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The Unbearably Unstoppable Neutrino

Why a thousand of the best physicists from around the world are trying so hard to catch just a few of nature's weirdest particles

By Robert Kunzig and Ferit Kuyas
Aug 1, 2001 5:00 AMApr 19, 2023 2:58 PM


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Once you create a neutrino, a tiny subatomic particle, it moves at nearly the speed of light, and it doesn't stop. It keeps going in a straight line to the edge of the universe. Straight through any stars, planets, or mountains; straight through any atoms, nuclei, or other particles that happen to lie in its path. Straight through any people too: At this moment— let's say you're reading this at night, somewhere in the United States, relaxed in your living room— every second, 40 billion neutrinos from the sun are rocketing up your left nostril and through your brain's frontal lobe, on their way through the roof of your house and then clear out of the Milky Way galaxy, having already passed through China, Earth's rocky mantle, the seat of your recliner, and your left thigh. Even huger numbers of neutrinos pierce you from above; they come from stars in the night sky, from cosmic rays, and above all from the Big Bang— 15 billion years old, those neutrinos are, and still traveling. The nuclear reactor in the next state is sending fresher neutrinos your way. You are at this moment, at every moment, a busy crossroads of neutrinos. But you're not special. Neutrinos, unseen and beyond counting, fill the universe. People call them ghostly, but ghosts aren't real. Neutrinos are real.

When physicists actually manage to stop a neutrino, and thus detect it, the moment is so special they call it an event. In 1998 an international team managed to stop a few thousand neutrinos in Japan, near the town of Kamioka, using a detector called the Super-Kamiokande. They caught the particles 2,000 feet down in an old zinc mine, in a cavern lined with stainless steel and filled with 50,000 tons of purified water. Those observations were meaningful far beyond just stopping some neutrinos in their tracks. The experiment suggested strongly that neutrinos— unlike ghosts and unlike photons— have mass, albeit an incredibly tiny amount. The Kamioka observations also supported an even stranger idea: that a given neutrino does not have one stable mass or one stable identity. Instead, as it flies along, it oscillates from one identity— what physicists call a flavor, which means a way of interacting with other particles— to another. "A Dr. Jekyll and Mr. Hyde sort of affair," observed one of the Kamioka researchers.

Neutrinos had always seemed weird, but the Kamioka results made the particles important in three distinct ways. First, if neutrinos oscillate, physicists will know they finally understand how the sun produces light in the form of photons— something of which they haven't been entirely sure. The sun is a large source of neutrinos— they're produced in the same fusion reactions that create photons— but detectors on Earth consistently capture far fewer solar neutrinos than the best theory of solar physics would predict. The Kamioka results suggest a happy resolution: Perhaps it's not the theory that is wrong; it's that the neutrinos are oscillating, changing to a different flavor on their way to Earth, a flavor the detectors can't detect.

Subatomic High-Speed Commute— UndergroundThe beam of neutrinos originating at CERN's Super Proton Synchrotron, located outside Geneva at the French-Swiss border, will shoot underground through the Alps, cross the river Po, traverse the Italian Apennines, and travel beneath the city of Florence before arriving at the neutrino detector at Gran Sasso, Italy. Travel time: 2.5 milliseconds.

Second, because neutrinos are so fantastically numerous, if they have even a tiny mass, they outweigh all the stars and galaxies, all the visible matter in the universe. They might make up as much as one fifth of the dark matter that physicists and astrophysicists have been seeking so assiduously. Finally, the standard model of particle physics— which describes all matter in terms of 12 fundamental particles (including three flavors of neutrinos) and four fundamental forces— does not allow for neutrinos with mass. But there are theories that do. Proving that neutrinos do have mass, and figuring out how much, would help physicists determine which if any of these theories is correct— and thus hasten the advent of an ultimate theory that describes all the forces as one.

