How do the tiniest constituents of matter behave?

We always say that the universe has its origin in the Big Bang, when everything was compressed into a point. But a universe-sized amount of matter compressed into a pinprick – what kind of stuff is that?

Pasi Huovinen

As we know, ordinary matter around us consists of atoms. They are made of electrons and nuclei, and the latter of protons and neutrons. Protons and neutrons have a structure too, and consist of quarks and gluons. When matter is heated, the connections between its constituents get weaker – ice melts, water boils. Eventually the atoms break up into electrons and nuclei – so-called plasma is formed. If we continue to heat the plasma, we reach a stage where even protons and neutrons do not hold together, but are broken into quarks and gluons. This state of matter is called quark-gluon plasma, and since quarks and gluons are point-like, without any structure, this is the stuff of the Big Bang.

The temperature required to form quark-gluon plasma is humongous, and the only way to form it here on Earth is by the collisions of heavy nuclei – like gold or lead – at velocities close to the speed of light. We cannot observe the quark-gluon plasma formed in these collisions, since its huge pressure blows the tiny plasma droplet apart immediately. Instead we have to deduce its formation and properties by studying protons, neutrons and other particles coming from it.

The first part of my project was to improve our understanding of the properties of these conventional particles at the late stages of the evolution of the droplet of plasma. We especially studied the class of particles called resonances and showed how their description should be improved to draw reliable conclusions about the properties of the formed droplet of matter. In addition, we studied how the repulsive interactions affect the properties of the cloud of particles emitted from plasma.

The second part of my project was to analyse the properties of particles coming out of the plasma: their momenta, i.e. velocities, the directions where particles fly, how the amount of particles flying to a particular direction correlates with particles flying to another direction, and how all this varies from one collision to another. All this allows us to eventually deduce what the properties of the plasma are. Such an analysis is very time-consuming. During my fellowship we were able to take only the first steps towards that goal, and the first results of this analysis are being published now, more than a year after the end of my fellowship.

In this way we improved our understanding, not only about how the tiniest constituents of the matter around us behave, but also about the early universe and the origin of us all.

How did you benefit from the POLONEZ fellowship?

During the POLONEZ fellowship I benefited from the Wrocław group’s expertise in hadrons and resonances. Knowing of their interactions is essential when modelling the heavy-ion collisions. I formed new connections with my Polish colleagues and initiated new collaborations. As an academic nomad, I should not downplay the very practical benefit of someone paying my salary for two full years either.

Last but not least, the POLONEZ fellowship allowed me to learn to know and appreciate Poland much better than previously. I enjoyed my two years in Wrocław.


Dr Pasi Huovinen was born in Varkaus, Finland in 1967. He gained a Ph.D. in theoretical physics at the University of Jyväskylä, Finland in 1999. Since then he has worked as researcher at various universities and research institutes both in the U.S. and Europe; his last position before the POLONEZ fellowship was at Goethe University in Frankfurt, Germany. Presently he is Research Professor (3-year position) at the Institute of Physics Belgrade, in Serbia. His main field of interest is nuclear matter in extremely hot and dense environments, its transition to quark matter, observation in heavy-ion collisions, and consequences to the Big Bang and neutron stars.