Penn researchers have a long history of tracking solar neutrinos, elusive particles which may tell us much about our sun and the nature of matter itself.
By Susan Lonkevich | Photos: Sudbury Neutrino Observatory
Sidebar | The Future of Neutrino Research
Each morning Dr. Josh Klein shows up for work in miner’s garb—hard hat, safety boots, the works—and waits for the “cage,” or elevator, to take him down one and one-quarter miles into the Earth.
“The cage is usually jam-packed,” says the 35-year-old Penn physicist, who has spent the better part of the past few years commuting down into a nickel mine in northern Ontario. “When you can put both feet on the floor, you’re pretty happy about it.” Around 3,000 feet, “Everybody’s working their jaws” to relieve the pressure on their ears. “If you have a head cold, it’s incredibly painful.”
Next he walks 1.25 miles to a lab, where the first step is getting clean. “First you wash your boots off twice. Then you go in, take off your hard hat, belt, all your clothes, go through showers, put on new clean clothes, and you put on a hairnet.” That’s to keep the lab free of mine dust. As little as a thimble-full, says Klein, could produce misleading signals in a sensitive detector that, through the help of Penn scientists, has played a pivotal role in the hunt for solar neutrinos.
If neutrinos were people, they’d be loners: the stranger who shows up at a party without so much as a bottle of Beaujolais for the host, talks to no one, and then mysteriously disappears out the back door into the night. Dr. Paul Langacker, Penn professor of physics, describes them as “oddballs” in the realm of subatomic particles. But, he says, “There’s more than meets the eye.” Neutrinos call to mind “somebody who at first glance is very quiet and unassuming, but is very deep and has a lot of consequences.”
Thus it was with much fanfare in mid-June that scientists working at the Sudbury Neutrino Observatory (SNO) announced the results of the latest hunt for these tiny, elusive particles. With an elaborate detector sunk two kilometers below the surface in an active nickel mine in Sudbury, Ontario, the multinational team found direct proof that solar neutrinos, produced in fusion reactions at the center of the sun, are also quick-change artists: On their eight-minute journey to Earth, about two-thirds of solar neutrinos change into different types—or flavors, as physicists call them—that are more difficult to detect. This explains why for more than three decades, scientists have been unable to detect neutrinos at a rate consistent with the standard model of how our sun works. “Our latest result is the smoking gun, I would say,” says Dr. Eugene Beier, a Penn physics professor who serves as U.S. spokesman for SNO. By accounting for the missing neutrinos, the findings—widely reported in the media, including The New York Times, The Washington Post and The Philadelphia Inquirer—prove that the solar model is correct, and open the door for further research.
Mysterious Ways
Neutrinos have no charge, apparently very little mass, and are essentially the most weakly interacting of all subatomic particles. Though you would never notice it, every second some 60 billion neutrinos from the sun zip through a space roughly the size of the button on your sleeve. That’s not even counting the neutrinos that come from other sources, including the atmosphere and radioactivity within the earth. (They can also be manufactured by scientists in particle accelerators and nuclear reactors.)
Why do scientists find the unassuming neutrino to be so compelling? In large part, the particle’s appeal lies in what it may be able to tell us about our own sun and other astronomical objects, as well as about the nature of matter itself.
The nuclear reactions that power our sun, for instance, produce most of their energy in the form of photons of light, but our sun is so dense that a photon produced there takes about 10,000 years to diffuse to the sun’s surface, notes Paul Langacker, a Penn theorist who has not been directly involved with SNO. “By the time a photon has bounced around for 10,000 years, it doesn’t tell you much about the reactions that produced it at the center. But some of the energy is emitted as neutrinos, and they come right out. By observing solar neutrinos, one has a direct probe of what happens at the center of the sun.”
As one of the basic building blocks of matter—and a strange one at that—neutrinos could also provide the answer to a key question: which of the solutions to the superstring theories—theories which seek to unify all the known forces in the universe—is actually correct?
Though the idea of going more than a mile underground to search for particles that come from the sun may sound odd, even unsettling, it’s actually a necessary step to screen out cosmic radiation that rains down on us constantly and would otherwise produce too many distracting signals in a neutrino detector. Neutrinos, because they react so weakly with other matter, pass easily through the Earth.
