The Telescope in the Ice

by Mark Bowen

Out of respect for his pioneering achievements with DUMAND, the irrepressible John Learned was accorded the honor of presenting the introductory overview of the field and one of the future-related talks, near the end of the workshop, on his latest quixotic vision: a gigantic amoeba-like instrument made up of two concentric spheres of light detectors, buried either deep underground or deep in the ocean, which he called The Neutrino Eye. He wore a Jerry Garcia tie decorated with fish, to symbolize his faith in water as opposed to ice, and he bore a certain resemblance to the iconic guitarist, with his eyeglasses, his beard, his rounded frame, and his slightly zany smile.

  John is not only the pied piper of neutrino astronomy, he is also the intelligent Forrest Gump. He was now collaborating on the Super-Kamiokande experiment, aka Super-K, the successor to Kamiokande, ten times the size, which had been completed in 1996. And he seemed to be on a sort of world tour with Yoji Totsuka, the leader of the project—who gave the most anticipated talk of the workshop. For, one month earlier, Super-K had announced the physics discovery of the decade: neutrino oscillation.

  This is the wraithlike, shapeshifting process that Bruno Pontecorvo had first proposed four decades earlier, in which a neutrino changes flavor as it flies through space: a muon neutrino will transform into an electron or tau neutrino and then into a third flavor and so on. Since oscillation will only occur if the neutrino has mass, it violates the standard model of particle physics, which holds that the particle shouldn’t weigh anything. This discovery was the first and is still the only glimpse into physics beyond the standard model and is a large part of the reason the neutrino is one of the major focuses of particle physics today.

  The Super-K discovery also provided the first clue to a puzzle that had been vexing the physics community ever since Ray Davis and John Bahcall had first encountered it in the 1960s.

  * * *

  Recall that in the mid-fifties, after having failed to detect “reactor” antineutrinos at the Savannah River nuclear plant, Davis had decided to train his sights on the Sun. In 1958, he learned about some new theoretical estimates for neutrino emission by the Sun that seemed to put solar neutrinos within his reach. He proceeded to build what seems to have been his fourth thousand-gallon chlorine-based detector, just short of half a mile deep in a limestone mine near Akron, Ohio—and came up empty again. It turned out that the brightness estimates were accurate, but his method was about a thousand times less sensitive to neutrinos in the energy range given off by the Sun than he had thought.

  Four years later, the young theorist John Bahcall came out with new estimates for neutrino production by the many different fusion reactions that take place in the Sun, each of which produces neutrinos of a specific energy. When Davis learned of Bahcall’s work, he suggested he calculate not only production, but also the rate at which a chlorine detector would capture the neutrinos in each energy range, and about a year later, Bahcall came up with a prediction higher than any of the previous by about a factor of twenty. This was promising enough to prompt him to join Davis in proposing a larger (of course) 100,000-gallon detector. The proposal took the form of back-to-back papers, published in 1964 in Physical Review Letters, the first by Bahcall on the theory, the second by Davis on the experiment.

  Bahcall introduced the so-called solar neutrino unit, or SNU (pronounced “snew”) as a convenient measure for Davis’s experiment. And as with most things neutrino, the SNU is rather small: 10-36 neutrino captures per target atom of chlorine per second. This was part of the reason they needed a large detector. Bahcall estimated that the Sun would emit about 7.5 SNU, which meant that a detector containing 1036 chlorine atoms would capture about 7.5 neutrinos every second. Davis’s 100,000-gallon tank may have seemed large, but it was half a million times smaller than that; it contained “only” 2 × 1030 chlorine atoms. That meant he could expect to detect 7.5 neutrinos only every half-million seconds, which is slightly less than six days, which adds up to only about 460 neutrino captures in an entire a year. His immense challenge, therefore, demanding incredible purity in the target liquid and painstaking separation chemistry, was to sweep tiny numbers of argon atoms (the byproduct of inverse beta decay between an electron neutrino and a chlorine atom) from among the astronomical number of perchloroethylene molecules (his target liquid) in a tank about one-sixth the size of an Olympic swimming pool.

