Since the late 1960s, experiments have hinted at a more-than-massless neutrino. Theoretical models of the sun predict that neutrinos should be made in staggering numbers. Neutrino detectors on the Earth, however, have repeatedly seen less than expected. Because neutrinos come in three varieties—known as electron, muon, and tau neutrinos—and because solar neutrino detectors have been primarily sensitive only to electron neutrinos, the preferred explanation over the years is that those "missing" neutrinos had changed, or oscillated, into a flavor for which the detectors had little or no sensitivity. And if a neutrino oscillates, according to the laws of quantum mechanics, then it must have a mass. And a massive neutrino, no matter how small that mass may be, is the kind of thing that makes life interesting for a physicist.

The evidence has been building for years, most notably from the Super-Kamiokande experiment in Japan. Then in August 2001, the Sudbury Neutrino Observatory (known as SNO), a detector facility located 6,800 feet underground in a mine outside Sudbury, Ontario, checked in with a direct observation suggesting that electron neutrinos from the sun really were oscillating into muon and tau neutrinos. SNO published its report in the August 13, 2001, issue of Physical Review Letters, and the paper immediately joined the ranks of the hottest in physics, racking up well over 400 citations in just two years and becoming a fixture in Science Watch’s physics Top Ten (table below, paper #1). SNO followed up on this remarkable show with two more Physical Review Letters reports in July 2002 (table below, paper #2 and #3), which have since been equally hot. Having scored the top three places in the Physics Top Ten in the previous issue (November/December 2003), the three papers continue their run among the most cited in physics of this edition (paper #2, #3, and #7).

The SNO collaboration, consisting of 135 physicists from 15 institutions, is led by Arthur B. McDonald of Queen’s University in Kingston, Ontario. McDonald, 60, received both his bachelor’s and Master’s degrees in physics from Dalhousie University in Nova Scotia, before moving on to Caltech, where he earned his Ph.D. in 1969. Through the 1970s he worked as a research officer at the Chalk River Nuclear Laboratories northwest of Ottawa. He was a professor of physics at Princeton University from 1982 through 1989, when he left to join Queen’s University, where he currently holds a University Research Chair.

  Can you give us a little history of SNO? How did it get started, and what was the original plan?

McDonald: It started as a proposal by Herb Chen at the University of California, Irvine, back in 1984. He was interested in using the deuterium in heavy water to study solar neutrinos. Deuterium had been used in experiments before for studying neutrinos, but Herb wanted to take it to another level. He wanted to use 1,000 tons or more to study solar neutrinos. There were about 15 of us who went to the early meetings and became the collaboration, and that was the start of it, with Herb Chen and George Ewan from Queen’s University, Canada, as the initial co-spokesmen. Herb passed away in 1987 but the collaboration continued to carry out the experiment.

  What was the state of the solar neutrino problem at the time? And how did deuterium address it?

McDonald: There was clearly a significant deficit as measured by Ray Davis’s experiment in the 1960s that used chlorine targets to detect neutrinos. What we had to work with, theoretically, was an estimate of the electron neutrino flux from the sun. And when you observe too few of those as measured at the Earth, as Ray’s experiment did, the question is, is that because the estimate is wrong—in this case by a factor of three—or is it because the neutrinos have changed from one type to another, with two-thirds changing into other types before reaching the Earth? The chlorine experiment, for example, was sensitive only to electron-type neutrinos, and so if the neutrinos changed, you couldn’t tell. If you use deuterium, which SNO does, you can measure not only the type of neutrino produced in the sun, but you can also measure all neutrino types. So you can tell whether neutrino transformation has taken place. If you measure all neutrino types and you see three times as many as you do measuring only electron neutrinos, then you know. And that’s what we eventually measured—18 years after Herb Chen proposed it.

  How compelling was the original data when you published it?

Highly Cited SNO Papers, including A.B. McDonald, published since 1999 (Ranked by total citations) Rank Field # of  core papers 1 Q.R. Ahmad, et al., "Measurement of the rate of ne, + d à p + p + é interactions produced by 8B solar neutrinos at the Sudbury Neutrino Observatory," Phys. Rev. Lett., 87(7): 1301, 13 August 2001. 466 2 Q.R. Ahmad, et al., "Direct evidence for neutrino flavor transformation from neutral-current interactions in the Sudbury Neutrino Observatory," Phys. Rev. Lett., 89(1): 1301, 1 July 2002. 271 3 Q.R. Ahmad, et al., "Measurement of day and night neutrino energy spectra at SNO and constraints on neutrino mixing parameters," Phys. Rev. Lett., 89(1):1302, 1 July 2002. 198 4 J. Boger, et al., "The Sudbury Neutrino Observatory," Nucl. Instr. Meth. Phys. Res. Sect. A, 449(1-2): 172-207. 11 July 2000. 86 5 A.B. McDonald, "The Sudbury Neutrino Observatory Project," Nucl. Phys. B-Proc. Suppl., 77: 43-7, May 1999. 30 SOURCE: Thomson ISI Web of Science®

McDonald: The first thing we published was the reaction in heavy water, which is specific to electron neutrinos, compared with measurements from the Super-Kamiokande experiment in Japan, where one-sixth of its sensitivity comes from muon or tau neutrinos. With an accuracy corresponding to about 3 standard deviations, we were able to show that neutrinos, in fact, appeared to be transforming from electron to muon and tau neutrinos. That was what we published in 2001. The next year we published the results from the reaction I mentioned that is sensitive to all neutrino types. That gave us six times more sensitivity to the muon and tau neutrinos, and we were able to show with about 5.3 standard-deviations accuracy that neutrinos were changing their types.