Scientists have long known that neutrinos come in three flavors, each named for the charged particle they are associated with: electron-neutrino, muon-neutrino, and tau-neutrino. What SNO has demonstrated is that they oscillate between these flavors on their path from the sun, and in order to do so, they likely have at least some mass.
Penn researchers played a major role in SNO, from the detector’s design, construction, and operation to the data analysis. The first experiment to confirm the existence of solar neutrinos—done in another mine, essentially using a huge vat of dry-cleaning fluid to “trap” the particles—was conducted 33 years ago by Dr. Raymond Davis Jr., who would later join Penn’s faculty. During the years between the two experiments, a host of Penn scientists have contributed to solar neutrino research. “I would say we’ve been more involved than any other institution on this issue,” says Beier.
Here Comes the Sun
Anna Davis pulls up to the train station, and her husband, Dr. Raymond Davis Jr., age 86, springs from the car to greet the reporter who has come to Long Island to interview him. He gives up the front passenger seat to the visitor for the brief drive to the century-old red farmhouse in Blue Point, N.Y., which they have occupied for the past 50-odd years.
Though Gene Beier cites Davis, a research professor in Penn’s physics and astronomy department since 1985, as a “high probability” for a Nobel Prize for his detection of the solar neutrino, he brings a stack of neutrino memorabilia to the dining room table and goes through it with a gentle enthusiasm, the way one might show off family pictures.
Davis, who according to his wife’s teasing, must have “sat up in his cradle and said, ‘I want to be a physical chemist,’” started working at Brookhaven National Laboratory after World War II. Soon after he arrived, he talked to the department chair in chemistry about some experiments he thought he could do. “He didn’t react terribly enthusiastically on any of them,” Davis recalls with a laugh, “so he recommended going to the library for a few weeks and thinking about it.” Davis started reading through the journals and found an interesting article about the neutrino, about which little was known. “I said, ‘There’s room for a lot of science if I choose this route and just start doing things.’”
This ultimately led to him building a solar neutrino detector inside the Homestake Gold Mine in Lead, South Dakota. Accompanied by his department chair, Davis went to the U.S. Atomic Energy Commission for money. “They gave us a budget so that I could build quite a big tank—I used to say it cost as much as 20 Cadillacs!”
In 1965 Davis put a 100,000 gallon tank in a mine shaft 4,850 feet below the surface and filled it with perchloroethylene, a common dry-cleaning fluid that could be manufactured cheaply in large quantities. Every once in a while, a neutrino would interact with a chlorine atom in the fluid, producing a radioactive isotope of argon, which could then be “counted” by the detector. A Caltech theorist named John Bahcall worked with Davis to analyze the data.
Scientists celebrated the fact that the solar neutrino had been detected, but were left scratching their heads about one key element of the Homestake experiment: Davis had detected only a third of the neutrinos that Bahcall predicted he should find. Which was wrong—the existing theories about neutrinos, the standard solar model, or the experiment itself? The question would consume the attention of physicists for the next three decades.
v In 2000 Davis won the prestigious Israeli Wolf Prize in physics for his experiment. While half of Wolf recipients go on to receive Nobels, Davis’s wife says he hopes he won’t fall into this category.
“In the first place,” she says, “he thinks he doesn’t deserve it. In the second place, once you’re a Nobel Prize winner, the whole world comes and asks your opinion on all kinds of stupid things you know nothing about.”
“Well,” says Davis, “I’ve gotten a lot of prizes.”
“There’s no room on your wall.”
Davis just laughs.
Soon after he announced his first results in 1968, Davis brought Dr. Kenneth Lande, Penn professor of physics, into the Homestake project. Lande recalls traveling with a small group of scientists to a meeting at Los Alamos around Christmas 1970. “We got on a plane—a leftover C-46—in the middle of the night, and it was freezing cold. The plane wasn’t heated. And the only person who was intelligently dressed in a winter coat was the man in front of me. That’s how I got to know Ray Davis.”
A couple years later, during an American Physics Society meeting in Washington, Lande went to lunch with Davis, who invited him to come see what he was doing at the Homestake mine. The next week, Lande arrived at the mine unannounced. “The foreman took me down. I showed up in the lab and said, ‘Hi Ray.’ He said, ‘What are you doing here?’ Ever since, I’ve been involved in the Homestake mine in one form or another.”
