The Why Files The Why Files --

Studying the invisible

POSTED 10 FEB 2005
'trino time
Bearded man in hard hat points to machine behind gate.Fermilab's neutrino-maker is expected to outstrip the K2K project in Japan, which sent a neutrino beam to a detector at Super-Kamiokande, producing the first confirmation of neutrino oscillation.

Super-K's 50,000-ton water detector had already measured natural neutrinos, and produced a rough measurement of neutrino mass. Fermilab's large neutrino gun gives it a better shot at solving the 50-year-old neutrino accounting problem, and pinning down neutrino mass.

Physicist Jim Hylen helped design Fermilab's neutrino gun. That grill guards massive conductors carrying a 200,000-amp current that steers the particle beam.

The question of neutrino mass arose due to the surprising shortage of solar neutrinos. The best explanation for that shortage is the theory that neutrinos spontaneously change "flavor." (Neutrino flavors -- electron, tau and muon -- are named for the particle that appears when a neutrino does interact with matter.) And because a particular neutrino detector sees only a particular flavor, when neutrinos change flavor, they effectively disappear.

diagram of how neutrinos are made

Why might this matter? Because physics theory will only allow neutrinos to change flavor if they have mass, but the standard model of physics says neutrinos don't have mass.

The flavor of a neutrino
Edward Kearns, a Boston University physicist who works on the Japanese neutrino detector, where this change in flavor -- called oscillation -- was first documented, says, "The fact that you see oscillation, as we saw, said there has to be something beyond the standard model. It's the kind of oscillation you would expect if they had mass, but the standard model has no provision for neutrino mass."

The flavor of neutrino...Albert Erwin, a University of Wisconsin-Madison physicist working on the Fermilab neutrino project, says neutrinos are "terribly interesting, just because of what we don't know." One "loose end" of the standard model, he says, involves a possible case of "charge-parity" violation -- the possibility that time could go backwards. Using the new neutrino gun, he says, "it may be possible to observe if the neutrino has something to do with that." Neutrinos may also help answer another riddle: Theories say equal amounts of matter and antimatter were created at the big bang. So why do we see so little antimatter?

For technical reasons we can't begin to grasp, physicists can't measure the mass of a neutrino; they measure the difference between the masses of two neutrinos. Complicated, but when you think about it, you realize the mass must be larger than the mass difference. Existing mass estimates, says Erwin, "Are numbers they throw around, and they depend on what month you look at them. I'm hoping the stuff we do [at Fermilab] will give a little better number."

The estimated minimum mass of the neutrino is 0.05 electron volts, thousands of times less than an electron. According to Fermilab physicist Gina Rameika, head of Fermilab's neutrino detector, the larger number of neutrinos at Fermi, combined with the long distance to the detector, should double the accuracy of the mass estimate.

Mass matters
Two crazy people stand on gigantic blocks of concrete.  Physics is beset by wanna-bes drooling for a chance to remodel the standard model, which is best described as an overarching theory explaining known particles and forces. And if neutrinos definitely prove to have mass, that remodeling contractor could walk off with a Nobel Prize (this would not be the first Nobel awarded to a neutrino physicist).

So as Fermilab began cranking up its new neutrino maker, we decided to visit. As usual, the 4-mile-long main accelerator was smashing protons against antiprotons. Since each stream of particles moves at 98 percent of the speed of light, and since energy and mass are interchangeable, the collisions produce a shower of weird, massive particles. Many are more unstable than a crack addict: after a few millionths of a second, they decay into something else.

But you can do a lot of basic physics in that eyeblink.

Like a couple of flummoxed tourists, Why Filers pose before the target, where fast-moving protons make pions and kaons. That concrete shielding protects against radioactive stuff in the area. Radioactive particles won't leave the site. Although neutrinos will, they are too "shy" to be dangerous.

Neutrinos on our mind
All this is cool enough, but we're in neutrino frame of mind, so we head downstairs -- down a clanky industrial elevator, actually -- to Fermilab's neutrino maker. Here's the plot in brief: Protons are converted into neutrinos, which are blasted toward two detectors, one close, one further away. Not all the neutrinos appear at the far detector, probably because they have changed into an undetectable tau neutrino. The neutrino shortage becomes the basis for calculating neutrino mass.

Fermilab will
convert protons into neutrinos.

It sounds simple, but like most big experiments in modern particle physics, it's a 10-year project involving hundreds of scientists and megabucks. "In the 1950s and '60s, it seems like more things were done, and they didn't take as long," says Catherine James, a Fermilab neutrino physicist. "There were 1-year experiments. All the easy things got done back then... but the questions that are left are the hard ones. When you work at high energy, everything gets bigger and harder."

Prosaic protons
Down in the basement, we track a skinny, stainless steel pipe as it descends through an artificial cavern. The pipe carries high-energy protons from Fermilab's main injector, a ring of speeding protons that also supplies the Big Daddy of accelerators, the Tevatron.

We approach the room where the protons hit a graphite target, producing particles called pions and kaons, which enter a giant tube, where they spontaneously convert to muons and muon neutrinos.Map of upper Midwest, showing distance traveled by beam.

We walk past the target, deeply shielded in concrete (neutrinos are not dangerous, but the protons and tritium in the tube are), and begin walking toward the near detector. On our left, in the gloom, is the concrete-shrouded tube, 2 meters in diameter and about 700 meters long. We see nothing but concrete, hear nothing but footsteps, and touch nothing but the damp, rough walls.

Passing through solid rock at the speed of light, neutrinos will reach the far detector in Minnesota in 0.0025 seconds. Map: Fermilab

Digging this tunnel cost about $40-million, says Fermilab health and safety expert Mike Andrews, who shepherds us through the gloom.

Is tunnel boring boring? It's time to play neutrino detective.

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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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