Hadron Physics: Understanding the building blocks of nature

Somewhere between Nuclear Physics and Elementary Particle Physics lies a fascinating new field of physical science. Hadron Physics studies the smallest particles of matter, and Tord Johansson, Professor of Nuclear Physics at Uppsala University, is leading an ongoing research project to understand the forces that hold the very stuff of nature together..

When it was finally split in the 20th century, we discovered the atom was not the smallest particle of matter after all. Inside atoms were nuclei with even smaller particles called protons and neutrons and these in turn were made by the binding together of the smallest particles of all – quarks – by another elementary particle called a gluon (from the English words ‘glue’ and ‘on’).

A hadron is simply a particle made of quarks that are held together by a strong force mediated by the gluons. In an atom, the protons and neutrons combine to form the very centre of the atom – the nucleus. And it is this nucleus that makes up 99.9% of the mass of all ordinary matter in the universe. In short, then, hadrons are the very centre of nature itself.

Tord Johansson is a leading figure in the HadronPhysics2 (HP2) Integrating Activity, a Europe-wide initiative to study the forces binding quarks and gluons together and how those forces act under different conditions and stresses. “We understand the forces present at high energies pretty well,” he says, “but very little about the strong force acting at low energies relevant to the matter surrounding us. That’s what this research is designed to discover more about.”

It’s a big project – Johansson’s team of 12 is just a fraction of the 2,500 researchers that make up HP2 across Europe – since it comprises the vast majority of the European physicists in the field. “It’s a truly challenging field,” says Johansson. “We are searching for things that don’t fit the expected patterns. Physics is a lot like that – discovering behaviours in nature that we don’t understand. Hadron Physics is one of these fields.”

Because of the groundbreaking nature of the research, Johansson says it’s been just as much of a challenge to design the experiments as deciding what those experiments should be beforehand. “Even when you’ve decided what you want to test, there are no off-the –shelf equipment you can use to test your ideas,” he says. “It’s about buliding up new experiments that work, sometimes with novel technologies, before you can even begin to find out what they might reveal.” Observing this great wide open of unknowns, Johansson emphasises the research is not about finding a definitive answer about nature but just about finding out more. “As the saying goes, the more you know, the more you find you don’t know!” he says.

“Physics is an infinite subject – we cannot hope to know it all from first principles. What we can do is try and discover more about it, understand a fundamental part of it better.” Of the research infrastructures relevant for HP2, Johansson highlights the PANDA experiment at the upcoming FAIR facility in Darmstadt, Germany as key ingredient in pursuing such understanding. He is, in the meanwhile involved in the WASA experiment at FZ-Jülich, Germany and the KLOE-2 programme at Frascati, near Rome. Both of which aim to study the low energy aspects of the interaction between the quarks and in particular to test theoretical predictions and how the symmetries, relevant to the stong interactions may be violated in the decay of hadron particles.

And what of the key findings? Johansson picks out two that have significantly improved our understanding of hadron physics. “Firstly, of course, we’ve discovered that objects in nature are made of very small building blocks – much smaller than atoms – and, what is even more amazing, that you cannot take these building blocks apart.” Indeed, the research has found that the more you move quarks apart, the stronger the force binding them becomes. The only way to liberate them seems to be to provide enough energy over the nuclear volume so the quarks change their very state and become plasma instead – the primordial state in which our universe began.

“Secondly, we now understand that the mass of the atomic nucleus is much greater than the total mass its quarks. When I was a student, the thinking was that the mass of the nuclei was, as you would expect, of the same order as its parts. But what we’ve found is that additional mass seems to be generated by the strong interaction between the quarks themselves making the nucleus almost 100 times heavier.” These findings still lack a true understanding and this is much what hadron physics is about.

Uppsala University has gained international recognition through the work of Johansson and his team researching the forces that bind quarks together. It has helped to put Hadron Physics at the forefront of scientific research, with other hi-tech countries such as the US and Japan making it a priority within their considerable research budgets. Perhaps that’s not surprising, when to gain further understanding of the forces holding nature together is to gain further understanding of the essence of nearly all the ordinary matter in our universe.

When asked about the future, Johansson remains philosophical about the field of science he loves. “There is no fixed end point for our research because we are not trying to reach any finite point in our understanding” he says. “Indeed, HP2, as its name suggests, developed from its predecessor HP1 which began some time ago. We are continuing our research indefinitely, in the hope of understanding a bit more of the nature of things that surround us.”

To find out more information – please contact tord.johansson@tsl.uu.se

Published: Wednesday, 29th June 2011 by Clive Somerville