Thomas Jefferson National Laboratory experiments focused on a previously unmeasured region of strong force coupling, a quantity that supports theories that account for 99% of the normal mass in the universe.
Much buzz was made about the Higgs boson when this elusive particle was discovered in 2012. Although it was promoted as giving mass of ordinary matter, interactions with the Higgs field generate only about 1% of the ordinary mass. The other 99% come from phenomena related to the strong nuclear force, the fundamental force that binds smaller particles called quarks to larger particles called protons and neutrons that make up the nuclei of ordinary matter atoms.
The strong nuclear force (often referred to as the strong force) is one of the four fundamental forces of nature. The others are gravity, electromagnetic force, and the weak nuclear force. As its name suggests, it is the most powerful of the four. However, it also has the shortest range, which means that the particles have to be extremely close before their effects can be felt.
Now, scientists have experimentally extracted the force of the strong force, a quantity that strongly supports theories that explain how most of the mass or ordinary matter in the universe is formed. The research was conducted at the US Department of Energy’s Thomas Jefferson National Accelerator Facility (Jefferson Laboratory).
This quantity, known as the extreme force coupling, describes how strongly two bodies or “couples” interact under this force. The strong force coupling varies with the distance between the particles affected by the force. Prior to this research, theories differed about how the strong force coupling would behave over large distances: some predicted that it would grow with distance, some would decrease, and some would remain constant.
Using Jefferson Lab data, the physicists were able to determine the strong coupling force over the largest distances to date. Their findings, which provide empirical support for theoretical predictions, recently appeared on the cover of the journal grains.
“We are pleased and excited to see our efforts recognized,” said Jianping Chen, chief scientist at Jefferson Laboratory and one of the authors of the research paper.
Although this paper is the culmination of years of data collection and analysis, it was not entirely intended to begin with.
Part of a spin experience
At smaller distances between quarks, the strong force coupling is small, and physicists can solve it in a standard iterative way. However, at larger distances, the strong force coupling becomes too large for the iterative method to work anymore.
“This is a curse and a blessing at the same time,” said Alexandre Dior, a scientist in the Jefferson Laboratory and one of the authors of the paper. “While we have to use more complex techniques to calculate this quantity, its absolute value unleashes a host of very important emerging phenomena.”
This includes a mechanism that accounts for 99% of the normal mass in the universe. (But we’ll get to that shortly.)
Despite the challenge of not being able to use the iterative method, Deur, Chen and colleagues extracted a strong coupling force over the largest distances between the affected bodies ever.
They extracted this value from a handful of Jefferson Lab experiments that were actually designed to study something completely different: the spin of a proton and a neutron.
These experiments were conducted at the Continuous Electron Beam Acceleration Laboratory, a DOE user facility. CEBAF is able to provide polarized electron beams, which can be directed at specialized targets containing polarized protons and neutrons in the experimental halls. When the electron beam is polarized, it means that the majority of the electrons are orbiting in the same direction.
These experiments fired a polarized electron beam at the Jefferson Laboratory at polarized proton or neutron targets. During the many years of analyzing the data after that, the researchers realized that they could combine the information collected about the proton and neutron to extract strong strong coupling at greater distances.
“Only the Jefferson Lab’s high-performance polarized electron beam, combined with advances in polarized targets and detection systems, allowed us to obtain such data,” Chen said.
They found that as the distance between the affected objects increases, the strong force coupling grows rapidly before stabilizing and becoming stable.
“There are some theories that have predicted that this should be the case, but this is the first time we’ve actually seen this experimentally,” Chen said. “This gives us details of how the strong force, on the scale of quarks that make up protons and neutrons, actually works.”
Compromise supports big theories
These experiments were conducted about 10 years ago, when the electron beam at Jefferson Lab was only able to deliver electrons up to 6 GeV in energy. It is now capable of up to 12 gigaelectronvolts. The low-energy electron beam was required to examine the strong force at these larger distances: the lower-energy probe allows access to longer time scales and, therefore, larger distances between affected particles.
Likewise, a high-powered probe is necessary to zoom in to capture views with shorter time scales and smaller distances between particles. Laboratories with high-energy beams, such as CERN, the Fermi National Accelerator Laboratory, and the SLAC National Accelerator Laboratory, have examined strong force coupling at these smaller spacetime scales, when this value is relatively small.
The magnified view provided by the high-energy beams showed that the quark’s mass is small, only a few MeV. At least, that’s the size of their textbooks. But when quarks are probed with lower energy, their mass effectively grows to 300 megaelectronvolts.
This is because the quarks collect a cloud of gluons, the particle that carries the intense force, as they move across greater distances. The mass-generating effect of this cloud accounts for most of the mass in the universe – without this extra mass, the basic mass of quarks can only account for about 1% of the mass of protons and neutrons. The other 99% comes from this gained mass.
Similarly, one theory posits that gluons are massless at short distances but actively gain mass as they travel further distances. The normalization of the strong force coupling over large distances supports this theory.
“If gluons remain massless in the long run, the strong force coupling will continue to grow unchecked,” Dior said. “Our measurements show that the strong force coupling becomes constant with increasing distance being investigated, a sign that gluons gained mass through the same mechanism that gives 99% of the mass to the proton and neutron.”
This means that strong force coupling over large distances is important for understanding this mass generation mechanism. These results also help verify new ways of solving the equations of quantum chromodynamics (QCD), the accepted theory describing the strong force.
For example, flattening the strong force coupling over large distances provides evidence that physicists can apply a cutting-edge new technique called the Anti-de Sitter/Conformal Field Theory (AdS/CFT) binary. The AdS/CFT technique allows physicists to solve non-recursive equations, which can help in strong force calculations over large distances where iterative methods fail.
Congruence in “matching field theory” means that the technology is based on a theory that behaves the same way at all scales of spacetime. As the strong force coupling levels decrease at greater distances, it is no longer dependent on the spacetime scale, which means that the strong force is compatible and AdS/CFT can be applied. While theorists have already been applying AdS/CFT to QCD, these data support the use of this technique.
“AdS/CFT has allowed us to solve problems of QCD or quantum gravity that have hitherto been intractable or nearly addressed using not very rigorous models,” Dior said. “This has yielded many exciting insights into fundamental physics.”
So, while these results are generated by empiricists, they affect theorists the most.
“I think 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 Jefferson Lab physics community, and in particular Dr. Alexander Dior, for this major advance in physics.”
It’s been years since experiments that mistakenly carried these results were carried out. A whole new set of experiments is now using the high-energy 12 GeV beam from Jefferson Lab to explore nuclear physics.
“One thing that I am very pleased with about all these old experiences is that we have trained so many young students and they are now leaders for future experiments,” Chen said.
Only time will tell which theories are supported by these new experiences.
Reference: “Experimental Determination of QCD Effective Fee αg1(s) “by Alexandre Dior, Volker Burkert, Jianping Chen and Wolfgang Korsch, May 31, 2022, grains.
DOI: 10.3390 / 5020015 جزيئات particles