Nearly massless and incredibly rare, the tau neutrino scorns its surroundings, seldom interacting with more common matter. These properties make it difficult to detect. Now, an international team of physicists has laid claim to the first "direct" detection of the tau neutrino.
Neutrinos were discovered after scientists failed to balance their subatomic books. In the 1930s, Wolfgang Pauli proposed that a very lightweight, weakly interacting particle was carrying away the energy that was missing from radioactive decays. The existence of the neutrino was confirmed a few decades later. Physicists believe there are three types of neutrinos, each named for the fundamental particle it interacts with: The electron neutrino interacts with electrons, the muon neutrino with muons, and the tau neutrino with taus. When physicists have fired beams of electron neutrinos at a target, they produce electrons. Likewise, muon neutrinos shot at a target generate muons. But no one has observed this for tau neutrinos.
At the Direct Observation of the Nu Tau (DONUT) experiment based at the Fermi National Accelerator Laboratory (Fermilab) near Chicago, scientists tried their hand with a 800 giga electron volt proton beam. They created what should be tau neutrinos and shot them through meter-long steel targets. One out of every trillion tau neutrinos interacted with an iron nucleus and created a tau particle, which, in turn, left a telltale track on layers of emulsions that acted like photographic plates. The yield: four taus that the DONUT team is quite confident came from tau neutrinos.
"It was a hard experiment, an expensive experiment, and a somewhat unfashionable experiment," says Stanford University physicist Martin Perl. Physicists already knew that tau neutrinos existed, from missing-energy analysis of tau particles, so some scientists saw no need to perform it at all. Perl disagrees. "It was very, very important to find out," he says. "Not only does it confirm [the tau neutrino's] existence, it shows that it interacts in a more-or-less normal fashion." DONUT team member Regina Rameika agrees. "It's just a relief, really," she says. "It's kind of one of those things you had to do."
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