When Davis was forced to retire from Brookhaven 16 years ago because of federal age limits that are no longer imposed on university professors, Lande suggested he continue his research at Penn. While continuing to collect data from the Homestake detector, they developed two new techniques for detecting neutrinos using a metallic element called Gallium as a “target.” The challenge was getting federal funding to build another detector in which to test them. Even after they arranged a collaboration with German scientists, the project was turned down. Eventually, Lande says, they were invited to join Russian scientists in using one of the two techniques at a detector in the former Soviet Union. By this point there had been four solar-neutrino experiments, including an earlier one in Japan involving Beier and Penn professor Alfred Mann. “Everybody saw too few neutrinos.”
Moonlight, Roses and Muons?
Dr. Alfred Mann is a passionate guy. But if the emeritus professor of physics promises to tell you “a romantic story” involving the origins of neutrino research, don’t bother playing violin music. When it comes to such matters, physicists’ passions are, well, different.
Back in 1930, an Austrian physicist named Wolfgang Pauli suggested an answer to one of the great scientific puzzles of his day: Why, during certain radioactive decays, was less energy released than expected? Pauli tentatively suggested that the reaction products must include another sub-atomic particle that interacted too weakly to be detected. In a letter addressed to a meeting of his colleagues in Germany, he wrote:
“Liebe Radioaktive Damen und Herren”:
(Dear Radioactive Ladies and Gentlemen):
… I have hit upon a desperate remedy to save the exchange theorem of statistics and the law of conservation of energy … I agree that my remedy could seem incredible because one should have seen those [neutrinos] very earlier if they really exist … Every solution to the issue must be discussed. Thus, dear radioactive people, look and judge … your humble servant, W. Pauli
Pauli, who didn’t dare publish his theory until three years later, “thought he had done a bad thing scientifically,” says Mann. “He had given this particle properties that prevented it from being observed. It was massless. It had no charge, and all it did was carry energy and momentum. You see, lay people mostly think about science as proceeding in a straight line, but science rarely proceeds in a straight line and we don’t live our lives in a straight line. We go in and out, we make mistakes, we do all kinds of foolish things—and occasionally some intelligent things.” It wasn’t until 1956, shortly before Pauli’s death, that the neutrino was first detected in a nuclear reactor, and Pauli’s intelligent solution to the puzzle was proven. “For a physicist, that’s romantic.”
Mann started out studying philosophy at the University of Virginia and began taking physics courses only to understand some of the modern philosophers better. Eventually he was recruited to work on the Manhattan Project. He came to Penn in 1949 and brought neutrino physics to the University in the early 1970s, working with Gene Beier on a series of projects.
Eventually Mann became interested in conducting lab experiments with neutrino-electron scattering. When a neutrino comes in contact with an electron, he explains, “It’s almost exactly as if these were two billiard balls, so they bounce off—except the interaction is a weak interaction. It isn’t a mechanical interaction. Because little momentum is exchanged between the two objects, the angle at which the electron bounces off is very small.”
In the course of doing these experiments, Mann says, “It occurred to me that maybe this was a way to demonstrate that what Ray Davis had been observing was neutrinos and to point them back to the sun.” To prove this, “you had to see the electron in its path so you know its direction, and secondly, you had to know precisely what time it was during the day or night and know where the sun was at that precise time.
“Again, a romantic story—I love romantic stories.” Mann spent a couple of years trying to find a mine that could be made into a national laboratory to test his theory. Unsuccessful in this attempt, he attended a conference in Utah in 1980, where he met a Japanese physicist, Masatoshi Koshiba from the University of Tokyo. “We hit it off,” he says, “and went through this whole marriage of ideas.” As a result, a group from Penn, including Mann and Beier, collaborated with several Japanese universities to carry out an experiment at the Kamiokande II detector that did point the neutrinos back to the sun and confirm the rate observed in Davis’s experiment.
“To complicate things, we got it all going, and we got several months of nice data—and then Supernova 1987a happened.” When a star uses up its fuel and can no longer generate the energy to hold off gravity, the core of the star collapses, producing an explosion that, if close enough to earth, can be viewed with the naked eye. The last time a supernova had exploded this close to the earth was nearly four centuries ago, so naturally scientists were excited by the event in 1987 and eager to find out if any neutrinos formed in the core of the star had been detected. Purely by chance, Mann says, the Kamiokande detector observed a burst of 12 neutrinos from the supernova.