To physicists these are serious stakes. And so the Neutrino Oscillation Industry, as one of them dubs it in his Web directory, now employs upwards of 900 physicists worldwide. They work on experiments that will cost, by one estimate, about $500 million to build. Each of these machines is focused on some different aspect of the problem, trying to extend or confirm the Kamioka findings in some different way; and each, because of the expense, is an international collaboration. For instance, many American physicists work at Kamioka. Neutrino detectors are expensive because they have to be large and underground: large, because neutrinos interact so rarely with atoms that you need many atoms to catch one; and underground, so those rare signals won't be drowned out by cosmic rays. Kamioka is not even the most impressive, geologically speaking. The neutrino observatory in Sudbury, Ontario, which just reported strong evidence that solar neutrinos oscillate, is located in a copper mine more than a mile underground.

Among the chanciest and most interesting of the current batch of experiments are several in which physicists will be creating their own neutrinos and beaming them to detectors hundreds of miles away. The idea is that such a long distance should give the neutrinos time to oscillate. Because the beam will contain a known quantity of neutrinos of just one flavor, any flavor switching should be evident. At CERN, the European Laboratory for Particle Physics outside Geneva, construction crews broke ground last fall on an experiment that, when it is turned on in 2005, will send a beam of neutrinos under the Alps, the Apennines, and half of Italy to detectors at an underground laboratory in Gran Sasso, a mountain east of Rome. That beam will travel through 454 miles of solid rock. But for neutrinos, that's nothing.

Late last winter, on a crisp, sunny day that hinted of spring and encouraged optimism about big projects, I toured CERN's construction site with an affable Swiss physicist named Konrad Elsener, who is leading a large team of physicists and engineers building the neutrino beam. As we drove to the site, the Alps were at our back and the slopes of the French Jura were before us; CERN straddles the border between France and Switzerland, in a valley it shares with farm fields, vineyards, and the western suburbs of Geneva. Elsener is an unassuming man of medium build, with a wide, slightly pudgy face, thin brown hair, and the casual attire— rugby shirt, black sneakers— that is the norm at CERN.

To make a neutrino beam, Elsener explained, you first make a proton beam. And to make a proton beam, you begin with a small metal bottle of hydrogen— a hydrogen atom is a single proton orbited by an electron. You bleed a little hydrogen into an iron chamber and apply a voltage to a filament at one end, causing it to spew electrons. Those electrons knock the electrons off the hydrogen atoms, releasing the protons— and a pinkish glow. Next you accelerate the protons— first in a straight pipe, then in a spiral of progressively larger, circular ones— by jolting them with radio waves. After they have made 20,000 laps or so around the third and last circle, which is 3.7 miles around, they are traveling at more than 99 percent of the speed of light. They are now almost ready to generate neutrinos; they just need to be aimed.

In the midst of farm fields, Elsener and I pulled up to a two-story building that sits above one section of that last accelerator, the Super Proton Synchrotron (SPS). Inside we rode an elevator 20 stories down to the accelerator tunnel. The tunnel is large, but the stainless-steel tube that contains the protons is just 8 inches across. Large magnets surrounding it, painted a cheerful red or a deep blue, keep the protons traveling in a circle (those are the red ones) and in a tight beam at the center of the tube (blue). The SPS has been operating for more than 20 years. Elsener led me along the tunnel to a point where a small borehole had recently been made in the outer wall, branching off at a tangent to the circle. We peered down the hole: Italy, Elsener explained, lay thataway.

When the neutrino experiment gets going in 2005, a magnet at this junction will extract 10-microsecond pulses of protons from the synchrotron. Peeling off in a sloping curve, down and to the southeast, the proton beam will straighten out after around half a mile. Its slope then will be 3.2 degrees, its compass heading 122.5 degrees— on a beeline through Earth's curved crust to Gran Sasso. Getting the protons aimed accurately before you use them to make neutrinos is paramount; the neutrinos are electrically neutral and therefore unsteerable. "As long as you're talking particles with a charge, you can guide them with magnetic fields," said Elsener. "As soon as you're talking neutrinos, they have to be pointed in the right direction."

