Unravelling atoms: the Centre for the Subatomic Structure of Matter

Almost all matter we can see and touch is made up of the protons and neutrons. But what are protons and neutrons composed of? The simple answer is quarks, of which there are six distinct kinds, held together by gluons.

The ‘strong force’ carried by gluons is about 100 times stronger than electromagnetism, which governs the interactions of atoms. It’s a major focus of the ARC Special Research Centre for the Subatomic Structure of Matter (CSSM).

Established 20 years ago at the University of Adelaide, the Centre is at the international forefront of investigating the ramifications of quantum chromodynamics (QCD), the theory which describes the strong force interactions that are fundamental to how our world works.

Three quarks become five

Electrons within atoms can become excited when energy is injected—a phenomenon that forms the basis of photosynthesis and lasers. And so can protons, neutrons and their extra-terrestrial relatives. Now CSSM physicists have shown that in some cases this can lead to a total rearrangement in their internal structure, which may well give rise to unusual properties.

Almost all matter we can see and touch is made up of baryons, subatomic particles composed of three quarks held together by gluons. Protons and neutrons are the common examples found on Earth.

There are six types of quark but we only encounter two, the up and down quarks. The next simplest three-quark particle is the Lambda particle, where one of the quarks in a proton or neutron is replaced by another type, the strange quark. Lambda particles can form on Earth in accelerator facilities or when cosmic rays strike. But they are unstable and decay almost immediately.

It was recognised in the 1960s that the energy of the lowest excited state of the Lambda particle is much lower than that calculated on the basis of an internal structure composed of just three quarks. This was recognised as a problem for QCD theory.

Since that time, the power of computers has increased exponentially as they have evolved into today’s supercomputers and the capacity of algorithms to handle this problem has also become more sophisticated, says CSSM Associate Director Professor Derek Leinweber.

He, together with colleagues including PhD candidate Ben Menadue, senior research fellow Dr Waseem Kamleh, and Centre Director Professor Tony Thomas, have now been able to calculate what should happen directly from first principles of QCD.

The team found the lower energy level is explained by the three quarks rearranging into a five-quark system—including an antiquark.

“The internal structure went from a single atom into something more molecular with two different quark-structures bonded tightly together. We didn’t put the extra quarks in. QCD created them in describing nature.”

Towards stable superheavy elements

For about 80 years since the discovery of the neutron, physicists have imagined the atomic nucleus simply as a weakly-bound collection of protons and neutrons (nucleons).

But protons and neutrons have an internal structure, and building this into models of how atomic nuclei are constructed can lead not only to a different and better understanding of the world around us, but also to predictions of new forms of matter, says CSSM director, Professor Tony Thomas.

In the 1970s, it was recognised that nucleons are composed of quarks bound by gluons. Given the relativistic forces at play between these subatomic elements, and how closely the protons and neutrons are packed, a suggestion arose that this internal structure might change as they bind together.

The diameter of a nucleon is about a hundred thousandth that of an atom, Tony says. And in the nucleus of an atom of lead, for instance, the distance between them is about the same.

“They are almost touching,” Tony says.

“So, they must be feeling the forces of the other quarks and gluons in nearby protons and neutrons.”

Tony and three international colleagues have done calculations reflecting this and recently published them in the leading American physics journal, Physical Review Letters.

They found that by incorporating such forces and the resulting internal changes, the outcomes closely matched the known binding energies of nuclei across the whole Periodic Table.

In fact, their results grew even more accurate as atoms became heavier. The group is now awaiting confirmation of this remarkable new picture of the atomic nucleus from measurements at Jefferson Lab in the US, the world’s leading electron scattering facility.

At present, as physicists generate ever-heavier nuclei, the new elements they form become increasingly unstable. But, based on this picture of changes of internal structure as protons and neutron bind, Tony thinks it possible we will find a group of stable superheavy elements—if we just go high enough.

Banner image: an artist’s rendition of the structure of the Lambda 1405 baryon resonance, illustrating the energy density of a typical gluon field fluctuation found in the QCD vacuum. Credit: Derek Leinweber, CSSM, University of Adelaide