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Case of the Missing Particles

How can you look inside the Sun to see how it shines?

In the mid-1960s, Ray Davis and John Bahcall thought they had a way. Drawing on advances made by other physicists earlier in the century, they intended to use notoriously elusive particles called neutrinos to verify ideas about the sun's inner workings. Theorist Bahcall calculated the number of neutrinos they expected to find, and experimentalist Davis tried to catch them. But for more than 3 decades, their results didn't jibe. In the chronology below, follow the case of the missing neutrinos which ultimately led not only to a triumph for Davis and Bahcall but also to a surprising breakthrough in particle physics. -- Susan K. Lewis

First Steps: 1920-1956

1920: Theory of Sunshine
British astrophysicist Sir Arthur Eddington proposes that the Sun generates heat and light by "burning" hydrogen into helium. According to Eddington, every time that 4 hydrogen atoms fuse to become a single atom of helium at the Sun's core, a tiny bit of mass is converted into energy just as Einstein indicated was possible in his famous equation E = mc2.
1930: Neutrino "invented"
Austrian physicist Wolfgang Pauli conjures up the notion of a novel subatomic particle to solve a puzzle about the apparent non-Conservation of Energy in radioactive beta decays. A few years later, Italian physicist Enrico Fermi dubs the particle -- which has no electrical charge -- the neutrino or "little neutral one". But there is no conclusive evidence that the particle exists. And most scientists think it may be impossible to ever detect.

Wolfgang Pauli lecturing in 1929. The next year when he devised the notion of the neutrino, he allegedly said to a friend, "I have done something very bad today by proposing a particle that cannot be detected. It is something that no theorist should ever do."

1939: Theory of Sunshine refined
In his landmark paper "Energy Production in Stars", Hans Bethe lays out details of how hydrogen is fused into helium in stars like the Sun. His work leads to an understanding that the fusion process releases not only energy but also the particles that Pauli "invented". Each time 4 hydrogen nuclei change into a helium nucleus, 2 neutrinos are emitted.
1956: Neutrino detected
In an endeavor dubbed "Project Poltergeist" conducted at the Savannah River nuclear reactor, Frederick Reines and Clyde Cowan prove that the neutrino actually exists.

Grand Experiment: 1964-1968

1964: Davis and Bahcall launch test
Ray Davis and John Bahcall propose that a study of neutrinos emitted from the Sun can show that nuclear fusion -- the "burning" of hydrogen nuclei to helium nuclei -- is indeed the source of the Sun's energy.
1964: Bahcall predicts number of neutrinos
John Bahcall creates the first detailed mathematical model of fusion reactions in the Sun's interior. As Bahcall later notes, he has to take account of "a smorgasbord of nuclear reactions at energies where measurements are difficult". He draws upon Hans Bethe's work, including Bethe's estimate of the Sun's core temperature. There are countless pitfalls in devising the model. Just a 1 percent error in the temperature figure alone means a 30 percent error in the predicted number of neutrinos. And the projected number is astounding: about a hundred billion solar neutrinos pass through your thumbnail every second according to Bahcall's model.
1965-1966: Davis builds experiment
Deep in the Homestake gold mine in Lead, South Dakota and sheltered from confusing background radiation, Ray Davis oversees construction of a giant neutrino trap: a tank of cleaning fluid roughly as big as an Olympic-size swimming pool. The cleaning fluid is mostly chlorine, which occasionally turns into a radioactive isotope of argon when struck by solar neutrinos. Bahcall has calculated that roughly 10 atoms of argon will be produced each week. And Davis is confident he can extract and measure them.
1968: Davis's initial results
The much-touted experiment appears a failure. Davis announces that he has detected only about ⅓ as many radioactive argon atoms as Bahcall predicted. Scientists call the discrepancy "The Solar Neutrino Problem". The Press calls it "The Mystery of the Missing Neutrinos".

When this photo inside the Homestake Mine tank was taken in 1966, Davis's mammoth neutrino trap was about half-built. From the finished tank holding 100,000 gallons of cleaning fluid, Davis hoped to isolate 10 atoms of argon each week.

Decades of Doubt: 1965-1985

In the 2 decades following their disappointing results, Davis fine-tunes his solar neutrino detector and Bahcall refines and checks his calculations. Hundreds of other physicists, chemists, and astronomers also examine Bahcall and Davis's work. No one can find significant fault with either the apparatus or the calculations. Yet along the way, there are hints of a solution to the problem:

