The Nature of the Neutrino

With a particle accelerator and 800 tons of baby oil, Janet Conrad and her colleagues may change our fundamental conception of the universe.

Janet Conrad is in the business of chasing ghosts, but they’re not ordinary specters. With the mysterious squarely in her sights, the Columbia physicist is working to unravel the secrets of a world that is all around us yet completely transparent, an intricate part of the universe but one just beyond the frontier of human knowledge.

Conrad’s target is the neutrino, a wispy, subatomic particle whose true nature has eluded physicists for decades. By way of elucidation, Conrad and her colleagues have built an instrument that hopes to capture some of the intricacies of the neutrino, to catch it changing partners in its elementary dance through the universe. By observing “oscillations” of the neutrino, spying it as it exchanges costumes from one character into another, Conrad and her experimental collaboration hope to help settle basic questions about the neutrino and its place in the cosmic fandango.

Flavor physics
The neutrino baffles not because it is scarce. Indeed, neutrinos are nothing if not ubiquitous—100 trillion of these tiny particles just passed through your body in the last second. They carry away about 10 percent of the sun’s energy but are so tenuous they may travel through trillions of miles of lead without interacting with any of the dense material’s atoms.

Its shyness earned the neutrino its name: the Italian physicist Enrico Fermi called it “little neutral one.” Hypothesized to exist in order to explain the apparent disappearance of energy in certain subatomic reactions, neutrinos were positively identified in a 1956 experiment.

But that discovery was only the beginning. Soon it was found that neutrinos come in two different “flavors,” the “electron neutrino” and the “muon neutrino,” distinguished by the nuclear reactions that give rise to their creation. Two years ago, a third type of neutrino was found, long expected—the “tau neutrino.” Together, physicists refer to the three flavors of neutrinos as a family.

In 1999, Conrad won the prestigious Presidential Early Career Award—the highest honor bestowed by the U.S. government to scientists beginning their careers—and she spent most of the $500,000 on phototubes (like the one she holds above) for the MiniBooNE experiment. Below: Inside the Mini-BooNE detector: a 40-foot sphere, lined with 1,280 phototubes, now filled with mineral oil.
A 1995 Los Alamos experiment stunned the physics community when it reported a few incidences in which a neutrino of one flavor transformed itself into a neutrino of another, leading everyone to speculate that there is much more to neutrino science than the simple flavor family known to physicists for decades. But, says Conrad, “it was a very controversial signal.

“A lot of people have questioned it, but they’re never been able to find a reason why it was wrong. . . . For a long time people said, ‘Well, we don’t know what to do with this result, so we’ll have to throw it out and not include it in our study.’ But that’s obviously not the right way to handle physics.”

Conrad and her Columbia colleague Mike Shaevitz decided to pursue the controversial signal. Together they developed an idea for a new neutrino experiment and joined forces with some scientists from the Los Alamos experiment.

Baby oil
Conrad earned her Ph.D. from Harvard in 1993 and went on to do postdoctoral work at Columbia before becoming a junior faculty member in 1996. She’s a co-spokesperson for the collaboration, called Mini-BooNE: phase one of the Booster Neutrino Experiment. There are about fifty people on the experiment, representing fourteen institutions.

Mini-BooNE is a large detector set up to detect neutrino oscillations—specifically, a change of the muon neutrino into an electron neutrino. Set at the Fermilab National Accelerator Laboratory in Illinois, it consists of a forty-foot sphere filled with 800 tons (250,000 gallons, or thirty tanker trucks) of mineral oil.“Basically,” says Conrad, “it’s baby oil.”

Conrad explains neutrino oscillations with an analogy. If two flutes play the same note but one is slightly mistuned, you’ll hear a reverberating harmonic sound that Conrad mimics as a “wah-wah.” Or strike a tuning fork, and strike another that is identical except for a tiny piece of gum on its tines. The added mass of the gum will alter the fork’s frequency, and together the two forks will produce their own “wah-wah.” In the quantum world, where particles act like waves, the oscillating noise represents the neutrino’s oscillations, and the added gum their mass difference. It’s a property called wave interference, and it’s seen in waves as diverse as those of water, sound, and light. “What we’re really seeing here,” says Conrad, “is an interference effect.”

To search for neutrino oscillations, Mini-BooNE will look for a characteristic pattern of light inside the tank. A beam of muon neutrinos, produced by interactions of protons from Fermilab’s Booster ring, will enter the tank. A precious few of the muon neutrinos will collide with the carbon atoms in the mineral oil and produce a highly energetic particle called a muon, which is essentially a heavy electron. The muon will produce light, called Cerenkov light, as it shoots through the mineral oil and out of the Mini-BooNE tank.

Over the course of a year, about a million of the vast number of muon neutrinos that enter Mini-BooNE’s tank should collide with carbon atoms. However, a few of these muon neutrinos are expected to have oscillated before this collision. If the prior Los Alamos experiment is correct, about a thousand electron neutrinos will collide with the carbon atoms, producing a different Cerenkov light pattern that will serve as Mini-BooNE’s important distinguishing signal.

The Cerenkov light is the key. Cerenkov light is a shock wave phenomenon produced by any particle that travels through a medium (mineral oil, in this case) at a speed faster than light would travel through the medium—the electromagnetic equivalent of a sonic boom. (This does not violate Einstein’s dictum that nothing can travel faster than the speed of light, which is true only in a vacuum.)

