The Thomas Jefferson National Laboratory experiments focus on a never-before-measured region of strong force coupling, a quantity that supports theories representing 99% of the ordinary mass in the universe.
The Higgs boson caused a stir when this elusive particle was discovered in 2012. Although it was touted as giving mass to ordinary matter, interactions with the Higgs field only generate about 1% of the mass ordinary. The remaining 99% comes from phenomena associated with the strong nuclear force, the fundamental force that binds smaller particles called quarks into larger particles called protons and neutrons that make up the nucleus of atoms of ordinary matter.
The strong nuclear force (often referred to as the strong force) is one of the four fundamental forces in nature. The others are gravity, the electromagnetic force and the weak nuclear force. As its name suggests, it is the strongest of the four. However, it also has the shortest range, meaning particles need to be extremely close before its effects are felt.
Now scientists have experimentally extracted force from the strong force, a quantity that strongly supports theories explaining how most of the mass or ordinary matter in the universe is generated. The research was carried out at the US Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Lab).
This quantity, known as the strong force coupling, describes the force with which two bodies interact or “couple” under this force. The strong force coupling varies with the distance between the particles affected by the force. Prior to this research, theories disagreed on the behavior of the force coupling at large distances: some predicted that it would increase with distance, others that it would decrease, and others that it would remain constant. .
With data from the Jefferson Lab, physicists were able to determine the intense force coupling at the greatest distances to date. Their results, which provide experimental support for theoretical predictions, recently made the cover of the journal Particles.
“We are happy and excited to have our efforts recognized,” said Jian-Ping Chen, principal investigator at Jefferson Lab and co-author of the paper.
Although this article is the culmination of years of data collection and analysis, it was not entirely intentional to begin with.
A spin-off of a spin experience
At smaller distances between quarks, the strong force coupling is weak and physicists can solve it with a standard iterative method. At larger distances, however, the strong force coupling becomes so large that the iterative method no longer works.
“It’s both a curse and a blessing,” said Alexandre Deur, Jefferson Lab scientist and co-author of the paper. “Although we have to use more complicated techniques to calculate this quantity, its very value triggers a host of very important emergent phenomena.”
This includes a mechanism that accounts for 99% of the ordinary mass of 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 extracted strong force coupling at the largest distances between the affected bodies.
They extracted this value from a handful of Jefferson Lab experiments that were actually designed to study something completely different: the spin of protons and neutrons.
These experiments were conducted in the laboratory’s Continuous Electron Beam Acceleration Facility, a DOE user facility. CEBAF is able to deliver polarized electron beams, which can be directed at specialized targets containing polarized protons and neutrons in the experimental halls. When an electron beam is polarized, it means that the majority of the electrons are all spinning in the same direction.
These experiments projected Jefferson Lab’s polarized electron beam onto polarized proton or neutron targets. Over the several years of analyzing the data that followed, the researchers realized that they could combine the information gathered from the proton and neutron to extract strong force coupling at greater distances.
“Only Jefferson Lab’s high-performance polarized electron beam, together with developments in polarized targets and detection systems, have allowed us to obtain such data,” Chen said.
They found that as the distance increases between the affected bodies, the strong force coupling increases rapidly before leveling off and becoming constant.
“Some theories predicted that it should be, but this is the first time experimentally that we’ve actually seen it,” Chen said. “It gives us details of how the strong force, on the scale of quarks forming protons and neutrons, actually works.”
Leveling Supports Massive Theories
These experiments were conducted about 10 years ago, when the Jefferson Lab electron beam was only able to deliver electrons at an energy of up to 6 GeV. It is now capable of reaching 12 GeV. The low-energy electron beam was needed to examine the strong force at these larger distances: a low-energy probe allows access to longer timescales and, therefore, greater distances between particles affected.
Likewise, a higher energy probe is essential for zooming in to capture views on shorter timescales and smaller distances between particles. Laboratories with higher energy beams, such as CERN, Fermi National Accelerator Laboratory and SLAC National Accelerator Laboratory, have already examined the strong force coupling at these smaller spacetime scales, when this value is relatively low.
The magnified view offered by the higher energy beams showed that the mass of a quark is small, only a few MeV. At least that’s their textbook mass. But when quarks are probed with lower energy, their mass actually increases up to 300 MeV.
This is because quarks gather a cloud of gluons, the particle that carries the strong force, when they move over greater distances. The mass-generating effect of this cloud accounts for most of the mass of the universe – without this extra mass, the classical mass of quarks can only be about 1% of the mass of protons and neutrons. The remaining 99% comes from this acquired mass.
Similarly, one theory posits that gluons are massless at close range but actually gain mass as they travel farther. The leveling of strong force coupling at large distances supports this theory.
“If gluons remained massless at long range, strong force coupling would continue to grow unchecked,” Deur said. “Our measurements show that the strong force coupling becomes constant as the probed distance increases, which is a sign that the gluons have acquired mass by the same mechanism that gives 99% mass to the proton and neutron.”
This means that strong force coupling at large distances is important to understand this mass generation mechanism. These results also help verify new ways of solving quantum chromodynamics (QCD) equations, the accepted theory describing the strong force.
For example, the flattening of strong force coupling at large distances proves 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 facilitate calculations of strong forces at large distances where iterative methods fail.
Conformal in “Conformal Field Theory” means that the technique is based on a theory that behaves the same on all space-time scales. Because the strong force coupling stabilizes at greater distances, it no longer depends on the spacetime scale, which means the strong force is compliant and AdS/CFT can be applied. Although theorists have previously applied AdS/CFT to QCD, these data support the use of the technique.
“AdS/CFT allowed us to solve previously unsolvable or very approximate QCD or quantum gravity problems using loose models,” Deur said. “It yielded a lot of exciting insights into fundamental physics.”
So, although these results were generated by experimentalists, they affect 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 community of physicists at Jefferson Lab, in particular Dr. Alexandre Deur, for this major advance in physics.
Years have passed since the experiments that accidentally carried these results were conducted. A whole new suite of experiments now uses Jefferson Lab’s high-energy 12 GeV beam to explore nuclear physics.
“One thing I’m very happy with with all these older experiments is that we’ve nurtured many young students and now they’ve become leaders of future experiments,” Chen said.
Only time will tell which theories these new experiments support.
Reference: “Experimental determination of the effective charge QCD αg1(Q)” by Alexandre Deur, Volker Burkert, Jian-Ping Chen and Wolfgang Korsch, May 31, 2022, Particles.