1969: A possible explanation
Working in the Soviet Union , Physicists Vladimir Gribov and Bruno Pontecorvo suggest that Davis and Bahcall's missing neutrinos can be explained by "neutrino oscillations". Perhaps as they travel to Earth, some of the neutrinos made inside the Sun oscillate -- or change -- into types of neutrinos that Davis's apparatus can't detect. It's been known since mid-century that different types of neutrinos exist.
But few physicists take stock in Gribov and Pontecorvo's idea. According to the Standard Model (the cornerstone of modern particle physics), neutrino types are distinct and can never change one into another.
1978 and 1985: Pursuing a bold notion
Building on Gribov and Pontecorvo's radical solution, Lincoln Wolfenstein in 1978 and Stanislav Mikheyev and Alexei Smirnov in 1985 show how electron neutrinos created at the Sun's core might switch quantum states as they interact with other matter in the Sun and travel outward to the surface.
1985: More missing particles
In an experiment called "Kamiokande" sited in the Kamioka Mozumi mine in Japan, Masatoshi Koshiba and colleagues detect far fewer atmospheric neutrinos -- neutrinos produced by the collision of cosmic rays with Earth's atmosphere -- than they expect to see. While atmospheric neutrinos are a different type from those produced by the Sun, the so-called "atmospheric neutrino anomaly" is similar to the solar neutrino problem. Where are the missing neutrinos?

More than 4,800 feet underground in the Homestake Mine, Ray Davis and John Bahcall pose by the tank.

Mystery Solved: 1998-2002

1998: Answer to riddle of atmospheric neutrinos
A scaled-up version of Kamiokande called Super-Kamiokande reports on more than 500 days of data collecting. The detector is so big that it can tell what direction atmospheric neutrinos are coming from. And it has picked up far fewer neutrinos traveling from the other side of the Earth than from the sky directly above Japan.
There is evidence that many of the atmospheric neutrinos from the other side of the Earth have changed into a different type of neutrino during their journey. This confirmation of neutrino oscillation carries a profound implication: the Standard Model of particle physics must be modified.
2001-2002: Proof of solar neutrino oscillation
The Sudbury Neutrino Observatory (SNO) -- the first neutrino detector that can pick up all 3 known types of neutrinos -- resolves conclusively that in the case of the missing solar neutrinos, the neutrinos are not, in fact, missing.
SNO finds that the total number of neutrinos from the Sun is remarkably close to what John Bahcall predicted 3 decades earlier. Ray Davis's experimental work is vindicated as well because SNO finds that only about ⅓ of the solar neutrinos that reach Earth are still in the same state that Davis could measure. Roughly ⅔change type -- or oscillate -- during the journey.

Ray Davis continued working on the solar neutrino experiment until well into his 80s. Here, a 1999 portrait of Dr. Davis at the age of 85.

2002: Nobel Prize recognizes achievement
The Nobel Prize in Physics is awarded to Ray Davis and Masatoshi Koshiba, a leader of the Kamiokande group. The Nobel citation praises them "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos."
The award is also a tribute to their colleagues and the many dedicated scientists whose work led to a fundamental shift in particle physics.

Ray Davis's wife Anna, 5 grown children, and 11 grandchildren were with him in Stockholm as he accepted the Nobel Prize.

The Ghost Particle

(Program Transcript)
[Narrator]: Right now, something very strange is happening to you: a swarm of ghosts is flowing straight through your body.
[Boris Kayser] (Fermi National Accelerator Laboratory): There's something like a hundred trillion of them streaming through each of us, every second... every second a hundred trillion.
[Henry Sobel] (University of California, Irvine): Most of them pass through without doing anything, so this was ghostly or poltergeist-like.
[Narrator]: The ghosts are "neutrinos" -- tiny particles that fill the Universe. Without them, the stars would not shine and the Earth would be a dead and frozen world. They make existence possible and contain the secret of our past.
[Boris Kayser]: Not only have they solved several mysteries, but they are also our parents.
[Narrator]: Capturing these ghosts is one of the greatest challenges scientists have ever faced.
[John Bahcall] (Institute for Advanced Study): When you think about it, it's almost unbelievable what we were doing. And I'm glad we didn't think about it too carefully when we were doing it.
[Narrator]: A scientific detective story on the trail of the elusive ghost particle...
[a child]: They're here!
[Narrator]:: ...right now on NOVA.

Google is proud to support NOVA in the search for knowledge: Google.

