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Image courtesy of Tom Kemp

The first neutral currents

2 September 1998

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Twenty-five years ago, on 3 September 1973, an historic paper appeared in the journal Physics Letters*. It described the first observation of "neutral-current" interactions, at the European laboratory for particle physics, CERN. This image shows one of those interactions, in the big bubble chamber called Gargamelle. A beam of the elusive particles known as neutrinos has crossed the chamber from the left. The neutrinos are electrically neutral, and so leave no tracks at all, but a spray of particles has appeared - as if from nowhere - emerging from a point towards the centre-left of the image. These particles are the debris of a nucleus in the bubble chamber liquid, which has been struck by one of the energetic, but invisible neutrinos. No muon (the charged relation of the neutrinos, which were muon-type neutrinos) appears in the debris. Instead, the neutrino has continued on its way, unchanged and unseen.

Neutrinos interact only through the weak force, and to remain unchanged the neutrino in this case must have interacted by exchanging a Z particle, the neutral carrier of the weak force. The observation of this and similar events in Gargamelle provided the first evidence for the existence of this particle, which had been predicted by the electroweak theory that unites the weak force with electromagnetism. Ten years later, the Z particle, which has nearly 100 times the mass of the proton, was created directly for the first time, also at CERN.

(*Physics Letters, 46B, 138 (1973))

Credit: CERN Photo

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