Gran SassoScientists hope that the BOREXINO neutrino detector, still under construction, will yield evidence of electron neutrinos that oscillate into muon neutrinos during their journey from the sun to Earth. The stainless-steel sphere, above, when completed will contain a transparent nylon detector core 28 feet in diameter and filled with 300 tons of a liquid scintillator. As neutrinos hit the scintillator, 2,200 photomultiplier tubes— about 600 are visible in this photograph, most still covered in black protective plastic— will record traces of light emitted by charged particles as they travel through the fluid.

As they pull into the straightaway, the protons will slam into a series of graphite shafts as thin as pencil leads. Pions and kaons— short-lived nuclear particles— will spray out the back of the graphite. As those particles fly another six tenths of a mile through an evacuated steel pipe, some of them will decay into muons and, at last, neutrinos. At the end of the tunnel, at a depth of about 400 feet, the whole motley of particles— protons that survived the graphite, pions and kaons that survived the steel pipe, muons and neutrinos— will plow into 50 feet of iron. This is called the beam dump. It will filter out everything but the muons and neutrinos. The muons press on into the rock beyond the beam dump for half a mile or so. The neutrinos continue on to Gran Sasso, emerging there into the underground lab operated by the Italian National Institute for Nuclear Physics. The overwhelming majority will continue on past the lab, to infinity.

There are three known flavors of neutrino, each named for the particle it produces on the rare occasion when it interacts with a detector: electron, muon, and tau. (Muons and taus are heavier relatives of the electron.) Elsener and his team will produce muon neutrinos. The same thing happens around us all the time: Cosmic-ray protons raining in from space collide with atomic nuclei in the atmosphere, producing both muon neutrinos and electron neutrinos that shower through our skulls— or, if they were made in the atmosphere above China, fountain up through the soles of our feet.

When the physicists at Kamioka collected those atmospheric neutrinos, they found far fewer muon neutrinos than cosmic-ray theorists had predicted, especially among the particles that had traveled all the way through Earth and entered the zinc mine from below. That was their evidence that neutrinos oscillate and also that they have mass: The oscillations are changes of flavor and mass. The muon neutrinos that crossed through the planet, the researchers concluded, had more time to change to a flavor the detector couldn't detect. That flavor pretty much had to be tau neutrinos— unless there is some new kind of neutrino out there.

The CERN neutrino beam to Gran Sasso— CNGS for short— is designed to prove the muon-to-tau hypothesis. It will be a pure beam of muon neutrinos, which will take 2.5 milliseconds to reach Gran Sasso. At Gran Sasso, there will be detectors waiting that, unlike the one at Kamioka, can detect the signature of tau neutrinos. If the researchers see that signature 2.5 milliseconds after a burst of muon neutrinos leaves Geneva, it will mean that at least one has mutated into a tau neutrino en route: case closed.

And yet the CNGS experiment has been controversial at CERN because it is expensive— $100 million— and because two other "long baseline" experiments have a head start. The Japanese are already beaming neutrinos at the Kamioka detector from a synchrotron 155 miles away; and Fermilab, west of Chicago, is hoping to switch on a 454-mile beam to an iron mine in Minnesota in 2004, a year ahead of CNGS. Both experiments will lack the resolution to detect the appearance of tau neutrinos directly, so they are aiming to detect the disappearance of muon neutrinos. But they plan to do so in an indirect way. "By the time CNGS takes data, the question will be resolved and double-checked," Alvaro de Rujula, a Spanish theoretical physicist at CERN, told me. "CNGS is entirely useless."

The 12 Fundamental Particles of the UniverseThe "standard model" of physics divides all matter into 12 particles that— at least for now— can't be divided into anything smaller. Ordinary matter is made of the particles in the top row. The heavier particles below were abundant right after the Big Bang, but now they're produced only at high energies— by cosmic rays, for instance, or in accelerators. Neutrinos aren't stable parts of atoms; they're produced only when other particles decay.