[Narrator]: What is the World made of? It's the most ancient scientific question. Today, scientists believe they have discovered the recipe for matter. It's called the "Standard Model". And it says that everything is made from just 12 basic ingredients, 12 fundamental particles. But the Standard Model fails to explain where all these particles came from in the first place. And so for years, scientists have searched for a clue that might explain the great mystery of the origin of matter. Today, they believe they may have found that clue. And it's thanks to two of America's greatest scientists.
For decades, Ray Davis and John Bahcall struggled to convince their colleagues that they had uncovered a basic flaw in the understanding of matter. It all began 40 years ago with a daring underground experiment. Ray Davis had tunneled deep into the Earth to build a trap for the most elusive thing in the Universe. It was an experiment which few thought could ever succeed.
[Boris Kayser]: He set out to do something which sounds totally impossible.
[Narrator]: And it produced a result which no one believed.
[Andrew Davis] (University of Chicago/Ray Davis's Son): Well, no, there's got to be something wrong with that experiment. That can't be right.
[Narrator]: Everyone was convinced that the 2 scientists had made an embarrassing mistake.
[John Bahcall]: It was a personal shock, a very painful one. We learned from that, but it was a painful shock.
[Narrator]: But Davis and Bahcall refused to give up. Today, the experiment which no one believed has led to an astonishing discovery which is causing scientists to re-think their fundamental theory of what the Universe is made of and where it all came from. And at the heart of the story lies the thing which Davis and Bahcall hunted for over 40 years -- a tiny particle called the neutrino. It is one of the 12 fundamental building blocks of matter. And yet from the day it was born, the neutrino has been an enigma.
That day was the 4th of December, 1930. The great Austrian physicist Wolfgang Pauli was getting ready for a party. But he found the time to write a letter -- one of the most famous in the history of Science.
It was addressed to colleagues attending a conference on a subject that was causing great puzzlement among physicists -- the phenomenon of radioactive decay.
[Wolfgang Pauli]: Dear radioactive ladies and gentlemen -- Unfortunately I am unable to come to Tübingen personally since I am indispensable here because of a ball to be held in Zurich.
[Narrator]: In the early decades of the 20th Century, physics had taken the first steps in understanding what the Universe was made of. Matter, they knew, consisted of atoms. But they had found that atoms were made of still tinier particles. Protons, with a positive electrical charge clustered in the atom's nucleus, which was surrounded by a cloud of negatively charged electrons. Protons and electrons appeared to be the two ultimate building blocks of all matter. But there was something strange about the way these particles behaved.
[Boris Kayser]: Pauli had to deal with a very, very puzzling situation. On the level of atomic nuclei and particles smaller than that, many things don't live forever. They disintegrate or they "decay", as we say.
[Narrator]: It was almost as if some atomic nuclei had too much energy, making them unstable. They would suddenly spit out a particle -- often an electron -- leaving behind a new nucleus with less energy. That was strange enough. But what was even odder was what was happening to the energy.
[Boris Kayser]: There is a very, very well-established principle in physics called the principle of Conservation of Energy. It says that you don't get more energy than you had before, and you don't have less energy. You don't lose energy that you had before -- energy does not disappear.
[Narrator]: But that's just what seemed to be happening. The energy the nucleus lost when it decayed should all have been taken up by the electron. There was nowhere else for it to go. But it seemed the electron did not carry away as much energy as it should.
[Boris Kayser]: In fact, what they saw was that in different decays -- always with the same original nucleus, always with the same final one -- the electron had differing amounts of energy, typically not all the energy that was released. Energy was somehow disappearing.
[Narrator]: But disappearing energy was simply not acceptable to Pauli. The energy lost by the nucleus had to be going somewhere. It was time to be bold.
[Wolfgang Pauli]: I have had an idea for a desperate remedy in order to save the validity of the energy law.
[Narrator]: Pauli's idea was that there had to be a third particle involved in radioactive decay -- a new kind of particle which no one had ever seen but which was carrying away the missing energy.
[Boris Kayser]: He proposed in addition to the little particles that were known at that time, there was another one that would be emitted in radioactive decay along with the electron. Pauli suggested that this new particle was very elusive, hard to detect, and this is why people have never seen it. But the particle would take up whatever energy the electron didn't, thus resurrecting and saving the principle of Conservation of Energy.
He did it, I'm sure, with great hesitation. But he did it. It was a very bold move.
[Narrator]: But there was a problem. Pauli's hypothetical particle -- dubbed the neutrino, "the little neutral one" -- had no electric charge. So it would not feel the electrical forces of attraction and repulsion which are what make solid matter solid. To a neutrino, solid objects would seem like empty space. It would pass through them without causing a ripple. And that went for scientific instruments, too.
Neutrinos were the "ghosts" of the particle world. Even if they really existed, scientists saw no way to detect them.
[Jonas Schultz] (University of California, Irvine): They concluded that it was a practical impossibility -- that no one would ever see these neutrinos. And I think that's what put people off for many years from even trying.
