Subatomic particle sighting celebrated with doughnuts

Nuclear physicists have tracked down one of the last missing subatomic particles, a tiny, reclusive but essential building block of the cosmos.

Nuclear physicists have tracked down one of the last missing subatomic particles, a tiny, reclusive but essential building block of the cosmos.

Decades after its existence was first postulated on paper, a team of scientists at the Fermi National Accelerator Laboratory near Chicago have found the first direct evidence of the tau neutrino.

The experiment was called the "direct observation of the nu tau", or Donut, and the multinational team of scientists celebrated the historic discovery with doughnuts.

There are 12 tiny particles described in the Standard Model of particle physics, three of them neutrinos, and the tau neutrino is the last to be detected. Electron neutrinos and muon neutrinos were discovered in 1956 and 1962. "It's a tremendous milestone," said Martin Perl, a Stanford University physicist and Nobel Prize winner, who first put forward the theory of the tau neutrino's existence in 1978. "Now it has been seen and it behaves in the way we expected."

Neutrinos are subatomic particles with no electrical charge and a tiny mass close to zero. That means they have very little interaction with matter. So although they are among the most abundant particles in the universe, they are very hard to track down. In effect, they are the unsociable loners of subatomic particles, drifting in the universe, much rumoured but rarely seen.

Byron Lundberg, a physicist and spokesman for the international team, said: "We finally have direct evidence that the tau neutrino is one of the building blocks of nature. It is one thing to think there are tau neutrinos out there. But it is a hard experiment to do."

Scientists used the world's most powerful particle accelerator, Fermilab's Tevatron, to squeeze neutrinos into a beam that was shot through sheets of metal with emulsion, something like a series of photographic plates in a multi-layer sandwich. Fermilab detected four instances of the tau neutrino, identifying a track with a distinctive kink in it.

There is another unseen particle, a tricky chap called the Higgs Boson. Fermilab and Cern, the European particle physics laboratory near Geneva, are now facing off in the challenge to track it down. And there may be an even more mysterious and elusive particle, known as the sterile neutrino.