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Jean Willy André Cleymans (1944 – 2021)

Jean Cleymans
(Image: CERN)

Jean Cleymans, a pioneer in the study of ultra-relativistic heavy-ion collisions and a leader of the South Africa–CERN collaboration, passed away on 22 February 2021 in a tragic accident in his hometown, Turnhout, in Belgium. Jean was not only a most distinguished theorist in the field of high-energy nuclear collisions; he also played a crucial role in establishing and operating several large international collaborations, bringing together CERN, JINR Dubna, GSI, iThemba LABS and a number of universities. He had initiated and played a leading role in establishing the South Africa–CERN and South Africa–JINR Dubna programmes, as well as the participation of South Africa in the ALICE collaboration. Jean was a member of the Programme Advisory Committee for Particle Physics at JINR Dubna.

Jean Cleymans was born on 8 August 1944 in Turnhout, a town he always stayed close to. He studied physics at the University of Louvain, where he received his doctorate in 1970. Subsequently, he worked as a Humboldt fellow in Aachen (Germany), as a NATO fellow at SLAC (California), and eventually as a CERN fellow. From 1975 to 1986, he researched at the University of Bielefeld (Germany), where he carried out his habilitation in 1977 and subsequently became Extraordinary Professor. In 1986, he moved to Cape Town, where he was appointed as Full Professor of Theoretical Physics. The development of theoretical physics in South Africa, the training of young students in high-energy physics, and the connection of the country to international research is, to a very large extent, the work of Jean Cleymans.

Jean made seminal contributions to numerous areas in heavy-ion physics. Of particular impact was his pioneering work on the statistical description of hadron production in nuclear collisions, which effectively started this approach. His numerical code for the analysis of hadron multiplicities remains an essential tool for experimental studies, and in recent years has opened up new perspectives with the availability of high-precision experimental data at RHIC and the LHC. He published more than 200 papers in leading physics journals. Jean was instrumental in organising numerous conferences and workshops in the field, as well as acting as a member of a number of important international advisory committees. He was also a prominent member of the former Hard Probes Collaboration, which provided the foundation for future collaborative work in that area. In addition, he organised well-received meetings in South Africa, bringing experts from throughout the world there and establishing connections with South African researchers.

Jean did much to make the field attractive to students in South Africa. He was an inspiring and passionate teacher and supervised 24 MSc and 17 PhD candidates. By joining the ALICE collaboration and establishing first the UCT-CERN Research Centre and then the national South Africa–CERN programme, he reached even more students, researchers and academics. Even after becoming Professor Emeritus of the University of Cape Town in 2010, Jean continued to lead these initiatives and saw them grow into a vibrant high-energy physics community in South Africa, with strong links to CERN.

For his achievements, Jean received numerous honours – in 1993 he was elected Fellow of the Royal Society of South Africa, in 1999 he received the prestigious Alexander von Humboldt Research Prize, and in 2000 the Prize of the Polish Ministry of Education for Research Excellence, to cite just three here.

Jean was always receptive to new ideas and willing to enter into joint work with others. His attitude was always inspirational, and he had close friends and collaborators in many parts of the world. The field has certainly lost one of its great and inspiring leaders, just as we have lost a great friend.

Krzysztof Redlich, Helmut Satz and the ALICE collaboration