John Riley Holt. 15 February 1918 — 6 January 2009

John Riley Holt was an experimental physicist who dedicated his working life to research in nuclear and particle physics at the University of Liverpool. He was born in 1918 in Runcorn, Cheshire, and in 1938 was awarded a first-class honours degree in physics at Liverpool University. He obtained his PhD in 1941 and became a member of Sir James Chadwick’s team working on the UK atomic weapon project. After the war he developed several new experimental techniques, which he used to make a systematic study of the deuteron stripping process with the use of the Liverpool cyclotron. After the Liverpool synchro-cyclotron became operational in 1954 he initiated a programme of precision measurements of cross-sections for proton–proton and pion–proton scattering. After the first observation of parity violation in 1957, his group completed an important experiment that observed parity violation in muon decay. When the construction of an electron synchrotron (NIN A, at the Daresbury Laboratory) was proposed, he became leader of the magnet design team. As NIN A became operational in 1966, John established a group to measure the cross-sections for the photoproduction of neutral and charged pions. The group then developed a collaboration with colleagues in other universities to measure the spin dependence of cross-sections for meson photoproduction by using a polarized photon beam and a polarized proton target. Before his retirement he contributed to the design of the experiment to determine the spin structure of the proton, performed by the European Muon Collaboration.