Ó Raifeartaigh, Lochlainn Séamus (1933–2000), theoretical physicist, scholar, and senior professor at the Dublin Institute for Advanced Studies, was born 11 March 1933 in Clontarf, Dublin, second eldest among three sons and three daughters of Tarlach Ó Raifeartaigh (qv), Gaelic scholar and civil servant, and his wife Neans, daughter of T. J. Morrissey, who became assistant secretary of the Department of Education. Neans had a keen interest in music and a gift for languages. She spoke Irish, French, German, and Swedish fluently, and studied Japanese for over ten years. Lochlainn Ó Raifeartaigh attended St Joseph's primary school in Marino, Dublin, and later Castleknock College (1946–50). In 1950 he completed school, securing a first place in mathematics in the Irish leaving-certificate examination and a university entrance scholarship to UCD. He had decided to study science, abandoning an earlier intention to study engineering. He had an outstanding undergraduate career at UCD, completing a joint mathematics and physics honours course as the best student in his year.
After graduating from UCD (1956), Ó Raifeartaigh obtained a research fellowship from the Dublin Institute for Advanced Studies (DIAS) to work with the great Irish relativist J. L. Synge (qv). After a year at DIAS, during which he attended lectures by Erwin Schrödinger (qv) and wrote three papers on relativity, Ó Raifeartaigh was awarded a travelling studentship to study under Walter Heitler (qv) in Zurich, who was a former professor at DIAS. After completing his Ph.D. on non-local field theories, Ó Raifeartaigh returned to DIAS as an assistant professor. Shortly afterwards (1963), as a result of a chance encounter with E. C. G. Sudarshan in Bern, Ó Raifeartaigh was invited to spend three months at the newly formed Mathematical Science Institute in Madras, India, to lecture on group theory. This was followed by an invitation to join Sudarshan's group in Syracuse, USA, in 1964. In 1968 Ó Raifeartaigh was offered a senior professorship at DIAS, which he accepted, returning to Dublin after spending some time at the Institute for Advanced Study in Princeton, USA.
He remained at DIAS for the rest of his life, building an internationally respected group in theoretical high-energy physics, and always working at the forefront of the subject. His method of collaboration was to interact and discuss physics at the blackboard. These discussions were not polite, considerate exchanges, but rather exciting, heated arguments which continued till the issues being debated had been clarified. The aim was to understand the essence of the problem being considered. Ó Raifeartaigh often said the main difficulty in tackling a problem was properly to understand it.
Lochlainn Ó Raifeartaigh had wide interests and was fluent in Irish, French, and German. He was a scholar in the history of his subject and wrote a masterly book, The dawning of gauge theory (1997), a historical account of the quest for a unified theory of the fundamental forces of the physical world. Ó Raifeartaigh received a von Humboldt research award in 1998 and the prestigious Wigner medal in 2000 for his pioneering contributions to particle physics. He was elected MRIA at the age of 29, and was also a member of Academia Europea. Ó Raifeartaigh was interested in the international aspects of science and was a co-founder of the Irish Pugwash movement.
His varied and important contributions to physics were marked by his passion for clarity and his instinct for uncovering the fundamental features in a problem. His best-known contributions are undoubtedly his ‘no-go theorem’ (1965) and his work on supersymmetry breaking (1975). The ‘no-go theorem’ brought to an end attempts to combine internal Lie group symmetries with Poincaré symmetry in a nontrivial way. This work used methods of group theory which were unfamiliar to most physicists at the time, and established Ó Raifeartaigh as a researcher of the first rank. The Ó Raifeartaigh mechanism for supersymmetry breaking was a result of a request from A. Jaffe at Harvard in 1974 to report on recent work by Wess and Zumino on supersymmetry in Aspen. This prompted him to study the work of Wess and Zumino carefully. Later, urged on by a remark of Pais that the problem of breaking of supersymmetry needed attention, Ó Raifeartaigh quickly showed that supersymmetry could not be broken by a single chiral superfield. Initial attempts by Ó Raifeartaigh to show that supersymmetry breaking by Higgs's mechanism was not possible failed, and eventually he was able to show that a minimum of three chiral superfields are needed for Higgs's mechanism to work.
Ó Raifeartaigh was a very productive and creative researcher. He published over 200 papers with over sixty collaborators. As new ideas came up, he studied them, discussed them with passion, and very often made important contributions to them. In all his work, Lochlainn Ó Raifeartaigh's love of physics, his interest in understanding fundamental problems clearly, and his ability to present results with precision and clarity shone through. He was an inspiring teacher for physics students in Dublin as well as abroad. His lectures on the Group Structure of Gauge Theories, published by Cambridge University Press, are very popular among students as well as researchers.
Ó Raifeartaigh died on 18 November 2000 after a brief illness. He married (1958) Treasa Donnelly, a student from QUB whom he had met while a member of An Cumann Gaelach (the Gaelic Society) at UCD. Treasa supported Lochlainn throughout his career; they had five children.