  You just delivered your latest results at the TAUP (Topics in Astroparticle and Underground Physics) 2003 conference in Seattle. What did you report?

McDonald: In our most recent paper, taking the same approach—that is, asking how many standard deviations we are away from no neutrino flavor change, which is the null hypothesis—the answer is greater than 7 standard deviations. This is getting to a point where the significance is so large it’s even difficult to calculate. The other very interesting element of this most recent work is we did the measurement by a completely independent technique. This time we put salt in the detector. By doing that it becomes possible to actually do a statistical separation of these two reactions in the detector.

Let me explain these two reactions. The charged-current reaction is specific to electron neutrinos. The neutral-current reaction is sensitive to all neutrino types. What we normally see in the detector is the sum of light bursts from these two processes; we see the sum of both types of event. When we put salt in the detector, we get a factor-of-three improvement in efficiency for detecting that second reaction, and the bursts of light show a different pattern on the light sensors for the two reactions. So rather than simply having to look at the sum of the two reactions to infer whether or not neutrinos are changing type, we now can do a statistical separation of the two reactions and have a completely independent measure of whether the results that we obtained previously were correct. And it turned out to be spot-on with what we originally measured.

  What does this experiment and the now-likely fact of neutrino oscillations actually say about the mass of the neutrino?

McDonald: It says there is a mass and it tells you the differences between the masses, but it doesn’t tell you the absolute value of the masses. That was a substantial topic in the conference I’m attending here in Seattle.

  Do neutrino oscillations require rewriting the Standard Model, or do they require an addendum to it?

McDonald: The basic Standard Model postulates essentially zero neutrino mass and no flavor change. What we need is a mechanism that explains neutrino mass and, in particular, why neutrino masses are so much smaller than the masses of other particles. The leading theory for doing this is referred to as the see-saw model, and it doesn’t substantially change the structure of the Standard Model. It just puts in components that had not previously been identified, and which go beyond the structure that works so well at the energies that have been explored so far in accelerators.

  Would this revision or addition tell us whether supersymmetric theories—the favored candidates for physics beyond the Standard Model—are more or less credible?

McDonald: Well, supersymmetric theories can also incorporate non-zero neutrino masses. So it’s a way of moving toward such theories. One question that now has to be answered—and experiments looking at a phenomenon called double-beta decay might be able to do so—is whether these neutrinos are, in fact, their own antiparticles. We also have to learn the overall mass scale for the neutrino masses. But, yes, these are things that can be accommodated in supersymmetric theories. Therefore, they can serve as ways of getting more complete information as to whether or not theories such as supersymmetry are correct.

  SNO was a decade and a half in the making. What kind of challenges does that present in terms of keeping funding going, worrying about competition, etc.?

McDonald: All of the above existed at various points throughout the process. Fortunately we had a very good team of scientists, and we knew we had a piece of science that could be done, and which would be very significant regardless of the outcome. Whether you observe a new property of neutrinos themselves or sort out the questions about solar models, it’s significant. Our team was motivated by these challenges and over the years managed to overcome them. Of course it also required that we have peer reviewers who accepted the validity of the science objectives, and it also required that we exhibit progress along the way that showed that we could do it. Well, everybody hung in there with it—the scientists and the supporting agencies.

Then there were the technological challenges, which required a major tour de force. You are trying to build the whole thing in better than a class 2000 clean room, within a factor of 10 or 20 of what you have in a semiconductor facility. And you’re doing it at the bottom of a high-powered nickel mine, which is taking about 5,000 tons of nickel a day out of the ground. And you’re working with $300 million worth of heavy water, which is a rather valuable resource.

  The heavy water is on loan?

McDonald: It is. For a dollar. It belongs to what’s called a Canadian Crown Corporation, which is Atomic Energy of Canada, Ltd. (AECL), and they loaned it to us. That is generally one of the things we’ve been saying about the tremendous leverage obtained by doing this experiment in Canada. The capital cost was around 73 million Canadian dollars, but the value of the heavy water was $300 million; the value of the location would be $150 million if we’d had to start from scratch and dig a deep shaft. So for $73 million, we’re able to leverage another $450 million in assets to do the measurements

So, with the valuable heavy water, you have to be sure it’s safe in the process of doing the experiment. And then you have to beat down the natural background of radioactivity to a level at which you can then see these incredibly subtle neutrino interactions. These are the kind of challenges that are inspiring to scientists. But the thing that keeps you going is the knowledge that you can really do excellent science if you can pull it off technologically.