[Narrator]: But according to Pauli, neutrinos were produced when atomic nuclei decayed. And eventually physicists discovered how to make atoms decay at will.
[Janet Conrad] (Columbia University): What happens when a nuclear bomb goes off is that there's a chain reaction of decays. So many decays happen all at once. And if Pauli was right and neutrinos are produced in each decay, then an intense pulse of these neutrinos should come out when the bomb goes off.
[Narrator]: Fred Reines was a young researcher working on America's nuclear deterrent. But he really wanted to do fundamental physics. And then he realized that the atomic weapons program was the perfect place to hunt the elusive neutrino.
[Jonas Schultz] He sat in an office for a long time, staring at a blank pad, trying to think of an idea. And he hit on the idea of looking for the neutrino. For him, it was intolerable that the neutrino could exist and not be seen. And he had to resolve that problem.
[Narrator]: Reines realized that nuclear bombs were not a very practical source of neutrinos. But nuclear reactors also harness radioactive decay to make power or fuel.
[Henry Sobel]: In a reactor, you get elements being produced. And when they decay, they give off neutrinos. So you get lots and lots of neutrinos. It's an enormous number -- 10 with 13 zeros after it (i.e., 1013) -- per second, going through every little square centimeter of your detector nearby the reactor.
[Narrator]: With such an intense source of neutrinos, perhaps Reines and his colleagues could finally detect the "ghost particle". They christened their enterprise "Project Poltergeist".
The name was apt. Without an electric charge, the neutrino was invisible to all scientific instruments. So how to detect it?
Reines realized that just as radioactive decay produced neutrinos, so neutrinos could sometimes produce radioactive decay. If a neutrino collided with a nucleus, there was a very slight chance that it might destabilize it and cause it to decay.
In Reines' experiment, the sign would be a distinctive double pulse of energy -- a signal that would enable him to detect the invisible particle by proxy.
[Henry Sobel]: There was a particular signature of this detection. You saw a pulse … and then you saw another pulse afterwards within a certain specified period of time. And that very characteristic signature enabled you to pull it out from the background.
[Narrator]: It was a matter of watching an oscilloscope, waiting for that double pulse.
On June 14, 1956, Reines and his colleagues announced that they had detected -- for the first time -- the particle which Pauli had theorized 26 years earlier!
[Boris Kayser]: They sent Pauli a telegram informing him of this discovery. And Pauli was very, very happy, saying something like, "All things come to him who knows how to wait."
[Janet Conrad]: So Pauli was right. And what he had discovered wasn't actually just an esoteric bit of information. It turns out that this new particle is absolutely crucial to the way the Universe works because the process which ignites stars involves the neutrino.
[Narrator]: Deep inside every star, scientists knew there must be a source of energy. They suspected that that source was nuclear fusion, a process in which small atomic nuclei fused together to form bigger ones. Just as neutrinos were produced in nuclear decay, so they were also emitted in nuclear fusion. And it was this idea that made neutrino hunters out of Ray Davis and John Bahcall.
40 years ago, physicist Ray Davis was already a renowned designer of experiments, a man who got the evidence scientists needed to test their theories. Theorist John Bahcall was just beginning his career. He was drawn to astrophysics -- the science of stars and galaxies. What brought them together was a shared desire to understand what made stars shine. And they believed that neutrinos would allow them to do this.
[John Bahcall]: For me and Ray, it was a great challenge to see if we could look inside of a star in the same way that your doctor can look inside your body with ultrasound or with X-rays. We wanted to do the same thing with neutrinos: use neutrinos; look right inside the Sun; and really see what the nuclear reactions are doing in the very interior.
[Narrator]: Deep inside the Sun's core, the crushing pressure forced Hydrogen nuclei to fuse together to form Helium and heavier elements, in the process releasing the energy that fuelled the Sun. Or at least that was the theory. Scientists had no direct evidence that this was happening.
[Boris Kayser]: Now, nuclear fusion would produce not only energy making the Sun shine, but also neutrinos. And lots of them. By looking at the surface of the Sun, you don't learn the details of what's going on deep inside. But by looking at the neutrinos from the Sun, you can.
[Narrator]: Lacking electric charge, neutrinos traveled from the Sun's core, unhindered, all the way to the Earth. They were "cosmic messengers". Find them and you would have proof that nuclear fusion really was the source of the Sun's energy.
So Davis asked Bahcall to work out exactly how many neutrinos the Sun made. It meant creating the first detailed mathematical model of the fusion reactions inside the core. It produced an astonishing result.
[Narrator]: We believed that the Sun should be emitting a huge number of neutrinos all the time. Every second through my thumbnail and your thumbnail, about a hundred billion of these solar neutrinos would be passing through. Every second, a hundred billion solar neutrinos through your thumbnail every second of every day of every year of your life. And you never notice it.
[Ray Davis]: What can you do?
[Narrator]: For Ray Davis, the challenge was clear. Confirm that John's fusion model of the Sun correctly predicted the number of solar neutrinos. In 1965, Ray Davis embarked on one of the most difficult experiments in the history of Science: to count the neutrinos coming from the Sun.
