Spot the difference: two neutrinos

15 July 1998

 
On 1 July 1962, a paper appeared in Physical Review Letters* reporting experimental evidence for the existence of two distinct types of the elusive particles known as neutrinos - one associated with the well-known electron, the other with the less-familiar, heavier version of the electron, the muon. In 1988, the three leading physicists who made this discovery at the Brookhaven National Laboratory - Leon Lederman, Melvin Schwartz and Jack Steinberger - shared the Nobel Prize for physics. Today, a variety of experiments routinely seek out the different types of neutrino. These images show computer reconstructions of neutrino interactions in the Soudan detector, 713 m below ground in an iron mine in Minnesota. In the detector, steel plates form a massive honeycomb structure permeated by a mixture of carbon dioxide and argon gas. Neutrinos do not affect the gas as they pass through, but if they interact in the detector, any charged particles that are produced will ionise the gas. The ionised trails generate electrical signals that are recorded so that a computer can then show where the particles went and how much energy they had. The image on the left shows the "shower" of tracks characteristic of an electron, made in the interaction of an electron-neutrino. The right-hand image show the long straight track of a muon, in this case created by a muon-neutrino. The Soudan detector provided some of the first evidence that neutrinos may in fact be able to change - or oscillate - from one type to another, as also indicated recently by the Super-Kamiokande experiment.

(* See Physical Review Letters 9 36 (1962))

Credit: Physics Photographic Unit, Oxford University / Soudan

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