The people involved don't see things that way, of course; they argue that only the direct detection of tau neutrinos can prove the oscillation case conclusively. But they do acknowledge another danger: They might not detect anything at all. It could turn out, for instance, that muon neutrinos need more time than it takes to get from Geneva to Gran Sasso to oscillate into taus. "You worry that you're sitting there in 2007, and you haven't seen a single tau neutrino," Elsener said, "not because the beam is bad or the detector is bad but because the physics is against it. Once in a while I worry about that. It would be a pity."

Just down an access road from the synchrotron building, Elsener and I visited the site where serious earthmoving for his proton beam tunnel started last fall. We peered this time into a large hole: a shaft well on its way to being 180 feet deep. From that shaft, said an enthusiastic young construction engineer named Anthony Pooley, tunnel borers will dig 500 feet back to the synchrotron and a mile and a half in the other direction, toward the southeast. Pooley pointed just east of Mont Blanc, white and massive on the southern horizon. "One of the really big issues for us is alignment," he said. "The tunnel axis has to be within 2 inches of the theoretical."

But CERN has built accelerator tunnels and neutrino beams before; that's not the hard part. "This is a classical neutrino beam," said Elsener. "The detectors are a different story. They're very innovative."

Two days later a train took me east and south of Mont Blanc and on into northern Italy, to Pavia, where I had an appointment to visit an innovative detector called ICARUS. Just before leaving Geneva, though, I talked with the man who conceived ICARUS, Carlo Rubbia. Rubbia was once director of CERN and is now head of the Italian National Agency for New Technology, Energy, and Environment. At age 67, his time is eaten up by "after-dinner talks and administration," but he still keeps one foot in research and an office at CERN. A big, energetic man, he has always been known there for his strong personality; today, with his face deeply lined and framed by graying, somewhat unruly hair and sideburns, he still projects a live-life-to-its-fullest, cut-the-crap manner. He arrived for our conversation with a briefing folder prepared by his secretary and answers already thought out to questions I had submitted in advance. This is unusual in a scientist. In 1984 Rubbia won the Nobel Prize, along with a Dutch physicist named Simon van der Meer, for discovering the W and Z bosons, the particles that transmit the weak nuclear force— and explain why a neutrino is so hard to detect.

Although neutrinos are particles of matter, they are immune to the forces that hold small bits of matter together— the strong nuclear force and the electromagnetic force. Neutrinos feel only the weak force, which causes radioactive decay. They are born in decay processes, such as "beta decay," in which a neutron emits a W boson, causing the neutron to transmute into a proton, an electron, and a neutrino. And neutrinos die as a result of the weak force when they interact with another particle and become something else. To do that, however, the neutrino itself has to pass a W or Z boson to the other particle.

Kamioka The Super-Kamiokande detector, seen here during construction, was designed to study neutrinos that originate in the sun or in collisions between cosmic-ray particles and Earth's atmosphere. The stainless-steel tank is 131 feet in diameter, holds 50,000 tons of purified water, and contains a stunning array of photomultipliers that detect neutrino interactions occurring inside the underground tank.

The photons that transmit electromagnetism and the gluons that transmit the strong force are massless bosons, and so those forces have a long range. But the weak-force bosons are extremely heavy— as Rubbia showed when he created them in the CERN synchrotron. They are about 100 times as massive as a proton and perhaps 100 billion times as massive as the neutrino that must emit them. In the world of quantum physics, thanks to the uncertainty principle, a flea can indeed throw a bowling ball to another flea. But the two fleas have to be very close together. A neutrino almost never gets close enough to another particle to fling a boson at it, simply because atoms, nuclei, and even the protons and neutrons inside nuclei, are mostly empty space. (If the nucleus of a typical atom were the size of a baseball, the nearest electron orbiting the atom would be half a mile away.) So a neutrino usually sails right through matter, leaving no trace of its passage.

