1973: neutral currents are revealed

The discovery of neutral currents at the Gargamelle bubble chamber was a major advance in understanding the fundamental particles of matter and their interactions.

Interactions determine the structure of matter from the cosmic scale of stars and galaxies to the microscopic scale of atoms and particles. They are mediated by carrier particles, and one of the aims of modern physics is to understand all the interactions in a single theoretical framework.

In the 1960s Sheldon Glashow, Abdus Salam and Steven Weinberg developed a theory that united two interactions, electromagnetism and the weak interaction that helps the Sun shine and is responsible for natural radioactivity. Their theory predicted that the weak interaction should be mediated by particles both electrically charged and neutral. This implied that weak reactions where the charges of the interacting particles were not shuffled around should exist. The problem was that such weak neutral current reactions as they were called, had never been observed. The quest was on to find them.

In 1972, in an experiment conducted by André Lagarrigue and colleagues, an invisible neutrino passed through the Gargamelle bubble chamber at CERN jolting an electron in its wake. Could this be an example of a neutral current event? No other experiments had observed similar events and confirmation was needed. After careful scrutiny and some more events of the same type, the discovery of neutral currents was announced in 1973.

Direct observation of the two charged particles, called W, and their neutral counterpart the Z, had to wait until the 1980s when experiments at CERN’s proton–antiproton collider observed them for the first time. Glashow, Salam and Weinberg received the Nobel prize in physics in 1979, with Carlo Rubbia and Simon van der Meer sharing the prize in 1984 for their decisive contributions to the discovery of W and Z particles.