In Mini-BooNE’s tank, any entering muon neutrino from Fermilab’s muon neutrino beam will collide with a carbon atom and produce a highly energetic muon. This muon will hurry through the mineral oil, producing a cone of Cerenkov light that, in turn, will travel to the edges of the tank. There, an array of photomultiplier tubes—essentially, inverse light bulbs—will receive the light and produce an electrical signal.

Should any muon neutrinos in the muon neutrino beam have oscillated to electron neutrinos before entering the tank, they will, once inside the tank, collide with carbon atoms and produce, instead of a muons, electrons. These electrons will scatter (bounce around) because they are so much lighter than the escaping muons and will quickly come to a halt as they collide with mineral oil atoms. This produces a Cerenkov cone with fuzzy inner and outer edges, smeared by the bouncing around. After two years of taking data, the group hopes to have results by summer 2004.

Missing mass?
Mini-BooNE expects to detect vastly more of the interesting oscillations than did the Los Alamos experiment, allowing physicists to pin down the characteristics of the neutrino. The information is of more than merely academic interest.

If neutrinos do indeed oscillate, basic principles of quantum physics imply that they also have mass. Because there are so many neutrinos in the universe, even a small neutrino mass could constitute a significant portion of the mass of the universe. That excites cosmologists who study the universe at large, who have found that part of the mass of the universe is “missing”; their determination of the rate of expansion of the universe is too slow to jibe with the amount of matter they observe in the universe. Neutrinos could account for at least part of that “missing mass.”

And if Mini-BooNE confirms neutrino oscillations, its results, together with previous findings from other experiments, would have yet another implication: a new, fourth flavor of neutrino. Such an enlargement of the flavor family would rock the field of high-energy physics.

“We would find ourselves forced into a situation where we have to introduce a fourth neutrino,” says Conrad. “We know that the fourth neutrino can’t be one that interacts the way all the other neutrinos do, or we would have seen it. Since we don’t see it, that means that, if it exists, it must not interact.” Physicists call such a neutrino “sterile.” In fact, Mini-BooNE might see such sterile neutrinos if their initial muon neutrinos simply disappear, having oscillated to the fourth flavor.

A fourth neutrino might confound experimental physicists, but it could be good news for theoretical physicists. A fourth flavor of neutrino, Conrad notes, routinely falls out of esoteric theories that try to unify the three microscopic forces (the nuclear force, the electromagnetic force, and the weak force responsible for radioactive decay) with the macroscopic force of gravity.

Tiny universes
The Mini-BooNE Collaboration includes eleven people from Columbia: Conrad and Shaevitz, three postdoctoral scientists, two grad students, two recently graduated students (including Conrad’s first graduate student, Bonnie Fleming ’98 ’02GSAS), and two undergraduate students. Atypically (for the physics community), 50 percent of this batch are women.

Conrad says the netherworld of particle physics, where particles oscillate and dance and, like Alice in Wonderland, nothing is quite what it seems, captures her imagination. “I find the idea of these tiny universes that you create just so exciting. That there could be something so completely different if you go to very high energies and to very, very short time scales from the world that we actually live in. . . . I end up anthropomorphizing these particles, and they become a part of my life.” A true physicist wouldn’t have it any other way.

Janet Conrad and the Joy of Physics:
When Experiment Pushes Theory

For someone who spends so much of her time thinking about neutrinos, Janet Conrad is surprisingly down-to-earth.

She found her way to physics through an early interest in astronomy—as a sixth grader, she watched Star Trek and gazed at the stars through a friend’s telescope. Her father, an agricultural scientist, advised that she would need to learn physics to be an astronomer. Then in her first class on quantum mechanics at Swarthmore College, she became enthralled by trying to imagine what’s going on in the physical world at “these little tiny scales.” But it was the accelerator facility at Fermilab (in Batavia, Illinois), which she first visited in her junior year, that truly won her over. From the heights of her scientific knowledge she says: “The detectors are just awesome.”

Whether at home in Nyack, teaching at Columbia, or working at Fermilab, Conrad is thinking physics at the highest level. Yet she has an extraordinary ability to explain her work to the intimidated layperson. “It’s not the way many people imagine,” she says. “Somehow when you think about being a scientist you think about people who work in very small groups, maybe by themselves, in lab coats, in sterile environments with black tables. But if you like to work with people, this is a very good field. And it’s messy, which is good for someone who likes to make a mess like I do.”

Along the way she has had to teach herself everything from electronics to plumbing. “Every year I’m doing something different,” she says. “I will be designing a detector and doing a lot of computer work or I will actually be trying to build the detector and install it. Then we’ll get the data and analyze it, which is actually the most fun time, and then we’ll take it out and present it to the world.”

Conrad clearly delights in her decision to be an experimentalist rather than a theoretician. “There’s this cycle in physics,” she says, “where experiment pushes theory and then theory pushes experiment, and I really like the moment in which experiment is pushing theory”—which is what is happening now in the area of neutrinos. “We have the data; we have it first. We can play with it as much as we like before we give it to them. That’s what I like about experimental physics—I want to be the first to get my hands on what nature tells us. I want to find out that secret for a little while. For a moment in my life only I know this one thing about nature—which is totally amazing.”

Photos: Janet Conrad and Mini-BooNE detector, Reidar Hahn.