To detect a neutrino, you have to put a lot of atoms in the path of a lot of neutrinos and watch closely. What you are watching for is not the neutrino itself but the charged particle it transforms into when it interacts with an atom. Unlike a neutrino, that particle— an electron or muon or tau particle, depending on the neutrino flavor— does leave a track. There are many ways of looking for it.

OPERA, for instance, a detector that will share lodgings in Gran Sasso with ICARUS, will try to stop some of the CERN neutrinos with 72 consecutive walls composed of 235,000 lead bricks. Lead is dense and cheaper than gold or tungsten. Layers of plastic scintillator between the walls will give off light when a charged particle passes through. A robot will then move along the detector and extract the brick from which the charged particle emanated. Inside each 3-inch-thick brick, sandwiched between layers of lead, will be 58 layers of photographic film; in the extracted brick, all 58 will be developed and examined under a microscope, again by a machine. If the charged particle is a tau particle produced by a tau neutrino, the film will record its distinctive track, a few hundredths of an inch long, which then abruptly changes direction. The OPERA physicists hope their detector will stop 5,000 of the million trillion muon neutrinos that CERN will be sending to Italy each year. Of those stopped, they expect four of five will have changed flavor to tau neutrinos.

Compared with OPERA, which is the apotheosis of proven neutrino-detection technology, ICARUS is a new and unproven device, although Carlo Rubbia first proposed it 24 years ago. Also unlike OPERA, ICARUS will look for other things besides tau neutrinos from CERN— atmospheric and solar neutrinos, for example. "Most discoveries are surprises," said Rubbia. "Our device is just a telescope, a new instrument that allows you to see all kinds of things. There is no preconceived notion that tells you what you should see— you see everything. It provides you a real image like a camera taking a picture, only it is an electronic image. It's a big TV."

In a hangar outside Pavia, 30 miles southwest of Milan, I rode a scissor lift to the top of ICARUS's first module, which was just being completed. My guide was Claudio Montanari, a sad-eyed, soft-spoken young physicist at the University of Pavia, who did his Ph.D. on ICARUS and knows its nuts and bolts. On top of the detector, as workmen came and went, we stepped over nuts and bolts and boxes and half-assembled electronics, and talked of what was beneath our feet.

The first module of ICARUS, due to be trucked to Gran Sasso later this year, is a rectangular box made of aluminum, 13 feet high and wide and 65 feet long. Liquid nitrogen flows through the aluminum to chill the contents of the box to -300 degrees Fahrenheit. On the inside walls of the box are three layers of steel wires, so thin you see them clearly only when light catches them from the side. The 26,000 wires emerge in bundles from a long line of ports on top of the box, each port with its own 5-foot-tall cabinet of data-processing electronics— because when this box is in operation, the wires will be spitting out 65 gigabytes of information a second. The data will come from 300 tons of liquid argon inside the box— that's why it has to be cold.

Liquid argon, to hear Rubbia or Montanari talk, is a material with many virtues. For starters, it's as cheap as gasoline (air is nearly 1 percent argon). More important, argon is a noble element: Its atoms have all the electrons they need— unlike oxygen, say— and don't want to grab any more. This means that when a neutrino produces a charged particle in ICARUS, and that particle in turn knocks electrons off a long track of argon atoms, all those electrons will be able to travel, unimpeded, under the pull of an electric field, to the wires on the sides of the box. From the time the electrons arrive at each wire, computers will reconstruct the particle track. Each wire will be like the scan lines of a TV. Montanari showed me a sample image, a richly detailed and evocative black-on-gray picture of a particle shower generated by a cosmic ray. "The technology is really beautiful," he said.