It meant building a laboratory deep underground -- in a goldmine in South Dakota -- to shelter it from confusing background radiation from space. The heart of the experiment was Ray's neutrino trap: 600 tons of cleaning fluid, a liquid full of Chlorine atoms. It was the ability of neutrinos to occasionally provoke the decay of one of these Chlorine nuclei that was the key.
[Boris Kayser]: When a neutrino strikes a Chlorine atom and does anything at all, it will convert the Chlorine into Argon. And this particular form of Argon will be radioactive. Ray Davis thought you could use the radioactivity of the Argon atoms to give themselves away.
[Narrator]: The idea was the more neutrinos flowed through the tank, the more Argon atoms they would make. So by counting the Argon atoms, Ray would be indirectly counting the neutrinos. But it was here that the immense difficulty of the experiment became apparent.
Trillions of neutrinos went through the tank every second. But they interacted so rarely that John Bahcall calculated just 10 Argon atoms would be made each week. Finding them seemed a ludicrously impossible task.
[Boris Kayser]: Davis was claiming that he could take a tank consisting of 350 zillion atoms of Chlorine and other stuff and extract from it only 10 Argon atoms. It's worse than a "needle in a haystack".
[Narrator]: Nevertheless, every few weeks Ray would bubble Helium through the cleaning fluid to sweep out the Argon atoms that had accumulated. He then brought them back to his New York laboratory to be counted.
[Anna Davis] (Ray Davis's wife): I used to joke that he traveled all the way across the Country with a little tube full of nothing. Which was not strictly true, of course. It turned out to be a very important piece of nothing.
[Narrator]: But as the first results began to come through, it was immediately clear that something was wrong. John had expected 10 Argon atoms per week. But Ray only counted 3. Most of the neutrinos were missing.
[John Bahcall]: Right from the beginning, it was apparent that Ray was measuring fewer neutrino events than I had predicted. Only about a third. And that was a very serious problem.
[Andrew Davis]: My father and I would always talk whenever I'd come home. And I mean it was certainly very perplexing that the number was low.
[Narrator]: It looked like Ray's daring experiment simply wasn't working.
[Andrew Davis]: I remember even people coming and saying, "Well, you know there's got to be something wrong with that experiment. That can't be right."
[Narrator]: The skepticism was understandable.
[Boris Kayser]: He set out to do something which sounds totally impossible. If you have a shot of the size of the tank of cleaning fluid that he used … and then you mention how many atoms are in one of those tanks and the fact that he extracts 10 or 3-or-4 and he counts them correctly. Oh, yeah? Give me a break!
[Narrator]: Most of Ray's colleagues were convinced that the missing neutrinos revealed not a problem with their theories, but a problem with his experiment.
[William Fowler] (1969): We think if Ray improves the sensitivity of his equipment, he'll find the neutrinos all right.
[Narrator]: And, in every other respect, physicists' theories of matter had indeed made great progress. By the 1970s, they had finally figured out the basic recipe of the Universe: the Standard Model of particle physics. It seemed that everything was made from just 4 basic ingredients, 4 fundamental particles -- one of which was the neutrino. But the Standard Model also said that each of these particles came in 3 different "flavors". So, in fact, there were 3 different kinds of neutrino.
{magician}: I actually have a neutrino... there you go. The problem is that once you look at one very carefully. Sometimes it behaves as if there are two. The 3 neutrinos... Can you open your hand again?
[Narrator]: According to the Standard Model, the 3 neutrinos had bizarre properties. Not only did they have no electric charge, but they also had no mass either which meant they would flit invisibly through the Universe at the speed-of-light.
As a recipe for matter, the Standard Model was a tremendous achievement. No matter what experiment scientists performed, the Standard Model correctly predicted the result … except, that is, for Ray's missing neutrinos.
{magician}: They're very slippery … very difficult to detect.
[Narrator]: All through the '80s, Ray continued to improve his detector. And year-after-year, the results were the same. He could only find ⅓of the neutrinos John Bahcall had predicted. He became sure there was nothing wrong with the experiment.
[Ray Davis] (1976): We have lived with it a long time and thought of all possible tests. We feel that our result is valid. And we realize it's -- as John Bahcall calls it -- "a socially unacceptable result."
[Narrator]: Inevitably, the focus of scientific skepticism began to shift.
[Janet Conrad]: If Ray Davis was right, then that would mean that John Bahcall must be wrong. And so he was under a lot of pressure to try to explain why it is that his theory must be right in the face of this experiment.
[John Bahcall]: Almost every theoretical physicist believes that we astrophysicists have just messed it up and it's our fault. We never understood what was happening in the center of the Sun no matter how much we pretended to do so.
[Janet Conrad]: I saw John stand up to give a talk. He began and started his very well-reasoned arguments. And it wasn't 5 minutes into the talk before somebody stood up and started arguing with him. I was very impressed at how well John handled this, even when this person in the audience suddenly broke into Hebrew and John was arguing back in Hebrew.
[Narrator]: Despite the barrage of criticism, John Bahcall was confident there was nothing wrong with his model. He continued to insist that the Sun must be producing far more neutrinos than Ray was detecting.
[John Bahcall]: And it didn't matter how convinced I was that they were wrong. Every year for 30 years, I had to demonstrate scientifically that yes, the expectation from the Sun was robust and therefore, you should take the discrepancy seriously.
