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Loners of the Universe: Why Neutrinos are Hard to Detect

On an ordinary day in Centerville, USA, two strangers, Jones and Smith, pass through town. Mr. Jones pulls up to the Centerville Motel and books a room for the night, chatting with the desk clerk as he signs the guest book and checks in. On the clerk's recommendation, he eats dinner at the Centerville Grille. Mr. Jones is the outgoing type, and by the time he leaves the restaurant, he has made the acquaintance not only of the waiter but of the couple at the table next to his. After dinner, they all stop for a drink at the Centerville Tap. In the morning, Mr. Jones pays his motel bill with a credit card, fills up with premium at the Centerville Service Station, buys the paper at the drug store, and has the Farmhouse Special breakfast at the Centerville Diner before moving down the road.

Mr. Smith, on the other hand, is a loner. He drives through town on the freeway, not even slowing down for the Centerville exit.

You are a detective. You have come to Centerville to track down Mr. Jones and Mr. Smith. Which one will be easier to trace? Clearly, you will have no problem discovering Mr. Jones's activities, because he interacted with at least a dozen different people in Centerville. But it will be almost impossible to learn about Mr. Smith, because he interacted with no one. No one even knows he passed through. We are known by our interactions.

Neutrinos are the Mr. Smiths of the universe. They sail through space, through walls, through planets, never even slowing down. Every minute, trillions of solar neutrinos flit through our bodies at nearly the speed of light. When they have passed through, they leave no trace, because they almost never interact with other particles. They are the loners of the particle world.

There are many particle types for the role of Mr. Jones. When they encounter other particles, they interact with them, sometimes beneficially and at other times in ways that damage those they encounter. But because neutrinos almost never interact, they can do no harm.

The same "loner" characteristics that make neutrinos harmless also make them very hard to detect and to study. From a million million tau neutrinos that crossed the DONUT experiment, less than ten left a signal by producing a tau lepton.

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