  It says something about the wisdom of the physics community that the project was funded despite Davis’s decade and more of null results (although, to put things in perspective, he once pointed out that the $600,00 it cost to do the experiment would have bought only about ten minutes of advertising on commercial television at the time). He installed his detector almost a mile deep in the Homestake Gold Mine in Lead, South Dakota, and revealed his first results in a talk to the American Chemical Society in September 1967.

  The good news was that he had finally detected neutrinos. The bad news was that he had only detected about a third as many as Bahcall had predicted. This was a remarkable achievement, nonetheless, considering the complexity of the experiment and the exquisite sensitivity of the solar models. In subsequent years, Bahcall would point out that an agreement this close should have been a cause for rejoicing, since his estimate of the neutrino flux was proportional to the central temperature of the Sun raised to the 20th power!

  But the physics community, as it will, focused on the two-thirds of the glass that were empty, rather than the one-third that was full. The tension between Bahcall’s theory and Davis’s experiment became known as the solar neutrino problem, although Bahcall liked to call it the solar neutrino opportunity, since it indicated to him that there was new physics in there somewhere—and it would turn out that he was correct.

  The indomitable Davis continued to run his experiment, Bahcall continued to refine his model, and neither much budged. By the time of the Super-K discovery thirty years later, Bahcall’s prediction was 7.6 SNU, and Davis’s measurement was 2.56 SNU, which gave a ratio tantalizingly close to 3:1. It is a testament to Bahcall’s intellectual integrity that he stuck to his guns: he never tried to modify his theory to match the experiment.

  * * *

  Bruno Pontecorvo had first suggested that neutrinos might oscillate from behind the Iron Curtain in 1958, about a decade before the solar neutrino problem reared its head. Being interested all things neutrino (and the inventor of Davis’s method, after all), he also kept tabs on “the problem.” Bahcall and Davis remembered him telling them at a conference in Leningrad, many years after they had published their proposal, that he had presented it at a special seminar in the Soviet Union at the time and that “he was the only person present who expressed the opinion that it would be a successful experiment.”

  Pontecorvo’s thoughts on oscillation evolved as neutrino physics evolved. When he first came up with the idea, he suggested that the neutrino might transform into its antiparticle and back again. After the muon neutrino was discovered in 1962, he guessed that electron neutrinos might change back and forth into muon neutrinos, and since the Sun was expected to emit only electron neutrinos, he predicted—in advance of Davis’s first results!—that “the flux of observable Sun neutrinos must be two times smaller than the total neutrino flux.” The discovery of the tau lepton in 1975 meant that there should be three flavors of neutrino and the deficit should be precisely three, just as Davis was measuring. (By the time the electron neutrinos from the Sun travel all the way to Earth, they will oscillate back and forth enough times for there to be an equal mix of the three flavors.)

  Pontecorvo did not live to see the discovery of oscillation. He died in 1993.

  * * *

  The Super-K collaboration made their discovery using atmospheric neutrinos. In other words, they used the atmospheric beam as an experimental beam, in contrast to AMANDA, which was using it as a test beam at the time. They did real physics with it—and impressive physics at that.

  Their instrument could detect only electron and muon neutrinos, effectively speaking, an
d by chance, the atmospheric beam is made up predominantly of those two flavors. Without oscillation, the instrument should have detected the same number of each flavor coming up through the Earth as it did coming down from the sky, but it detected fewer muon neutrinos coming up than down. Since the up-going particles traveled farther to reach the detector—all the way through the planet in some cases—those neutrinos had more of a chance to oscillate into tau neutrinos, which were essentially invisible to the detector. The electron neutrinos did not oscillate significantly over that distance.

  There was some complex logic involved in cementing their case, partially because it was tricky to work with the mixed atmospheric beam. (Manmade accelerator beams, which are pure and well-characterized, are often better suited to the pursuit of pure physics goals. It is sometimes said that cosmic rays are like illegitimate children: you can never be sure who their parents were.) But the collaboration covered all the bases. One smoking gun was that the fraction of muon neutrinos that disappeared depended upon the direction they came from: if they came straight up, which meant they had traversed the entire diameter of the Earth, more disappeared than if they came from closer to the horizon.

