The Why Files The Why Files --

Studying the invisible

POSTED 10 FEB 2005
A lovable particle
He and she pose before a mass of concrete shielding.If scientists love a mystery, then the neutrino may be the most lovable particle of them all. In 1930, when a physicist invented this ghostly particle, he called it a "desperate remedy" to balance an equation.

The little critter evaded detection until 1956, and it remains one of the biggest mysteries of physics. Electrically neutral, neutrinos are shy, elusive, and everywhere: Right now, millions are zipping through your body, and you wouldn't even know it without you read The Why Files. Neutrinos may not bother you. They certainly don't bug us. But physicists get cranky when the equations don't balance.

The Why Files makes a field trip to the world's largest neutrino gun.

Pioneering physicist Wolfgang Pauli "invented" the neutrino in 1930 to make equations behave. In 1938, physicist Hans Bethe suggested that nuclear fusion in the sun must make a gazillion neutrinos. Large circles seen from above represent injector at Fermilab.But when physicists measured solar neutrinos in 1968, they saw only about half as many as expected. (Although only a tiny proportion of neutrinos interact with matter -- nearly all of them zip right through the Earth -- once in a great while they will whang into an atomic nucleus, creating secondary particles you can see -- if you know how to look.)

Fermilab's main injector shoots protons at a target, creating pions and kaons, which decay into muons and neutrinos. A few neutrinos hit the near detector; most pass through on their way to the far detector in Minnesota. Photo: Fermilab

One thing these folks at Fermi National Accelerator Laboratory know how to do is to look for tiny, weird particles. After all, researchers at this northern Illinois laboratory run the world's largest particle accelerator, where they discovered the bottom and top quarks, two fundamental components of protons, electrons and neutrons.

Fermifolk also know how to make a scientific profit from big, expensive holes.

Neutrinos on the cheap
If you're a neutrino-obsessed tightwad, you can measure neutrinos coming from the sun or cosmic rays, but you would need to work deep underground, using rock to block other particles, and you'd need a giant detector containing thousands of tons of metal or liquid.

Over the years, physicists in Japan, Canada, Europe and the United States have been there, done that, but they still see the "wrong" number of neutrinos coming from the sun.

Darkness envelopes slimy, sloping walkway towards light.Is there light at the end of this tunnel? Photo shows grotto alongside tube where neutrinos form at Fermilab's new neutrino gun.

At some point, such a well-documented discrepancy ceases to be annoying and starts to be intriguing. Neutrinos, as we'll see, raise big, simple-sounding questions: Do neutrinos have mass? If so, how much?

Answers to these questions could overturn -- and thereby advance -- some fundamental principles of physics. Thus, the urge to make neutrinos and shoot them at a detector, rather than to sit back and wait for neutrinos from space.

All of which explains why two normally desk-bound Why Filers are stumbling down a dark, soggy tunnel at Fermilab. With the light at the end of the tunnel quickly receding behind us, we scrape against the concrete walls and slog through puddles as we explore Earth's largest neutrino maker.

Want to tag along?


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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