Though he later wrote a book for laypersons on the phenomenon, Shadow of a Star, Mann shakes his head at the publicity the event garnered. “To set up and plan and do everything to see the solar neutrinos in the Kamiokande II detector is one thing. To observe the supernova neutrinos waspure chance. We happened to be in the right place at the right time. But the excitement generated by the supernova neutrinos far outshadowed what we did with respect to the solar neutrinos at the time. Which always irritated the hell out of me.”
Another Penn scientist, Paul Langacker, was the first to synthesize all the solar-neutrino data. “I’m a theorist,” he explains. “I’ve always had an interest both in fundamental unifying theories and making contact between theory and experiment. So I sort of sit a little bit between two camps and talk to both.” Neutrinos are one aspect of a worldwide program of experiments he’s been involved in interpreting over the years.
About a decade ago, he says, “I felt the time was right to look at [the neutrino experiments] in a global way [and] distinguish whether there was some problem with the astrophysics or the neutrinos. We eventually were able to argue, looking at the total pattern of experiments, that it was very unlikely any modification of the theory of the sun could account for what we were seeing. The overall pattern didn’t smell like any astrophysical anomaly and really favored the neutrino interpretation.” That couldn’t be confirmed, however, until this summer.
Though construction of the Sudbury Neutrino Observatory began in 1990, the ideas behind it were conceived of 17 years ago by Herbert Chen, a University of California-Irvine researcher who died of leukemia before the project’s completion. While Gene Beier was working in Japan, Chen consulted him periodically for technical advice. During one long phone call while Chen was sick in the hospital, Beier says, “He made a comment that sounded like he wanted to know if I was interested in joining. I told him I couldn’t at the time, because it would compromise our efforts in collaborating with the Japanese.”
In 1987, the year that Chen died, Beier received another invitation to work on SNO. “So I took a small group of people from Penn: Dr. William Frati (research professor of physics), Mitchell Newcomer (instrumentation specialist), and Richard Vanberg (director of the high-energy physics engineering group) to help design the project.” Taking a sabbatical in 1989-90, he moved out to Toronto to raise money.
Klein, a research assistant professor whom Beier brought to Penn seven years ago for post-doctoral work, laughs when he thinks about his early expectations for the pace of the project. “At the time there was me and another post-doc [from the University of Toronto] who had gotten here [several months] earlier than I did. He thought we were going to be taking data in, you know, nine months. So he got in his truck and drove as fast as he could to get down here.” Five years later, they finally began taking data. In the meantime they helped design and build highly sensitive electronic components of the detector, and assembled the equipment on site.
“The thing to remember when you go underground is that you only have one shot every day of getting down there,” says Klein, who later was in charge of data analysis for SNO. “If you go down there and you’ve forgotten a wrench, and you need a wrench to do your work, you sit around for 8 to 12 hours until the next day, when you can go back and get a wrench. That’s because the mine cares very little for moving people around. It cares about moving equipment. “There is actually a ladder,” he says. “If you want to climb a mile and a half, you could do it. We once tried to calculate whether or not you had enough calories to be able to survive the climb.”
At the center of this elaborate operation is an acrylic tank filled with 275,000 gallons of “heavy water,” or D2O, which is especially sensitive to neutrinos. Even so, out of the trillions which pass through the tank each day, only a handful will be scattered or stopped. When this happens a tiny flash of light is produced and the events are recorded by 10,000 photosensors—the eyes of the detector—which are embedded on panels in a geodesic dome surrounding the tank. The entire sphere is surrounded in ordinary water in an enormous rock cavity blasted out of the mine.
The first reaction observed at SNO was the absorption of only electron neutrinos by deuterium nuclei in the heavy water. The neutrino inside the nucleus changes into a proton and spits off an electron, which moves off so rapidly that it produces a flash of light. Researchers detected 35 percent of the neutrinos they were expected to observe, according to the standard solar model.
They went on to compare this number with the results of an earlier experiment at a different detector, which counted electron neutrinos and a much smaller portion of the other types, muon and tau. It detected 45 percent of the neutrinos predicted. With the method used in the earlier detector, muon- and tau-neutrinos are about one-sixth as likely to be detected as electron-neutrinos. Comparison of the two experiments convinced scientists at SNO that about 65 percent of the “solar flux” is from non-electron neutrinos. “You add that to our 35 percent and you get 100 percent,” say Klein. “And so you find out very satisfyingly, and, for some people, surprisingly, that the predictions of how many neutrinos the sun puts out are about right.”