  It seemed very likely that oscillation explained the solar neutrino problem; however, its simple discovery was only circumstantial evidence, not quantitative proof. It would take yet another large underground detector, the Sudbury Neutrino Observatory (SNO), to solve the puzzle quantitatively. This instrument consisted, as usual, of a 1,000-ton spherical tank of ultraclean water, buried more than a mile deep in a zinc mine in Sudbury, Ontario. What made it unique, however, was that the water was heavy water, in which the two hydrogen atoms in the water molecule are replaced by a heavy hydrogen isotope named deuterium. The water, which was worth several hundred million dollars, was on loan from the Canadian government’s atomic energy corporation.

  SNO could detect electron neutrinos coming from the Sun in the usual way, employing Cherenkov radiation. And the presence of the deuterium allowed it to detect the total flux of all three flavors simultaneously. In 2001, the SNO collaboration announced the solution to the problem that Davis and Bahcall had been working on for more than three decades: the flux of electron neutrinos matched Davis’s measurement, and the flux of all three flavors together matched Bahcall’s prediction.

  Bahcall remarked at the time that he felt the way “prisoners that are sentenced for life do when a DNA test proves they’re not guilty.” “For thirty-three years, people have called into question my calculations on the Sun.… I feel like dancing!”

  The following year, Davis shared half the Nobel Prize in Physics with Masatoshi Koshiba, who had been leader of the Kamiokande experiment when it detected the neutrinos from Supernova 1987a. The other half went to Riccardo Giacconi, for his pioneering contributions to X-ray astronomy. The prize is limited to three people, and Giacconi certainly deserved his share, but many still believe it was a travesty that John Bahcall was left out.

  Davis was eighty-eight when he received the prize. Owing to his failing health, his son delivered his Nobel lecture for him. “The collision between solar neutrino experiments and the standard solar model has ended in a spectacular way,” said the son for the father. “Nothing was wrong with the experiments or the theory; something was wrong with the neutrinos.”

  * * *

  At the conference banquet in Zeuthen, a barbecue in the institute’s courtyard, I sat across from John Learned, who was very much in his element, flush with the success of Super-K and friends with almost everyone there. He knew many of the Russians from the days before Baikal sprang from DUMAND, and he and Totsuka seemed to have a genuine affection for each other, beyond the dictates of protocol. Totsuka-san, a warm, large-hearted man and by all accounts an inspiring and effective leader, was clearly enjoying a high point in life.

  Francis Halzen points out that the Super-K discovery was a big blow to the accelerator community and that it had a huge impact sociologically by prompting them to focus on the neutrino. Not only had a relatively inexpensive “non-accelerator” experiment made a momentous discovery in pure particle physics, it had revealed the first glimpse of physics beyond the standard model, a quest the accelerator scientists had joined roughly twenty years earlier.

  It took almost another twenty “for the discovery of neutrino oscillations, which shows that neutrinos have mass,” to result in a Nobel Prize. In 2015, the physics prize was awarded jointly to a younger member of the Super-K collaboration, Takaaki Kajita, and Arthur McDonald of the Sudbury Neutrino Observatory. By that time, unfortunately, Yoji Totsuka had died of cancer. McDonald, too, was a “second generation” member of his experiment. The visionary who had conceived of SNO and gotten it into the ground was Herb Chen of UC Irvine, a protégé of Fred Reines who had also died young of cancer. Experimental particle physics, especially neutrino physics, takes uncommon persistence.

  Speaking of which, these were the tenth and eleventh Nobel Prizes connected to Bruno Pontecorvo’s prescient insights.

  15. The Peacock and Eva Events

  Francis knew that they needed to get gold-plated events out of AMANDA-B10 if they wanted to keep the momentum going for IceCube. His group set their sights on that goal as soon as John Jacobsen sent the filtered data around.