[Narrator]: It became more and more puzzling. Nobody could see what was wrong with John's theory or find fault with Ray's experiment.
[John Bahcall]: This is the one I love -- this is you swimming.
[Narrator]: But at least they finally had everyone's attention. Their neutrino anomaly had become the biggest mystery in particle physics.
[Patrick Moore] ("The Sky at Night", BBC, 1983): Everything indicates that this apparatus is accurate and can tell us how many neutrinos are coming from the Sun. But observation and theory don't agree. They're simply aren't enough neutrinos. And that's causing a great many raised eyebrows.
[John Bahcall]: I think we need a new experiment in order to decide who is right and who is wrong.
[Narrator]: In Kamioka, Japan, there was another experiment. But it wasn't to study the Sun. It wasn't even designed to look at neutrinos. And at first, it only seemed to deepen the mystery.
In 1983, the Japanese started looking for a rare kind of nuclear decay. They had built an experiment called "Kamiokande" deep inside a mountain to shield it from radiation from space. But there was one thing the mountain couldn't shield them from: neutrinos!
[Janet Conrad]: Neutrinos can be produced by all kind of nuclear interactions. And one that is particularly interesting is one where the particles that are flying through the Universe. The high-energy particles called "cosmic rays" hit our atmosphere. And when they collide with the atmosphere, a spray of particles comes out and that includes neutrinos. And those neutrinos are called atmospheric neutrinos.
[Narrator]: These atmospheric neutrinos were a nuisance, easily confused with what they were really looking for. But then they noticed something strange about the atmospheric neutrinos. They were detecting far fewer of them than they expected.
[Kunio Inoue] (Tohoku University): They found that atmospheric neutrino is not coming as we expected. Surprisingly, we found that neutrinos coming from the atmosphere is smaller than the expectation. We called it an "atmospheric neutrino anomaly".
[Janet Conrad]: The atmospheric neutrino anomaly which the Kamiokande scientists were seeing looks in many ways similar to the solar neutrino deficit. They were looking for a certain type of neutrino, and they see a lot less than was expected. And you can't blame this one on John Bahcall, okay? This has nothing to do with the Sun. And so once this happens, the scientists start to think, "Hmm, maybe something's going on here."
[Narrator]: Physicists went back to basics. The Standard Model said there were 3 different types of neutrino. The first were electron neutrinos, the type the Sun produced and the only neutrinos Ray's experiment was designed to detect. But there were also muon neutrinos and tau neutrinos. Could this be the key to the problem?
[Dave Wark] (Imperial College London): Very suggestive, of course, that Ray's experiment sees ⅓ of what John thought it should. There are 3 flavors of neutrinos. So it's not a great leap of the imagination that those 2 numbers might be connected.
[Narrator]: Still, the connection was not obvious. True, Davis could only detect 1 of the 3 neutrino flavors. That was the only flavor the Sun could create. But that was not all there was to it.
[Dave Wark]: There was a theoretical proposal that neutrinos might change from one type of flavor into another type of flavor. This is called neutrino oscillations. You emit the neutrino as one particular flavor. But later on when you detect it, it might be another.
[Narrator]: In this theory, neutrinos would be continuously changing from type-to-type as they traveled through space. What started as an electron neutrino would later look like a muon neutrino … still later a tau neutrino … and then an electron neutrino again.
[Dave Wark]: And in fact, it can change back-and-forth and back-and-forth and back-and-forth. That's why it's called a neutrino oscillation. And this is sort of like a pendulum.
[Narrator]: Was this why Ray saw only a third of the neutrinos that John predicted the Sun was making? In the time it took them to travel from the Sun's core to the Earth, had electron neutrinos oscillated into muon and tau neutrinos which his experiment couldn't detect? That would explain everything. But there was just one problem.
[Dave Wark]: The problem with that is that in the Standard Model, neutrinos are mass-less. And mass-less neutrinos can't do this. They can't change from one type of neutrino to the other.
[Narrator]: It all had to do with time. For anything to change, time must pass. But the Standard Model said the neutrino was a mass-less particle traveling at the speed-of-light. And according to Einstein, if you are traveling at the speed-of-light, time stands still so nothing can change.
[Boris Kayser]: When a particle moves fast, its clocks -- its internal timing mechanism -- slows down. And as it approaches the speed-of-light, the clock slows down until it's not moving at all. A particle which is mass-less is moving at the speed-of-light, so it has no sense of what time it is.
[Narrator]: Without mass, a neutrino would be frozen in time traveling at the speed-of-light, but unable to change.
[Boris Kayser]: Neutrino oscillation is a time-dependent phenomenon. It requires a neutrino clock that requires that the neutrino travels slower than light and that the neutrino have a mass.
[Dave Wark]: And so as an explanation for the Davis experiment, it's not very attractive because if you don't believe neutrinos have mass, then they can't oscillate. Then, you know, whether there's a factor of 3 here or 3 there, it doesn't matter. It can't be the explanation.
[Narrator]: But then scientists made a discovery that completely transformed all their ideas about the neutrino. Back in Japan, they had completed a vastly scaled-up version of the Kamiokande experiment called "Super-Kamiokande".
