Testing a fundamental force: New study advances our understanding of the universe

Proton spin structure function g₂ depending on the invariant mass O. Credit: Natural Physics (2022). DOI: 10.1038/s41567-022-01781-y

Research by a team of physicists from the University of New Hampshire is helping to better understand how protons, which make up 95% of the mass in the visible universe, interact with each other. The results provide a benchmark for testing the strong force, one of the four fundamental forces of nature.

“There’s still a lot left unanswered about these two things, the proton and the strong force,” said David Ruth, Ph.D. physics candidate and lead author. “It brings us one step closer to that understanding. It’s a necessary part of two very fundamental things in the universe.”

The strong force governs how that which is internal to the nucleus of the atom – the neutrons, the protons and the quarks and gluons which compose them – binds together. It is the least understood of the four fundamental forces of nature, which include gravity, electromagnetism and the weak force.

In the study, recently published in the journal Natural Physics, the researchers tested two state-of-the-art, competing theoretical calculations of the strong force with an experiment probing the spin of protons in a regime, or mode of operation, where the quarks, or elementary particles, that compose them are at great distance the one another. Their experimental results agreed with one of the calculations but not with the other.

This type of physics work requires strong collaboration between theorists and experimenters. So the next step in strong forces research is for theorists to take a closer look at why the calculations don’t agree. They explain that these calculations are very complex, each theory group makes different choices about how to do them and some of the calculation choices made by theorists turned out to be different. To better understand the strong force, they need to know which is right and which is wrong.

“If we really want to understand our world, we need to have a solid theory of this force,” said Karl Slifer, professor of physics and astronomy and senior collaborator. “I don’t know what the applications will be, but this understanding could drive new technologies in the future.”

Slifer can imagine the work moving from theoretical or experimental to practical applications, much like when our understanding of nucleon interactions gave rise a century ago to applications such as fission, fusion and nuclear power. .

The hugely complex research took a decade to conduct and a small army of graduate students, post-docs and technical staff six months to set up and another six months to run. The experiment was conducted at the Department of Energy’s Thomas Jefferson National Accelerator Facility and, at the time, was the largest installation ever in Hall A at Jefferson Lab.