Professor Francesco Sannino, the head of the Centre for Cosmology and Particle Physics Phenomenology (CP3-Origins) and Danish IAS Co-Director, University of Southern Denmark, deals with the smallest building blocks in the universe. Matter in the universe is made up of atoms, which are constructed of protons, neutrons, and electrons. Protons and neutrons are made up of quarks and gluons – so-called fundamental particles that cannot be divided into smaller parts. Sannino is working on understanding the fundamental interaction between particles, including quarks. Using a different theoretical approach, he tries to create a foundation for an extension on the standard model – the theory of particle physics.
The standard model describes the interactions between particles, but it is not a fundamental theory in that sense because the model breaks down at extremely small distances between the fundamental particles. Therefore, particle physicists work on expanding the standard model in the ultimate hope of creating a so-called theory of everything. Over the past few years, Sannino has worked on a different theoretical approach that can underlie alternative expansions of the standard model. In an article published on December 28, 2017 in the scientific journal Physical Review Letters and a related article in the journal Physical Review D in September 2017, Sannino and his colleagues presented a theory that can be tested experimentally and that forms a new framework that other researchers can use in their work.
“It is a bit like saying that we previously had a certain set of LEGO bricks, and now we add a whole new pile of bricks to our set. This means that we now have more building blocks, which opens new perspectives and possibilities for constructing models that you might not previously have been able to see,” said Sannino.
A different paradigm for interaction between quarks
Today, so-called asymptotic freedom is the dominant paradigm of interaction between quarks. The theory states that quarks are born free because they do not sense each other when the distances between quarks are extremely short. On the other hand, it is impossible to pull quarks apart and isolate the particles. The force between quarks can roughly be compared to a string. The more you pull quarks apart, the more the string pulls them back, but the closer the quarks are to each other, the freer they feel.
In 2004, three American physicists were awarded the Nobel Prize in Physics for the discovery of asymptotic freedom in the theory of interaction between quarks and gluons. Unlike the standard model, the theory is fundamental in the sense that asymptotic freedom, in theory, is valid at all length scales. However, no experiment has explored what happens among quarks at extremely short distances.
This is where Sannino’s theoretical approach comes into the picture. The theory operates with a different paradigm – so-called asymptotic safety. Sannino, however, thinks that asymptotic freezing would be a more fitting description. Asymptotic safety basically means that quarks are “almost” free and unaffected by other quarks when they are neither too close together nor too far apart. This means that the quarks always continue to interact at very small distances but the interaction freezes rather than ceases.
Enables new theories of nature
In theory, asymptotic safety’s advantage, compared to asymptotic freedom, is that it is possible to make the standard model asymptotically safe and thus secure the fundamental aspect of the theory. However, achieving asymptotic safety requires the existence of several new particles that interact with known matter in the universe. In a newly published article from Physical Review Letters, Sannino and his colleagues argue that expanding the standard model with particles with specific interactions makes it possible to realize an asymptotically safe and fundamental theory.
“Our results show that it is possible to construct new fundamental theories for nature based on asymptotic safety. It is interesting because it changes the viewpoint of interaction between quarks and potentially means that we can expand the standard model to be fundamental. At the same time, the results make it possible for other researchers to use our theoretical framework in creating and testing their own theories,” explained Sannino.
The possible existence of several unknown particles is also good news for researchers who explore new physics, for example, at the Particle Accelerator LHC at the research center CERN in Switzerland, where large experiments chase signals and evidence of unknown particles.