Rare kaon decay tests Standard Model

22 February 2002

An experiment at Long Island’s Brookhaven laboratory has just reported a long-awaited result: the observation of only the second decay of a positive kaon into a positive pion accompanied by a neutrino and an antineutrino.

The E787 experiment at the laboratory’s Alternating Gradient Synchrotron began 12 years ago with the aim of detecting this extremely rare kaon decay. The first sighting was reported four years ago, and some 6 trillion events have since been analysed in the quest for confirmation.


The Standard Model of particle physics forbids the direct decay of positive kaons into positive pions. Instead the decay must proceed via a two-step process involving massive gauge bosons. That’s what makes it so rare.

Theory predicts that such two-step decays should occur just a few times for every hundred billion kaon decays. The theoretical uncertainty on rare kaon decays is very small, so measuring them is an important test of the Standard Model, as well as being a sensitive indicator for new physics.

A new collaboration, known as KOPIO, has formed round the core of E787 to study another rare kaon decay – that of the long-lived neutral kaon to a neutral pion accompanied by a pair of neutrinos. KOPIO has support from Canada and Japan and is awaiting congressional approval in the US for National Science Foundation funding.

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