to account for that additional high-x data, and then we reran the analysis. And we found that with the inclusion of the high-x data, there was a statistically significant difference between the positive and negative ∆g replicas, the key finding being that the negative ones were heavily disfavored.”

Hunt-Smith went on to add that the JAM analysis showed that the ∆g was constrained by not just the high-x data from deep-inelastic scattering, but also by the results of polarized-jets experiments from a different set of proton-collision studies at RHIC, as well as the recent HadStruc lattice QCD data.

The results are not completely definitive, but the analysis makes a negative ∆g gluon far less likely than ever.

“I don't think anybody else has included these Jefferson Lab data in any analysis. I think it's a very important feature of this analysis,” Thomas said. “This is a very comprehensive dataset of regions that nobody else has been able to get to, and it's the first time it's been included in a comprehensive global analysis.”

Sato said that while the JAM paper makes negative ∆g less likely, it doesn’t completely close the door to the possibility. He added that additional observational data from instruments such as Jefferson Lab’s ongoing CEBAF 12 GeV research program or the future Electron-Ion Collider would likely be capable of replacing remaining theoretical assumptions. Jefferson Lab is a partner with Brookhaven National Laboratory in designing and establishing the Electron-Ion Collider at Brookhaven.

“Then it is possible that the door will be 100% closed” he said. “But now we are basically closing that gap.”

Further Reading
Theory and Experiment Combine to Shine a New Light on Proton Spin

By Joseph McClain

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