That image was made with a 10-ton prototype. Eventually Rubbia and his team want to build a detector of at least four modules containing 1,200 tons of argon. One of the toughest technical challenges will be keeping the argon uncontaminated. "You have to have liquid argon that is very pure," Montanari said. "Typically it contains oxygen— and what that does is capture electrons." Even one part per million of oxygen could wipe out a neutrino image. Montanari and his colleagues hope to reduce that contamination to less than one part per billion by constantly filtering the argon. It is not certain this will work on so massive a scale, which is one reason some physicists are skeptical of ICARUS. Although the project is fully funded as a Gran Sasso experiment, unlike OPERA it has not yet gotten formal approval from CERN.

This irritates Rubbia. "How you make progress in these things is by introducing new technologies," he said. "This is a new technology; we are the only ones developing it. It's really very original— and it's a lot of fun. I swear to you it's tremendous fun. If the damn thing works, we are in business. If it doesn't work, no committee is going to make it work. I'm working like a horse; everyone is working like mad to get this thing working."

From Nobel Prize winner to postdoctoral student, neutrino physicists are not brought to the office each morning by a restless desire to push back the frontiers of knowledge. "I cannot think of the tau neutrino every day," said Elsener. Montanari has spent 10 years, his entire professional career, on ICARUS. "You start interested in the scientific matter— you want to see if some hypothesis is true," he said. "But of course you also want to demonstrate that you can make these things work, which is not obvious. For me the main motivation now is just to see the detector working. After that I can get back to physics."

GenevaThis so-called magnetic horn is one of two that CERN will use to help make its neutrino beam travel in a straight line. When protons from the synchrotron slam into the graphite target, the charged particles that fly out the back— and that will soon decay into neutrinos— tend to fan out like buckshot. The magnetic fields inside the horns will straighten them out, so that, hopefully, they will hit their target in Italy, 454 miles away.

Neutrinos began in 1930 as a bookkeeping trick— a "desperate way out," as their inventor, Wolfgang Pauli, said himself. Pauli was made desperate by his analysis of beta decay, which was then thought to consist of a neutron decaying into a proton and an electron. He found that the energy and momentum of the proton and electron did not add up to those of the neutron— whereas the most fundamental laws of physics said they had to. Rather than chuck those laws, Pauli preferred to imagine a third, unseen particle coming out of the neutron, whose energy and momentum would exactly balance the books. At the time it seemed too convenient. In 1934, when Enrico Fermi gave beta decay a mathematical description and the new particle a name (Italian for "little neutral one"), the journal Nature rejected his paper, saying "it contained speculations too remote from reality to be of interest to the reader.''

No one doubts the reality of neutrinos now, but they are still disconcerting, and they will become even more so if it turns out that they oscillate. To understand the phenomenon, you have to think of a neutrino not only as both particle and wave— which is typical quantum mechanics— but also as a combination of waves. Each wave has a slightly different frequency, and each frequency corresponds to a certain mass. When the neutrino is born, the waves are in step. But because of their different frequencies, the waves gradually drift out of step as the neutrino travels through space and time. The waves start to interfere with each other; at times one frequency dominates, at times another. At times the neutrino has one mass, at times another. At times it has one flavor, at times another. When you pin down one quality, the other blurs: It's the old quantum mechanical uncertainty again. But this is the very essence of a neutrino— as it is of no other fundamental particle.

Seven decades after Pauli's sleight of hand, physicists are still trying to figure out the basic properties of neutrinos. "You don't have many constituents of matter," said Montanari. "You have 12. Neutrinos are one fourth of the population. How they interact with other particles, whether they have a mass or not— these are not trivial questions."

Find links to the neutrino detection experiments discussed in the article and then some at www.hep.anl.gov/ndk/hypertext/nuindustry.html.

For more about the Sudbury Neutrino Observatory, see www.sno.phy.queensu.ca.

Why all the fuss about neutrinos? Find out here: www-e815.fnal.gov/~bugel/why.html.

Still not sure how neutrinos are born? Check out pdg.web.cern.ch/pdg/cpep/neutrinos.html.

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