The strong force, also referred to as the strong nuclear force, is one of the four basic forces in nature, along with gravity, the electromagnetic force, and the weak nuclear force. As its name implies, the strong force is the strongest of the four; however, it also has the shortest range, meaning that particles must be extremely close before its effects are felt. Its main job is to hold together the subatomic particles of the nucleus (protons and neutrons). In new research, physicists at the Thomas Jefferson National Accelerator Facility experimentally extracted the strength of the strong force. Known as the coupling of the strong force, this quantity describes how strongly two bodies interact or ‘couple’ under this force. Strong force coupling varies with distance between the particles affected by the force. Prior to this research, theories disagreed on how strong force coupling should behave at large distance: some predicted it should grow with distance, some that it should decrease, and some that it should become constant. With the new data, the researchers were able to determine the strong force coupling at the largest distances yet.
The strong force is the fundamental force that binds smaller particles called quarks into larger particles called protons and neutrons. Image credit: Gerd Altmann.
At smaller distances between quarks, strong force coupling is small, and physicists can solve for it with a standard iterative method.
At larger distances, however, strong force coupling becomes so big that the iterative method doesn’t work anymore.
“This is both a curse and a blessing,” said Dr. Alexandre Deur, a physicist at the Thomas Jefferson National Accelerator Facility and the Department of Physics at the University of Virginia.
“While we have to use more complicated techniques to compute this quantity, its sheer value unleashes a host of very important emerging phenomena.”
Despite the challenge of not being able to use the iterative method, Dr. Deur and colleagues extracted strong force coupling at the largest distances between affected bodies ever.
They extracted this value from a handful experiments at the Continuous Electron Beam Accelerator Facility (CEBAF) that were actually designed to study something completely different: proton and neutron spin.
CEBAF is capable of providing polarized electron beams, which can be directed onto specialized targets containing polarized protons and neutrons in the experimental halls.
When an electron beam is polarized, that means that a majority of the electrons are all spinning in the same direction.
These experiments shot polarized electron beams at polarized proton or neutron targets.
During the several years of data analysis afterward, the researchers realized they could combine information gathered about the proton and neutron to extract strong force coupling at larger distances.
“Only CEBAF’s high-performance polarized electron beam, in combination with developments in polarized targets and detection systems allowed us to get such data,” said Dr. Jian-Ping Chen, a physicist at the Thomas Jefferson National Accelerator Facility.
The authors found that as distance increases between affected bodies, strong force coupling grows quickly before leveling off and becoming constant.
“There are some theories that predicted that this should be the case, but this is the first time experimentally that we actually saw this,” Dr. Chen said.
“This gives us detail on how the strong force, at the scale of the quarks forming protons and neutrons, actually works.”
The team’s results were published in the journal Particles.
_____
Alexandre Deur et al. 2022. Experimental Determination of the QCD Effective Charge αg1(Q). Particles 5 (2): 171-179; doi: 10.3390/particles5020015
