Calculating the strong attraction of a charming particle

A theoretical study by RIKEN physicists, published in Physics Letters B, has accurately determined the interaction between a charmonium and a proton or neutron for the first time.

From two galaxies colliding to an electron jettisoned from a nucleus, all interactions in the universe can be described in terms of just four fundamental forces.

Gravity and the electromagnetic force are the two we are familiar with in everyday life, while the weak and strong forces operate over minuscule distances—roughly the size of an atomic nucleus or smaller.

As its name implies, the strong force is the strongest of the four forces over minuscule distances, and it binds protons and neutrons (collectively known as nucleons) within the atomic nucleus. It operates between quarks and gluons, which are the building blocks of nucleons.

"Nuclear power plants generate electricity by harnessing the strong force, and the sun's energy is basically derived from it," says Yan Lyu of the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program. "So it's important to understand how the strong interaction works."

The strong force is described mathematically by quantum chromodynamics. While the theory is complete, how it operates in specific situations is an area of active research, both theoretically and experimentally in facilities such as particle accelerators.

Now, Lyu and co-workers have used quantum chromodynamics to theoretically investigate strong interactions between a nucleon and a type of short-lived particle called a charmonium.

Nucleons are made from up and down quarks, which are two of the six types of quarks. A charmonium, on the other hand, consists of a charm quark and a charm antiquark.

"The discovery of the first charmonium in November 1974 is sometimes referred to as the November Revolution because it was such a groundbreaking event in particle physics," says Lyu. "The two scientists involved were awarded Nobel prizes just two years later."

While charmonium was discovered more than half a century ago, many questions remain about how it interacts with other particles.

"One fundamental question is how charmonia interact with nucleons," says Lyu. "That's the basic question we addressed in this study."

Performing calculations on Fugaku, one of the world's most powerful supercomputers, Lyu and collaborators found that the interaction between the two particles was attractive at all distances. They were also able to pin down its magnitude much more accurately than previous estimates.

Experimental confirmation of the results may not be far off. "Experimentalists at the Large Hadron Collider in Europe said they're planning to measure the interaction between a nucleon and a charmonium particle in a few years," says Lyu.

The team plans to broaden their investigation. "We expect our findings will also be applicable to other systems," says Lyu. "That's something we're investigating now."