The power of the strong force

The power of the strong force

Credit: Thomas Jefferson National Accelerator Facility

There was a lot of fuss about the Higgs boson when this elusive particle was discovered in 2012. Although it was touted as a mass of ordinary matter, interactions with the Higgs field generate only about 1 percent of the ordinary mass. The other 99 percent comes from phenomena related to the strong force, the fundamental force that binds smaller particles called quarks into larger particles called protons and neutrons that make up the nucleus of the atoms of ordinary matter.

Now, researchers at the US Department of Energy’s Thomas Jefferson National Accelerator Facility have experimentally tested the power of the strong powera body that supports theories that explain how most mass or ordinary matter in the universe is generated.

This quantity, 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 study, theories disagreed about how strong coupling should behave at great distances: some predicted it would grow with distance, some that it should decrease, and some that it should become constant.

With data from Jefferson Lab, the physicists were able to determine the strong force coupling at the greatest distances yet. Their results, which provide experimental support for theoretical predictions, were recently featured on the cover of the journal particles.

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

While this article is the result of years of data collection and analysis, it wasn’t quite the intention at first.

A spin-off of a spin experiment

At smaller distances between quarks, strong coupling is small, and physicists can solve this with a standard iterative method. At greater distances, however, 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 staff scientist at Jefferson Lab and co-author of the paper. “While we have to use more complicated techniques to calculate this quantity, its sheer value unleashes 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 use the iterative method, Deur, Chen and their co-authors have extracted strong force coupling at the greatest distances between affected bodies ever.

They got this value from a handful of Jefferson Lab experiments that were actually designed to study something completely different: proton and neutron spin.

These experiments were performed in the laboratory’s Continuous Electron Beam Accelerator Facility, a DOE user facility. CEBAF is able to deliver polarized electron beams, which can be directed in the experimental rooms at specialized targets containing polarized protons and neutrons. when a electron beam: is polarized, meaning a majority 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 the years of data analysis that followed, the researchers realized that they could combine the collected information about the proton and neutron to obtain a strong force coupling at greater distances.

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

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

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

Flattening supports huge theories

These experiments were conducted about 10 years ago, when Jefferson Lab’s electron beam was able to deliver electrons with an energy of up to 6 GeV (it is now capable of 12 GeV). The lower-energy electron beam was needed to investigate the strong force at these greater distances: a lower-energy probe gives access to longer timescales and thus greater distances between affected particles.

Likewise, a higher energy probe is essential to zoom in for views of shorter time scales and smaller interparticle distances. Labs with higher energy beams, such as CERN, Fermi National Accelerator Laboratory and SLAC National Accelerator Laboratory, have already investigated strong force coupling at these smaller spacetime scales, when this value is relatively small.

The zoomed-in view of higher energy beams has shown that the mass of a quark is small, only a few MeV. At least that’s their textbook mass. But when lower energy quarks are examined, 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 move over greater distances. The mass-generating effect of this cloud accounts for most of the mass in the universe — without this extra mass, the mass of textbook quarks may only make up about 1% of the mass of protons and neutrons. The remaining 99% comes from this acquired mass.

Similarly, one theory holds that gluons are massless at short distances, but effectively gain mass as they travel further. The leveling of strong force coupling at large distances supports this theory.

“If gluons remained massless at great distances, strong coupling would continue to grow uncontrollably,” Deur said. “Our measurements show that strong coupling becomes constant as the distance probed gets larger, which is a sign that gluons have gained mass through the same mechanism that gives 99% of the mass to the proton and neutron.”

This means that strong coupling at great distances is important to understand mass generation mechanism. These results also help verify new ways to solve equations for quantum chromodynamics (QCD), the accepted theory describing the strong force.

For example, the smoothing of the strong force coupling at great distances provides evidence that physicists can apply a new, advanced technique called Anti-de Sitter/Conformal Field Theory (AdS/CFT) duality. The AdS/CFT technique allows physicists to solve equations non-iteratively, which can help with strong force calculations at large distances 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 spacetime scales. Because strong coupling flattens out at greater distances, it no longer depends on the spacetime scale, meaning the strong force is compliant 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 enabled us to solve problems of QCD or quantum gravity that were hitherto unmanageable or very crudely tackled using not very rigorous models,” said Deur. “This has provided many exciting insights into fundamental physics.”

So while these results were generated by experimenters, they have the most impact on theorists.

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

Years have passed since the experiments that accidentally produced these results were conducted. An entirely new set of experiments now uses Jefferson Lab’s higher energy 12 GeV beam to explore nuclear physics.

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

Only time will tell which theories support these new experiments.

Nuclear physicists on the hunt for pressed protons

More information:
Alexandre Deur et al, Experimental determination of the QCD effective charge αg1(Q), particles (2022). DOI: 10.3390/particles5020015

Quote: The Strength of the Strong Force (2022, August 3), retrieved August 3, 2022 from

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