Bruno Pontecorvo

Bruno Pontecorvo, a former student of Fermi, was the first to propose that neutrinos might change identity as they travel — the idea now known as neutrino oscillation, and the thread that runs through much of modern neutrino physics.

From Rome to the wider world

Pontecorvo trained in Fermi’s celebrated Rome group before emigrating, working in France, the United States, Canada, and Britain. In 1946 he proposed a practical way to catch neutrinos: a radiochemical method in which a neutrino converts a chlorine atom into argon, to be extracted and counted. The technique would later be realised by Raymond Davis Jr. at Homestake.

A move east

In 1950 Pontecorvo and his family relocated to the Soviet Union, a defection that surprised his colleagues and shaped the rest of his career. He continued to work at the frontier of neutrino theory at Dubna for four decades.

The idea of oscillation

From the late 1950s Pontecorvo argued that neutrinos could oscillate, by analogy with the mixing already known among neutral mesons. The concept took its modern flavour-mixing form in the 1962 work of Maki, Nakagawa, and Sakata; the combined framework is encoded today in the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix that bears his name. By 1967 he had worked out how oscillation would affect neutrinos from the Sun — anticipating the solar neutrino problem before it was measured.

Vindication

For decades oscillation remained a hypothesis. It was confirmed at the turn of the century by Super-Kamiokande and SNO, the discovery honoured by the 2015 Nobel Prize — after Pontecorvo’s death. His insight that neutrinos mix and oscillate proved one of the most consequential ideas in particle physics, and it opens the Neutrino Oscillations series.