[Masatoshi Koshiba] (University of Tokyo): This is really marvelous opportunity. So I decided that we go ahead, we change the detector, improve it, to make our detector really capable of new type of neutrino observation.
[Narrator]: Super-Kamiokande was truly colossal: a 40-meter high tank holding 50,000 tons of ultra-pure water, surrounded by 11,000 photo-multiplier tubes. Super-Kamiokande could still only detect 2 of the 3 neutrino flavors. But because it was so big, the scientists could tell what direction the neutrinos were coming from.
[Henry Sobel]: A neutrino comes into your detector and produces a charged particle. The direction that the charged particle goes pretty much matches the initial direction of the neutrino. So by reconstructing the track of the charged particle, you can tell where the neutrino came from. You can make a plot, and you could say, "How many neutrinos do I have coming from there? From there? From there? And from there?" You make a plot on the sky.
[Narrator]: And when they plotted where the neutrinos were coming from, the Kamioka team made an astonishing discovery.
[Janet Conrad]: Neutrinos are produced in the atmosphere above you. Neutrinos are also produced in the atmosphere on the other side of the Earth "below" you. And those neutrinos can travel through the 13,000 kilometers up to your detector. And so the Super-Kamiokande experimenters expected to see neutrinos coming in from above and neutrinos coming in from below in equal numbers.
[Narrator]: But that's not what they found.
[Kunio Inoue]: Neutrino flux coming from above and coming from below should be the same. But what we have observed was that neutrinos coming from below is about half of that coming from above.
[Henry Sobel]: The number of neutrinos that are coming down and going through a small distance in getting to us is about what you'd expect. But the number of neutrinos that are coming up though the Earth -- which are going through tens of thousands of kilometers -- are fewer of them than you'd expect.
[Narrator]: For the charge-less neutrino, the solid rock of the Earth is just empty space. So the only difference between the atmospheric neutrinos coming from above and those from below was the time they had been traveling before they reached the detector. Which meant that contrary to all theory, neutrinos must have a sense of time.
[Dave Wark]: Just that fact that the neutrinos coming down from above still get here but the neutrinos coming up from below don't, tells you that neutrinos have mass because they tell you a neutrino knows how far it's gone. And the only way it can know how far it's gone is if its clock isn't stopped. Which means it can't be traveling at the speed-of-light. Which means it must have a mass.
[Narrator]: It was a bombshell. The Standard Model had gotten neutrinos completely wrong. They did have mass and they could change flavor. So had the missing solar neutrinos been there all along, just changed into the 2 flavors Ray's experiment could not detect?
There was only one way to know for sure. All eyes turned to a nickel mine in Sudbury, Ontario. Here, 2 kilometers below ground, a team of British, Canadian, and American scientists were building a new kind of solar neutrino detector -- one sensitive to all 3 flavors.
It was the deepest such experiment ever built. And it also had to be the cleanest because the confusing background radiation came not just from space but from the rocks themselves.
[Dave Wark]: If we got an amount of dust like that into our detector, it would just ruin it. It would destroy its sensitivity to the neutrinos by blocking them out with other signals. And so we have to build the detector with fantastic levels of cleanliness. We just have to get rid of all of this stuff.
[Narrator]: All these precautions made the Sudbury Neutrino Observatory -- "SNO" for short -- one of the least radioactive places in the Universe.
[Dave Wark]: When the SNO detector was finished, the exact center of the SNO detector has the lowest level of radiation of any point in the Solar System.
[Narrator]: After 9 year's construction, SNO started taking data in November, 1999, looking for proof that neutrinos could change flavor.
[Steve Biller] (Oxford University): When I joined the experiment, I was betting there was no neutrino oscillations. It just seemed too bizarre.
[Narrator]: At the heart of the detector was an acrylic sphere containing 1,000 tons of "heavy water" -- a substance which neutrinos could interact with in 2 distinct ways. One reaction -- like Ray's tank of cleaning fluid -- was sensitive only to electron neutrinos. This would be a direct check on the accuracy of Ray's experiment.
[Dave Wark]: But there's a different reaction which doesn't care what kind of neutrino it is. So it allows you to see all of the neutrinos. Now, measuring that reaction allows you to check John directly. If you see the number of neutrinos that John predicts, then he really does know how the Sun works.
[Narrator]: For 40 years, John Bahcall's predictions of the number of neutrinos coming from the Sun had flown in the face of his friend's experiment. Was that now at last about to change?
Over 19 months, 10 billion trillion neutrinos passed silently through the SNO detector. Just 2,000 of them reacted with the heavy water.
[John Bahcall]: Almost every theoretical physicist believes the astrophysicists have just messed it up.
[Andrew Davis]: There's no other really likely explanation than that one of those 2 guys was wrong.
[Patrick Moore]: There simply aren't enough neutrinos. And that's causing a great many raised eyebrows.
[Ray Davis] (1976): ..."socially unacceptable result."
[Narrator]: In June, 2001, the SNO team announced their estimate of the total neutrino flux from the Sun, taking -- for the first time -- all 3 flavors of neutrino into account.
[Steve Biller]: It was almost too good to be true. The Sun works as we expect, which is good. And there is this funny business that neutrinos coming from the Sun that arrive at the Earth are not all electron neutrinos -- that they have somehow changed in their nature.
