Torbjörn Sjöstrand

ProfessorKnown as name: Torbjorn Sjostrand

Research areas and keywords

UKÄ subject classification

  • Subatomic Physics


I study theoretical particle physics, which is the research area concerned with the smallest constituents of matter and how they interact with each other. More precisely, I am interested in phenomenology, i.e. the intersection where theory and experiment meet, and ideas flow in both directions.

Chance plays a big role in the microcosm. When two particles are hurled against each other with high energy, a collision can lead to long and complicated chains of random processes, which in the end can give hundreds of newly created particles moving out in different directions. No two collisions are identical, even if they may share common patterns. It is like a parlour game on a huge and complicated playing field, where the random choice of the dice is complemented by special rules for many squares. In the particle world the dice is called "quantum field theory", the rules are called "the Standard Model", and the playing field is a region approximately 0.0000000001 mm across. There are problems, however: even if the rules can be written in mathematical formulae so compact that they do not even fill a single sheet, the mathematics still is so complicated that nobody knows how to apply it. Often an approximate approach based on perturbation theory works, but for the strong force, QCD, this is usually not enough.

We then need models, which we believe are close to the truth, but still are sufficiently simple to be useful, and where different assumptions can be varied in a controlled manner. Such a model begun to be developed in Lund, in the group around professors Bo Andersson and Gösta Gustafson, and is accordingly called "the Lund model". One way to study this model, which rapidly proved to be the best one, is to let a computer simulate collisions according to the imagined set of rules, and produce final states with properties that can be directly compared with experimental events on a statistical basis. Such computer programs are called Monte Carlos, after a known place where chance reigns. My Ph.D. thesis 1982 was based on the development of the first Lund Monte Carlo, from physics ideas to code.

Gradually the description has been refined, and extended to encompass almost all known aspects of collision processes. Again it has meant the development of new physics models, and finding smart ways to implement these as working code. Comparisons with existing data, and suggestions for new studies with matching predictions, can help separate promising ideas from dead ends. The computer programs have come to be used by experimental particle physicists across the world, for comparisons with and interpretations of data. In the search for new physics phenomena, beyond the today accepted Standard Model, the programs can predict the consequences of different hypothetical scenarios. The interesting effects often are tiny, so lessons from computer simulation are relevant for detector design and search strategies.

I have led the development of the program called Pythia, which has played a leading role within the field for several decades, and also been the foundation on which many other programs have been developed. Over the years a number of students have contributed to the development, and some of these still are active collaborators, spread across the globe. The program has a size of around 100,000 lines of code, plus about as much for documentation and data files.

It is no chance that the program has been named after the priestess at the famous oracle in Delphi: replies can be ambiguous, and it is as important to formulate precise questions as to correctly interpret the replies. But, partly with the help of the current Pythia, we hope to understand more about the rulse of the game in the Universe.

Recent research outputs

Fischer, N. & Torbjörn Sjöstrand 2017 Jan 1 In : Journal of High Energy Physics. 2017, 1, 140

Research output: Contribution to journalArticle

Torbjörn Sjöstrand, Ask, S., Christiansen, J. R., Corke, R., Desai, N., Ilten, P., Mrenna, S., Prestel, S., Christine Rasmussen & Skands, P. Z. 2015 In : Computer Physics Communications. 191, p. 159-177

Research output: Contribution to journalArticle

Christiansen, J. R. & Torbjörn Sjöstrand 2015 In : European Physical Journal C. Particles and Fields. 75, 9, 441

Research output: Contribution to journalArticle

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