This summer SNO researchers analyzed another reaction, in which the neutrino breaks apart the deuterium nucleus, with an equal sensitivity to all neutrino flavors—and they added salt to the heavy water to dramatically increase the detection efficiency. They plan to compare the number of all types of neutrinos counted before and after the addition of the salt for results that Beier predicts “will increase the significance of the measurements we’ve done so far.”
The SNO experiment is the first to provide direct evidence for neutrino oscillation. Though they are “born” as electron-neutrinos inside the sun, when they move through space they become a mixture of three different types of neutrinos, each of which could have its own mass and travel at a different velocity.
It helps, says Klein, to think of neutrinos as consisting of waves, which can be added together like musical notes to produce a harmony. “As they move, the lighter one will move a little faster and the superposition of the waves changes,” he explains. So what began as an electron-neutrino looks, at another point, like a muon-neutrino. It shifts and looks like an electron-neutrino again, shifts some more and looks like a muon-neutrino.
The SNO observations also indicate that neutrinos have masses, though they are probably very small, perhaps 100,000 times smaller than the mass of the electron. So far scientists have only been able to measure the mass differences of neutrinos, rather than absolute values. The prevailing theory of elementary particles “says that neutrinos are massless, there are three types, and that’s all there is.” By showing that “the three types of neutrinos are not really separate,” says Klein, SNO has provided “a big example of a case where the [theory] is failing.”
While data collection continues at SNO, two particle sleuths have savored at least a few moments of satisfaction from their detective work. Soon after their announcement, Gene Beier and Josh Klein traveled up to Ithaca, N.Y., to visit Nobel-winning physicist Hans Bethe in his nursing home and talk at length about their findings. The 95-year-old Cornell scientist had always taken an interest in neutrino research, and from time to time would ask Beier for updates about the project. “One of the last times I saw him prior to this summer,” Beier says, “he made a comment to the effect that ‘I hope you get this result before I die.’ When I got it, I felt as though I should deliver it in person. It was one of those great experiences.”
Reflecting on the magnitude of SNO’s findings, Klein notes: “Thirty years ago, Ray Davis sat down and said, ‘I’m going to figure out this really cool way of understanding the sun by looking at these things which come directly from the core.’ What he found out was there is something about the neutrino which made it very hard [to do this]. We spent 30 years figuring out what the problem was. And we need to figure it out some more. But right now it looks like we can actually take these neutrinos and start to learn something about the sun, which is what the original program started out to do.”
SIDEBAR
The Future of Neutrino Research
In Lead, South Dakota, population 3,500, a banner hangs at City Hall proclaiming February 23, 2001, as Neutrino Day. Physicists, as Dr. Kenneth Lande will attest, get treated very well around here. Mining operations at the Homestake gold mine, where solar neutrinos were first detected, will end this year, but Lande, a Penn physics professor, is part of an effort to convert the space into the Homestake National Underground Science Laboratory, which would be the “premier world location for underground science.” Its organizers are seeking $281 million from the National Science Foundation. “Every death is followed by a birth,” says Lande, speaking from 4,850 feet below the ground, where he’s “trying to organize the transition from mine to lab.” He says 35 “letters of interest” have already come in from physics, biology, and geology researchers who want to use the lab in a variety of experiments, some involving neutrinos. He expects Penn to play a major role in the facility.
In the meantime, scientists at the Sudbury Neutrino Observatory (SNO) will continue refining their understanding of how neutrinos transform from one type to another. After SNO, other questions remain to be answered about these particles: What are their masses? Does another hidden “flavor” exist? According to Lande, there may be a fourth type of “sterile” neutrinos, which don’t engage in the kinds of reactions the others do.
Another possibility the next generation of experiments will examine, says Lande, is whether there are variations in neutrino signals depending on the time of day or night, or as a function of the seasons. During the day an underground detector is separated from the sun’s output of neutrinos by a mile or so of Earth. At night, there would be 8,000 miles between Earth and the sun’s output. “What’s the effect on neutrinos as they go through the Earth? Are there some more neutrino flavor changes as they pass through Earth?”
Beyond those inquiries, Lande says, now the neutrino can be used to probe the center of the sun and find out whether it varies its behavior over time. The neutrino can also be used as a probe to see inside a supernova formed when a star collapses: “Because some exciting things happen there in a very short period of time.”