[Narrator]: For decades the question had been, "Who was right -- Ray or John?" The answer was, "They were both right. It was the Standard Model that was wrong."
[John Bahcall]: I was called right after the announcement was made by someone from the New York Times and asked how I felt. And without thinking, I said, "I feel like dancing, I'm so happy!" And the one thing that my kids kept sending each other e-mails about all week was, "Did you see where it said in the New York Times that Dad felt like dancing?" They kept making fun of me about that, but I was deliriously happy.
It was like for 3 decades, people had been pointing at this guy and saying, "This is the guy that wrongly calculated the flux of neutrinos from the Sun". And suddenly that wasn't so. And it was like a person who had been sentenced for some heinous crime, and then a DNA test is made, and it's found that he isn't guilty. And that's exactly the way I felt.
[Narrator]: The 30-year struggle to resolve the problem of the missing neutrinos has revealed weird properties that defy the predictions of the Standard Model. They hint at a new theory of matter -- one even more profound.
[Janet Conrad]: Scientists have searched for so long -- my whole scientific career -- to find a problem with the Standard Model. It has been very resilient. And that is why it is so exciting to suddenly come up with this new information that neutrinos have mass, because that doesn't fit within our theory. And so it's like opening a door. And of course when you open a door, behind that you find a lot more doors.
[Narrator]: Today, neutrino physicists like Janet Conrad are building a new generation of neutrino experiments.
[Janet Conrad]: I strongly suspect that the neutrino has more up its sleeve than we have observed so far.
[Narrator]: Do neutrinos have other strange properties? Could there be more than 3 flavors of neutrino? Could neutrinos even be the answer to the greatest riddle in physics: "Where did all the matter in the Universe originally come from?"
The Standard Model says that when the pure energy of the Big Bang condensed into the stuff of the Universe. For every particle of matter produced, there would have been a corresponding anti-particle of anti-matter.
[Steve Biller]: The problem with that sort of universe is we wouldn't exist because matter and anti-matter would annihilate each other and there'd be nothing left. And so the reason that we exist is, in fact, because there is an imbalance. That there is much more matter than anti-matter in the Universe. And although we know this is a fact, we don't fully understand why this is.
[Janet Conrad]: It may be that the neutrinos -- which have provided the first chink in the Standard Model -- can provide us the clue to this too. Because when I describe a particle and an anti-particle, I'm describing them by saying that they have opposite charge. So, for example, the electron -- which has negative electric charge -- has an anti-particle partner that has positive electric charge. It's called the positron.
But the neutrino is actually very special. It is the only set of particles in the Standard Model that don't carry any charge. And so they can escape the definition. They can be their own anti-matter particle. A neutrino and an anti-neutrino might actually be different aspects of the same beast.
[Dave Wark]: In the Big Bang, we would have made huge numbers of neutrinos. And if neutrinos have mass, it is possible that the matter in the Universe today arose because of the decay of massive neutrinos created in the early Universe. So we may be the grandchildren of neutrinos. All the matter that makes us up may have arisen purely through the decay of neutrinos.
[Steve Biller]: So in a bizarre way, it may be that neutrinos tell us why we exist.
[Narrator]: Their hunt for the most elusive thing in the Universe may have brought scientists close to uncovering the origin of everything around us. And it all began 40 years ago with Ray Davis' pioneering underground experiment.
[Steve Biller]: Ray Davis is a hero to everybody in this field. This was really the first time somebody really seriously tried to measure such an impossible thing coming from the Sun.
[Boris Kayser]: Ray Davis's persistence in the face of seemingly wrong experimental results, contradictions between not only his experiment and theory but also his experiment and other experiments... He stuck to his guns. And he was right.
[Narrator]: Ray continued to work on his experiment well into his '80s until he was forced to stop by the onset of Alzheimer's disease. Then one day in October, 2002, Anna Davis got an early morning phone call.
[Anna Davis]: My nephew -- who works for Minnesota Public Radio -- called us at 6:00 in the morning and said, "Congratulations." And I said, "For what?" He said, "Don't you know? Ray got a Nobel Prize in Physics."
We gathered all of our 5 children, their spouses and our 11 grandchildren, flew them all over to Stockholm. And everybody had a wonderful time for 8 days.
[Andrew Davis]: The entourage of Davises was 23 people including the Nobel Prize winner himself. So it was … it was really special.
[Narrator]: Ray Davis shared the Nobel Prize with the scientist behind the Kamioka experiments -- Masatoshi Koshiba.
[Masatoshi Koshiba]: I was happy. I was happy … that's all.
[Narrator]: The awards were a tribute to all the scientists whose work over 40 years had gradually uncovered the true nature of the neutrino.
[Dave Wark]: In the end, to have put that much of your life into something and have it work -- and not just work but to work so beautifully! -- that is just the most tremendous feeling a scientist can have.
Then it just hits you what you've done -- that you've actually learned something about the Universe that nobody ever knew before. And now you get to tell them.
[Boris Kayser]: We are descended from neutrinos, yeah? We are descended from neutrinos. What a kick! It's true, we think.

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