At Manchester, Butler worked with G. D. Rochester, studying cosmic rays using a cloud chamber. During this research, they found two unexpected events, one in October 1946 and one in May 1947, showing previously unknown particles. These V particles were a thousand times heavier than the electron and long-lived on nuclear timescales. In order to increase the rate of detection with a higher flux of cosmic rays, the equipment was moved to the Pic du Midi de Bigorre observatory in the Pyrenees. The existence of the particles was confirmed by a group from the California Institute of Technology led by Carl Anderson, and the Butler group continued to study them and found that there were two types, hyperons and mesons. Butler's and Rochester's discovery may be seen as the first step towards understanding of the quark structure of matter. In 1953, Butler left Manchester to lead a high-energy nuclear physics group at Imperial College in London. He was promoted to professor in 1957, and head of the physics department in 1963. While at Imperial College London, Butler chaired the consortium established to design the British National Hydrogen Bubble Chamber, which was operated at the Rutherford Appleton Laboratory and CERN. From 1964, he chaired the CERN Track Chamber Committee. He was elected a fellow of the Royal Society in 1961, and served as dean of the Royal College of Science between 1966 and 1969. Over this period he became increasingly interested in educational policy, joining the academic planning board of the University of Kent in 1963, the Schools Council in 1965, and the University Grants Committee in 1966. He eventually served on the Schools Council for 19 years. In 1970, Butler resigned from Imperial College and his active involvement in physical research ceased. He accepted the role of director of the Nuffield Foundation, a charitable foundation influential in education. During his time there he set up a group for research and innovation in higher education, a programme for law and society, and the centre for agricultural strategy at the University of Reading. His role in the establishment of the Open University was also important, since the Nuffield Foundation funded a course on genetics as a test bed for other courses. He was appointed to the council of the Open University in 1971 and stayed as a member until 1995, serving as vice-chairman from 1986 to 1995. In 1975, Butler was appointed vice-chancellor of Loughborough University, retaining that post until he retired in 1985.
Honours
Butler was knighted in 1983 for his services to education.
