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Prestigious prize awarded to particle physicist

Portrait. Photo.
Torbjörn Sjöstrand. Photo: Private.

Torbjörn Sjöstrand, post-retirement professor at the Department of Astronomy and Theoretical Physics, has been awarded the EPS High Energy and Particle Physics Prize. This desirable prize, which has previously been given to several Nobel laureates, is awarded by the European Physical Society.

Congratulations on the prize, Torbjörn, how does it feel?

Well, of course it feels great. In everyday life, which has become particularly grey over the past year, every dash of colour is welcome.

Why do you think you were awarded this prize?

Because I developed a computer program which is used all over the world by thousands of particle physicists. In more detail, my research is about describing collision processes between high-energy elementary particles. These take place for example at the Large Hadron Collider facility at CERN on the outskirts of Geneva. In such collisions, hundreds of new particles can be formed, according to complex patterns that are only partially understood. I have dedicated over 40 years to developing a computer program that attempts to describe what is happening in the greatest possible detail.

Why is this computer program called Pythia?

In ancient Greece, there was a famous oracle’s temple in Delphi. A woman from the area took on the role of the oracle Pythia who, by inhaling volcanic fumes, began to speak incoherently. The priests were in charge of interpreting her speech and presenting the responses to advice-seekers. The priests were often able to provide useful answers, but “misinterpretations” of the oracle’s predictions also occurred. Similarly, the Pythia computer program is expected to provide useful answers to many of the questions particle physicists have. But it is still true that common sense is required to interpret the answers provided, so as not to end up in the wrong place.

Can you tell us a little more about the prize?

It has been awarded every other year since 1989, either for experiment or theory in particle physics. The prize is awarded at a major international meeting in late July, this year as a fully virtual event. As far as I know, there is no prize money involved, but it is the honour that counts.

Which of your research findings do you think were most significant in this context?

What the jury particularly highlighted were the studies of bremsstrahlung. This term is perhaps mainly associated with major synchrotron light facilities such as MAX IV in Lund, where electrons which are accelerated or decelerated emit intense light. On a smaller scale, the same mechanisms enable your cell phone to send radio signals. However, what I studied was not that kind of bremsstrahlung, but rather the kind associated with the strong force. The one that holds quarks together in protons and neutrons, and the latter together in atomic nuclei. There are similarities but also important differences.

Several Nobel laureates have previously been awarded this physics prize; will it soon be your turn?

Of course it is exciting to receive the same prize as several people who went on to become Nobel laureates, but the list of prize-winners also includes many who did not. Physics is a broad field, and particle physics is only one of several areas which are all to be rewarded, so competition is stiff. In addition, the Nobel Prize rules stipulate that a specific well-defined achievement is to be recognised, rather than long and faithful service, which is more the case for my prize.

You share the prize with the Faculty of Science’s 2012 honorary doctor, retired Cambridge professor Bryan Webber; how come?

We have each worked on the development of two different computer programs, in a friendly rivalry. As there is often no precise, unambiguous answer, the access to different models helped to drive development forward.

How will the prize affect your future research?

Probably not that much on a personal level. I recently retired but continue to work as a post-retirement professor. This means that I will still have freedom in future to continue with various studies. It is perhaps more important for my colleagues that this field is recognized. At the start of my career, I often worked alone, but over the years more people have joined me, and now we are a dozen researchers, mostly in Lund and Europe but also in the USA, India and Australia.

How does a particle physicist celebrate a prize of this distinction?

In normal circumstances, probably with a cake and sparkling wine at the department. But right now that isn’t possible. I’ll have to see.

Read more about the prize on

Text: Johan Joelsson.