  Wisconsin was the strongest group in the collaboration, owing in part to the immense resources of the institution and in part to Francis’s way of attracting excellent students. (Inside the collaboration, Madison is known as The Evil Empire.) At least five people were working with the data: Albrecht Karle had moved the previous year from his post-doc in Zeuthen to an assistant scientist position at Madison. Gary Hill had taken up the post-doc Bob Morse had offered him after his winter. There were at least two grad students involved: Rellen Hardtke and Ty DeYoung (mentioned previously for his worship of John Jacobsen’s code writing abilities). And Morse, who couldn’t advise graduate students, since he wasn’t a professor, had a knack for recruiting smart undergraduates, in this case a young man named Phil Romenesko. The most important strength of this group of five was that they worked well together.

  As the collaboration returned to their separate homes from the Zeuthen meeting, they were still waiting for one last piece to the puzzle, the so-called string geometry: where the strings were located in relation to each other. This was a persistent question with AMANDA, owing to the madcap way in which they sometimes chose where to drill the next hole. Kurt Woschnagg had presented a geometry at the meeting, based on the timing of laser flashes broadcast between modules, but it conflicted with the admittedly slapdash surveys that had been done as the holes were being drilled, so he had to go back to the drawing board. He came up with a new geometry within a few weeks of the Zeuthen meeting, and the ground was laid for looking at the B10 data in earnest. Madison was ready to pounce.

  Dennis Peacock, the head of Antarctic Sciences at the National Science Foundation, was scheduled to visit the campus on August 26, and Francis wanted to have something to show him. (AMANDA’s original champions, Peter Wilkness and John Lynch, had left the foundation by this time.) The Madison group, mainly Albrecht and Ty, had developed a reconstruction algorithm and some primitive cuts and had tested them on about four and a half days’ data, but when they scanned through the resulting events by eye they found nothing of interest. Phil Romenesko had then crunched through another thirty days, but by the day before Peacock’s visit that group of events still had not been scanned.

  Rellen was studying for an exam, and Ty was out of town, so Francis asked Gary to look through Phil’s sample, and Phil managed to join him. At eleven in the evening on the night before Peacock’s visit, Gary sent the following e-mail to the rest of the Madison group:

  Subject: The “moment of truth” arrives …

  Gidday all, well we spent about 5 hours last night scanning through 564 leftover events from the 30 days data reconstructed with the new geometry … in fact we went over them at least three times … just to be sure.


  … what did we find? Well, I think you should each experience your own “moment of truth” (we certainly did!), so check them out …

  Have fun … Gary and Phil …

  P. S. Although we really think everyone should look through all the events, we did condense them slightly into the following file: /d13/ghill/data/mot.rc.f2k … in case you want a subset to show the NSF chap …

  Francis did not check his e-mail the next morning. Peacock came into his office, and they spent “the usual half-hour talking about this and that and about IceCube and the future, all the big programs like you talk about with big bureaucrats. And then, you know, we said, now we’re going to show you the status of the data analysis, and … I sit down with him at Rellen’s work station…”

  Rellen had checked her e-mail. She ran the two beautiful events that Gary and Phil had found across her computer screen.

  Francis recalled that moment for me during a collaboration meeting the following November: “You cannot imagine how exciting this was.… Rellen shows these events, and I see them, and … you know, you look at these events and you know that this is something—this is like cocaine, right? I mean, this is what you have been waiting for ten years for.…

  “The events we found with B4 … I mean you would bet your wallet, but not, you know, the head of your son.… On these I would have bet the head of my son.

  “You know my first reaction? This is in really bad taste. I thought they were playing a joke on me. Seriously. But Rellen is not the type, right?… None of them.… They’re not as serious as they look, but they wouldn’t do that. I think they’re not confident enough to play a joke like that on me. So I didn’t say anything. And it was only afterwards, I talked to Albrecht, I realized.… I was floating on air for weeks.”

  And he laughed. To this day, Francis insists that this was the most exciting moment in the life of this project.

  Sometime in there, he also observed that “theory is just a sideshow, really,” and admitted that AMANDA’s moment of truth may have been more exciting for him than for the experimentalists in the collaboration, since he had never experienced the direct contact with hard reality that one does in an experiment. Some call it “touching the mystery.” Albert Einstein told friends that something in him snapped and he experienced heart palpitations for several days when he discovered that his general theory of relativity explained a previously inscrutable anomaly in Mercury’s orbit about the Sun.