By Don Lincoln, Fermilab
In the 1950s, an Italian physicist by the name of Bruno Pontecorvo had been toying with an idea that could possibly explain atmospheric neutrino prediction. The idea was what we now call neutrino oscillations, which is the startling idea that neutrinos could change their identity. This means that you could have a beam consisting solely of, for example, muon neutrinos.
Neutrino Oscillation
The muon neutrinos would morph over time into a mixture of electron, muon, and tau type neutrinos. At a certain moment in time, the beam could be (at least in principle) all of the electron types, in spite of starting out as the muon type. And there is nothing all that special about muon neutrinos oscillating. A beam of pure electron neutrinos could oscillate into tau neutrinos, or muon neutrinos.
Pontecorvo’s idea was startling. After all, other subatomic particles don’t change their identity. An electron stays an electron. But maybe that’s not true for neutrinos. Nobody knew if this neutrino oscillation idea was real until 1998.
The SuperK Experiment
The experiment that filled in the gaps was the Super Kamiokande experiment in Japan. Super Kamiokande, or SuperK as scientists call it, is a cylinder about 130 feet high and 130 feet wide, located a little over half a mile underground. It contains 50,000 tons of super pure water. The entire tank is surrounded by thousands of things called photomultipliers, which are essentially ultrafast eyeballs, which can detect brief blinks of light.
In SuperK, atmospheric neutrinos pass through the detector. Most of them do nothing, but a small number smash into a water molecule and turn either into an electron or a muon, depending on what kind of neutrino they are.
These electrons and muons then make a blink of light in the detector, which the photomultipliers can see. Electrons and muons make different kinds of blinks, so the neutrino types can be identified. And, finally, depending on which of the photomultipliers detect light, you can figure out the direction the neutrino was going when it hit the detector.
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How the Atmosphere Is Spread
What SuperK did that was very clever was to remember that the atmosphere is spread over the surface of the Earth. Some atmospheric neutrinos are made about 12 miles directly above the detector, while others are made on the opposite side of the Earth, about 8,000 miles away.
That means that the neutrinos made directly above get to the SuperK detector very quickly, while the ones from the other side of the Earth take much longer. And, with that much longer time, the neutrinos made in the atmosphere on the other side of the Earth have more time to oscillate before they hit the SuperK detector.
Where the Atmospheric Neutrino Come From
So, that’s what they did. Using their huge tank of ultrafast eyeballs, meaning the photomultipliers, they could identify where in the atmosphere the atmospheric neutrinos were coming from, from above, to partway around the world, to halfway around the world. And they could calculate how much time it would take for neutrinos made in the atmosphere to get to their detector.
Basically, what they expected to see was that neutrinos made directly overhead would not have much time to oscillate, while the ones from the other side of the world would have plenty time.
And that’s exactly what they saw. By simply plotting the ratio of muon to electron neutrinos as a function of where they were made, they saw that ones coming from directly overhead occurred in a ratio of about two to one, while the ones from the other side were much closer to one to one. SuperK had seen that at least atmospheric neutrinos experienced neutrino oscillation.
Solar Neutrinos and SNO
Now the story wasn’t complete, because these researchers couldn’t say anything about solar neutrinos. In 2001, another experiment, done in Sudbury, Ontario, filled in the final piece of the puzzle.
The Sudbury Neutrino Oscillation detector, or SNO, was a 20 foot diameter cylinder located a mile underground, filled full of water. It was also surrounded by photodetectors. They were also able to see the same signal that SuperK did.
The SNO measured solar neutrinos and found the expected result of about a third as many electron neutrinos hitting the detector as expected. That wasn’t exactly a big deal. After all, it was about the year 1999 and Ray Davis had seen that over 30 years before. But SNO was going to make a change that, well, changed everything.
What the SNO Found
They drained out the water from their sphere and replaced it with what is called heavy water. Heavy water is kind of like ordinary water, but with an important difference. Ordinary water is H20, meaning each water molecule contains two hydrogen atoms and one oxygen one.
In heavy water, the hydrogen is replaced with deuterium. Deuterium is what is called an isotope of hydrogen. While hydrogen consists of a single proton, deuterium consists of a proton and a neutron. That’s why it’s called heavy. The reason that the SNO experiment replaced water with heavy water is because a heavy water detector would be able to see not only electron neutrinos, but also muon and tau neutrinos.
So, the SNO experiment started with their upgraded and fancy heavy water detector, and what did they see? They saw about three times as many neutrino interactions as they had seen with ordinary water. The ordinary water detector saw only the electron neutrinos from the Sun, but the heavy water detector saw all three types of neutrino interactions.
Common Questions about SuperK and SNO
Bruno Pontecorvo’s idea said that neutrinos could change their identity. This means that you could have a beam consisting solely of one type of neutrino.
It was intended as a detector for the atmospheric neutrinos that passed through it. Some neutrinos smashed into a water molecule and, depending on their type, turned either into an electron or a muon. This would make a blink of light in the detector, with different kinds of blinks from electrons and muons. Thus the neutrino types could be identified. Also, the direction in which every neutrino was going can be figured out when it hit the detector.
The SNO measured solar neutrinos and found the expected result of about a third as many electron neutrinos hitting the detector as expected.
