The Why Files The Why Files --

Studying the invisible

POSTED 10 FEB 2005
The shy particle
Woman in hard hat points to pin on concrete wall.After 700 meters of dark tunnel, we are ready for a change, and change comes in the form of a bright, dry cavern with an array of pipes, cables and electronic bric-a-brac. By now, most of the pions and kaons have decayed into neutrinos and muons. A wall of stone and concrete will block everything except the neutrinos, which will continue zooming toward the near and far detectors.

Physicist Catherine James points to a mark showing where the stream of neutrinos will enter solid rock on its way to the detectors. Rock and concrete block other particles in the beam.

It's eerie. We'd never see these shy ones, even if the beam was operating, but over by the concrete wall, Catherine James is pointing to a pin marking the center of the beam path.

At this point, you might be wondering why neutrinos can zip right through solid rock and steel. Ditto for us. We asked Kurt Riesselmann, a physicist who works in Fermilab's public affairs office, for an explanation. His email response started by listing the four types of particle interactions: gravity, electromagnetic force, the weak force and the strong nuclear force. Gravity is too weak to play a role at the subatomic level, he wrote, and while "the other three forces are much stronger, not all particles are susceptible to these three forces. Electrons, for example, are subject to the electromagnetic and weak force, but not the strong nuclear force. ... Neutrinos are only subject to the weak force, which is... well... weak."

huge, brown steel plates in a row, hanging in air.
280 steel plates, each 1-inch thick, in the near detector at Fermilab. Plastic ribbons between the plates will catch particles made when a neutrino interacts with iron atoms. You are looking at 980 tons of steel...

Detection collection
When an electron or a proton (both have an electric charge) buzzes through mass, it responds to the many electromagnetic fields in and around the atoms, and is soon blocked, Riesselmann explained. "But to the neutrino, the electric charge doesn't matter. It doesn't see or feel it. All it sees is the 'weak charge' by the electrons and the nucleus, and it is much, much weaker than the electric charge. And unlike electric force and gravity, the weak force has a much shorter range. Hence the neutrino can cross lots and lots of atoms without seeing the weak force."

Could the shy particle suffer from social phobia?

Data runs through hundreds of white wires into electronic apparatus.Cables collect data from the near detector. This is no place for crossed wires!

Once in a great while, however, the neutrino does interact through the weak force, and only then does it become detectable. To improve the odds of such an interaction, you place more stuff -- more mass -- in front of the shy particles.

And that explains the honking-big detectors -- 980 tons here at Fermilab, 6,000 tons in Minnesota -- that the researchers need to catch neutrinos. (The Minnesota detector is larger because the neutrino beam spreads as it travels, and with fewer particles hitting a given area, it's essential to increase the chance of interaction.) Neutrino detection is a game of chance: When 5 trillion neutrinos pass through the far detector, the scientists expect to detect one measly neutrino.

Striking mass
When one of Fermilab's neutrinos does interact with an iron atom in a detector, it will create a muon, an energetic particle that will immediately strike a special type of plastic that responds by making a photon -- a particle of light. The photon will trickle through a fiber optic cable to a light-sensitive gizmo (called a photomultiplier tube) that sends a pulse of electricity to the data-gathering system. Message: Houston, we have a neutrino!

detecting neutrinos diagram neutrino goes through various barriers and ends with data gathering

It sounds like a story from Genesis: neutrino and iron beget muon, muon and plastic beget photon, photon and photomultiplier tube beget electron, and electron and computer beget data. At any rate, this detector actually works: during a test run, it has already "seen" some flying neutrinos (which also proves that the project has succeed in making neutrinos). After the official start-up later this month, real data will start trickling in: How many neutrinos will hit the giant rack of steel in Minnesota?

White circle with dot in middle on wall marks neutrino exit. Adios, neutrinos! This high-tech road sign shows the route to Minnesota. Neutrinos will penetrate solid rock on their 735-kilometer path to the far detector.

As we prepare to ascend to the sunlight, James points to a circle of spray paint on the rough concrete wall. Here, she says, is where the neutrinos will begin their 735-kilometer underground trek toward the far detector, half a mile underground in an old, Minnesota iron mine.

The massive importance of neutrinos.

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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