Quarks, gluons can now be visible at low energies, reveal scientists

For many years, scientists believed that fundamental particles like protons and neutrons that form an atomic nucleus, can’t be divided further into smaller units. However, in the following years, physicists discovered quarks and gluons. 

While quarks are particles that combine to form protons and neutrons, gluons act like glue, binding the quarks together. 

So far, scientists have been studying the atomic nucleus using two models. In the first model, at low energies like in most typical nuclear experiments, they describe the atomic nucleus in terms of protons and neutrons. This is the classic way of understanding the nucleus.

However, at high energies like in very powerful particle collisions, protons and neutrons break down into quarks and gluons. So, scientists describe the nucleus using quark-gluon models.

For years, scientists have been trying to connect the two models. Finally, a team of researchers from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ-PAN) has achieved this feat. They have developed an atomic nucleus model that shows quark-gluon interactions at low energies.

“Until now, there have been two parallel descriptions of atomic nuclei, one based on protons and neutrons which we can see at low energies, and another, for high energies, based on quarks and gluons. In our work, we have managed to bring these two so far separated worlds together,” Dr. Aleksander Kusina, one of the researchers, said.

Connecting the two atomic nucleus models

Just like light enables us to see things around us, scientists used charge particles like electrons to study what happens inside an atomic nucleus. They collide electrons with the atomic nucleus and study the interactions which at low energies reveal the presence of protons and neutrons, whereas at high energies make the quark-gluon interaction visible. 

The quark-gluon model effectively describes atomic nuclei at high energies. However, the problem is that its results don’t match with those observed in low-energy experiments, where protons and neutrons are the only particles visible.

To overcome this challenge, the researchers collected and examined the data from high-energy collision experiments conducted using the Large Hadron Collider at CERN. They looked for the parton distribution functions (PDFs), numerical values that explain the distribution of quarks and gluons inside protons and neutrons of an atomic nucleus at high energies.

“With PDF functions for the atomic nucleus, it is possible to determine experimentally measurable parameters, such as the probability of a specific particle being created in an electron or proton collision with the nucleus,” the researchers said. 

Using the PDF method, they could understand the distribution of quarks, gluons, and correlated nucleon pairs in 18 atomic nuclei. However, they found that among nucleons that can exist as protons-protons, protons-neutrons, and neutron-neutrons — the proton-neutron pairs were the most common correlated pairs. 

What’s even more surprising is that this result aligns with what is observed in low-energy experiments, especially those involving heavy nuclei like lead or gold, which also show a dominant presence of proton-neutron pairs. 

This finding brings the models of atomic nuclei at low and high energies on the same page, for the first time. It shows that along with protons and neutrons, quark and gluon interactions can also explain the properties of an atomic nucleus even at low energies. 

This will “open up new perspectives for a better understanding of the structure of the atomic nucleus, unifying its high- and low-energy aspects,” the researchers note.

The study is published in the journal Physical Review Letters.