The strength of the strong force

When this elusive particle was discovered in 2012, much fuss was made about the Higgs boson. Although it has been touted as providing ordinary matter mass, interactions with the Higgs field produce only about 1 percent of ordinary matter. The other 99 percent comes from phenomena associated with the strong force, the fundamental force that binds smaller particles called quarks into larger particles called protons and neutrons, which make up the nucleus of atoms in ordinary matter.

Now researchers at the US Department of Energy’s Thomas Jefferson National Accelerator Facility have experimentally extracted the strength of the strong force, a quantity that supports theories that explain how most of the mass, or ordinary matter, in the universe is created.

This variable, known as the coupling of the strong force, describes how strongly two bodies interact or “couple” under this force. Strong force coupling varies with the distance between the particles affected by the force. Prior to this research, theories were divided as to how strongly force coupling should behave at long range: some predicted that it should increase with distance, others that it should decrease, and others that it should become constant.

Using data from the Jefferson Lab, the physicists were able to determine the strong force coupling at the largest distances ever seen. Their results, which provide experimental support for theoretical predictions, were recently published on the cover of the journal particles.

“We are happy and excited that our efforts are being recognized,” said Jian-Ping Chen, senior scientist at the Jefferson Lab and co-author of the publication.

Although this paper is the culmination of years of data collection and analysis, it was not entirely intentional at first.

A spin-off of a spin experiment

For smaller distances between quarks, the strong force coupling is small, and physicists can solve it using a standard iterative procedure. At larger distances, however, the strong force coupling becomes so great that the iterative method no longer works.

“This is both a curse and a blessing,” said Alexandre Deur, a research associate at the Jefferson Lab and co-author of the study. “While we have to use more complicated techniques to calculate this quantity, its sheer value unlocks a host of very important emerging phenomena.”

This includes a mechanism that accounts for 99 percent of the ordinary mass in the universe. (But we’ll get to that in a moment.)

Despite the challenge of not being able to apply the iterative method, Deur, Chen and their co-authors extracted strong force coupling at the largest distances between affected bodies ever.

They extracted this value from a handful of Jefferson Lab experiments designed to study something else entirely: proton and neutron spin.

These experiments were performed in the lab’s Continuous Electron Beam Accelerator Facility, a DOE user facility. CEBAF is able to provide polarized electron beams that can be aimed at specialized targets containing polarized protons and neutrons in the experimental halls. When a beam of electrons is polarized, it means that most of the electrons are all spinning in the same direction.

These experiments shot Jefferson Lab’s polarized electron beam at polarized proton or neutron targets. During subsequent multi-year data analysis, the researchers found that they could combine the information gathered about the proton and neutron to extract strong force coupling at longer distances.

“Only Jefferson Lab’s powerful polarized electron beam combined with developments in polarized targets and detection systems allowed us to obtain such data,” Chen said.

They found that as the distance between affected bodies increases, the strong force coupling increases rapidly before leveling off and becoming constant.

“There are some theories that predict this should be the case, but this is the first time we’ve actually seen this experimentally,” Chen said. “This gives us details on how the strong force on the scale of the quarks that make up protons and neutrons actually works.”

Flattening supports massive theories

These experiments were conducted about 10 years ago, when Jefferson Lab’s electron beam was able to provide electrons with an energy of up to 6 GeV (it is now capable of providing up to 12 GeV). In order to study the strong force at these larger distances, the lower-energy electron beam was required: a lower-energy probe allows access to longer time scales and thus to larger distances between affected particles.

Similarly, a higher-energy zoom-in probe is essential for views of shorter timescales and smaller inter-particle distances. Higher-energy beam labs such as CERN, Fermi National Accelerator Laboratory, and SLAC National Accelerator Laboratory have already studied strong force coupling on these smaller space-time scales, when this value is relatively small.

The magnified view offered by higher energy rays has shown that the mass of a quark is small, only a few MeV. At least that’s their textbook crowd. But when lower-energy quarks are studied, their mass effectively grows to 300 MeV.

This is because the quarks collect a cloud of gluons, the particle that carries the strong force, as they travel longer distances. The mass-producing effect of this cloud accounts for most of the mass in the universe – without this extra mass, the textbook mass of quarks can only be about 1% of the mass of protons and neutrons. The other 99% comes from this acquired mass.

Similarly, one theory posits that gluons are massless at short range but effectively acquire mass as they continue to move. The leveling of strong force coupling over long distances supports this theory.

“If gluons remained massless over long distances, the strong force coupling would continue to grow uncontrollably,” Deur said. “Our measurements show that the strong force coupling becomes constant as the probed distance increases, indicating that gluons have acquired mass through the same mechanism that gives 99% of the mass to the proton and neutron.”

This means that strong force coupling over long distances is important to understanding this mass creation mechanism. These results also help verify new ways of solving equations for quantum chromodynamics (QCD), the accepted theory that describes the strong force.

For example, the flattening of the strong force coupling at long distances provides evidence that physicists can apply a new, cutting-edge technique called Anti-de-Sitter/Conformal Field Theory (AdS/CFT) duality. The AdS/CFT technique allows physicists to solve equations non-iteratively, which can be useful in large-distance calculations of strong forces where iterative methods fail.

The Conformal in “Conformal Field Theory” means that the technique is based on a theory that behaves the same on all space-time scales. As the strong force coupling flattens out at longer distances, it is no longer dependent on the spacetime scale, meaning the strong force is conformal and AdS/CFT can be applied. While theorists have already applied AdS/CFT to QCD, this data supports the use of the technique.

“AdS/CFT has allowed us to solve problems in QCD or quantum gravity that were previously unsolvable or very crudely addressed with not very rigorous models,” Deur said. “That brought many exciting insights into basic physics.”

Although these results were generated by experimenters, they influence theorists the most.

“I believe these results are a real breakthrough for the advancement of quantum chromodynamics and hadron physics,” said Stanley Brodsky, professor emeritus at SLAC National Accelerator Laboratory and QCD theorist. “I congratulate the physics community at Jefferson Lab, especially Dr. Alexandre Deur, on this major advance in physics.”

Years have passed since the experiments that happened to produce these results were conducted. A whole new set of experiments is now using Jefferson Lab’s higher-energy 12 GeV beam to explore nuclear physics.

“One thing I’m excited about with all of these older experiments is that we trained a lot of young students and now they’ve become the leaders of future experiments,” Chen said.

Only time will tell which theories support these new experiments.