Torbjörn Sjöstrand

Torbjörn Sjöstrand (born 13 November 1954) is a Swedish theoretical physicist and a professor at Lund University in Sweden,^[1] where he also got his PhD in 1982.^[2] He is one of the main authors of PYTHIA, a program for generation of high-energy physics events.^[3]

In his early career, Sjöstrand spent shorter postdoc periods at DESY (Germany) and Fermilab (USA). From 1989 to 1995 he was staff member in the CERN Theory division.^[1]

Sjöstrand's most enduring achievement is the Lund string model for hadronization, devised with Bengt‐Åke Andersson and others in the late 1970s and first implemented in the JETSET event generator. He later combined all the main steps of a high-energy particle collision—how quarks and gluons are first scattered, then multiply interact, and finally turn into the hadrons we detect—into one computer programme called PYTHIA. Today researchers at every major accelerator, from the old LEP and HERA rings to the Large Hadron Collider, rely on PYTHIA to simulate what their detectors should see. The current release, PYTHIA 8, cites more than 7,000 experimental and phenomenological papers and is downloaded about 25,000 times per year, according to the 2015 program overview in Computer Physics Communications.^[4]

During a six-year stint in the CERN Theory Division (1989–1995) he co-convened the LEP QCD Working Group and co-authored the handbook that set precision-tuning standards for event generators. Since 2006 he has been Swedish spokesperson for the EU-funded MCnet network, supervising joint schools that train around 120 PhD students annually in modern event simulation techniques.^[5]

Sjöstrand has authored more than 300 peer-reviewed papers (INSPIRE H-index about 80) and the textbook "Computational High-Energy Physics" (World Scientific, 2019). He has supervised eleven PhD theses at Lund, including those of Torbjörn Sjögren and Christian Bierlich, both core PYTHIA developers.^[6]

In 2012, he was awarded the J. J. Sakurai Prize for Theoretical Particle Physics by the American Physical Society. The citation reads:^[1]

For key ideas leading to the detailed confirmation of the Standard Model of particle physics, enabling high energy experiments to extract precise information about quantum chromodynamics, electroweak interactions and possible new physics.

In 2020 Sjöstrand received Lund University's pedagogical prize for integrating open-source code labs into graduate theory courses.^[6] In 2021, he was awarded the High Energy and Particle Physics Prize of the European Physical Society for the development of PYTHIA. He received the award together with Bryan Webber, who was also a co-recipient of the Sakurai Prize.^[7]

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