The 2015 Nobel Prize in physics was recently awarded for the discovery of neutrino oscillation. What makes this strange property so intriguing?
The Swedish Academy of Sciences recently announced that the 2015 Nobel Prize in Physics would be received by Takaaki Kajita and Arthur McDonald. Specifically, the prize was awarded for “the discovery of neutrino oscillations, which shows that neutrinos have mass”. For the last few decades, these elementary particles have been at the heart of one of the most frustrating puzzles in modern physics — the “solar neutrino problem”. Experiments that detect these solar neutrinos have persistently contradicted our understanding of the structure of the Sun, and the nuclear reactions within it, until relatively recently where experiments were able to resolve this problem. These experiments were the subject of the 2015 Nobel Prize in Physics.
But what are neutrinos, and what are these interesting oscillations that they undergo?
Neutrinos are produced in the decay of certain subatomic particles, as well as by radioactive decay and nuclear reactions, such as those that power the Sun. As energy production in stars became better understood around the middle of the century, it was predicted that the Sun must be constantly emitting a huge number of neutrinos.
The solar neutrino problem
In the late 1960s, physicist Raymond Davis, Jr. devised an experiment to try to detect these elusive neutrinos. In a huge underground tank of tetrachloroethylene (a liquid compound rich in chlorine atoms, also used in dry-cleaning), Davis aimed to count the radioactive argon atoms that would be formed when a neutrino hit a chlorine nucleus.
Despite trillions of neutrinos reaching the detector every second, neutrinos interact so rarely, that it was predicted the experiment would only be able to produce around 10 argon atoms per month. Amazingly, Davis was able to capture and count these few atoms, extracted from thousands of litres of liquid. But when Davis and his colleagues re-checked both the experiment and the calculations, they found that the number of solar neutrinos detected was consistently about one-third of the prediction.
In more recent times, better models of solar physics, combined with more advanced experiments such as SAGE, still produced the same anomalous result. The missing two-thirds of the expected neutrinos became known as the solar neutrino problem — and the solution would take several decades to find.
One theory proposed to explain the discrepancy involved a transformation of these neutrinos. There are three flavours of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. Davis's experiment was only able to detect electron neutrinos, the kind emitted by the Sun.
If the neutrinos were able to change, or oscillate, from one flavour to another, this could possibly hide some of them from the detector, and explain the discrepancy. But there was a problem with this theory — it wasn’t predicted by the Standard Model of particle physics.
The Standard Model is a beautiful part of modern physics that has stood up to many decades of experimental tests with high precision. It has successfully predicted many important experimental results, including the discovery of new particles such as the Higgs Boson. But when it came to the solar neutrino problem, the Standard Model didn’t appear to have any answers.
Half of this year’s Nobel Prize was awarded to Takaaki Kajita of Japan, director of the Super-Kamiokande collaboration, for their studies of neutrinos produced by cosmic rays. The experiment helped to reveal the ability of neutrinos to transform back and forth between different flavours — a phenomenon known as neutrino oscillation.
The Super-K experiment
In 1998, Kajita presented data showing that muon neutrinos produced by cosmic rays can disappear as they travel from the atmosphere to the detector. The Super-Kamiokande detector (Super-K for short) was able to measure not only the energy of incident neutrinos, but also the direction they came from. This meant they were able to tell if they had come from the atmosphere above, or, from below — having passed through the Earth.
Comparing neutrinos created in the atmosphere above, to those created on the other side of the Earth, physicists were able to demonstrate that some of the neutrinos were disappearing before reaching the detector.
But did this anomaly really signal a disappearance of these neutrinos, or simply their oscillation from one flavour to another? The only way to know for sure was to build a detector that could detect all three neutrino flavours.
Solar neutrinos at the Sudbury Neutrino Observatory
SNO's crucial innovation was the ability to detect all three kinds of neutrinos. This allowed the experiment to measure the true number of neutrinos emitted by the Sun, regardless of whether or not they had transformed into different flavours.
When a neutrino interacted with a deuteron in the heavy water, it could transform the deuteron into two protons, much like Davis' experiment could turn chlorine into argon. This transformation is only possible for electron neutrinos. But the detector could also sense a neutrino splitting a deuteron into a proton and a neutron, which is possible for all neutrino flavours.
Confirming the existence of neutrino oscillation
When the SNO Collaboration (led by Arthur McDonald) published their findings in 2001 and 2002, the rate of electron neutrino detections from the Sun was still lower than expected. But, they also found that the total number of neutrinos, counting all three flavours together, agreed very well with our understanding of the rate of nuclear reactions in the Sun.
This was strong evidence that solar neutrinos also undergo a transformation into different neutrino flavours, just like the Super-K experiment had demonstrated for neutrinos generated by cosmic rays in the atmosphere. The 'missing' solar neutrinos were not missing at all, just hiding in disguise. The expected solar neutrinos were all there, but during their travel from the sun’s core to the detector they had transformed, or oscillated, into different flavours of neutrinos.
How much does a neutrino weigh?
The oscillation of these missing neutrinos isn’t the only thing the Standard Model got wrong. The second reason this year’s Nobel Prize was awarded was for proof that neutrinos have mass. This conclusion follows from the fact that we can see neutrinos transform from one flavour into another. In the Standard Model of particle physics, though, neutrinos are not expected to have any mass.
Special relativity tells us that a spacecraft crew travelling at close to the speed of light experiences time that is dilated, or slowed down, relative to an observer in a stationary reference frame. Time dilation applies to subatomic particles too.
Since a massless particle, like a photon, travels at the speed of light, we can never observe any kind of time-dependent transformation or decay of that particle. This is because that particle does not feel the passage of time, so to speak. But because we see neutrinos transforming, they cannot be travelling at the speed of light. So neutrinos must have mass, even if it is very small.
Beyond the Standard Model
The laws of physics are often touted as absolute. But neutrino oscillation presents compelling experimental evidence that the Standard Model falls short in its description of nature. We already knew that the Standard Model wasn’t perfect; it doesn't account for gravity or dark matter yet, either. But this doesn’t mean it needs to be discarded, simply that it is incomplete. The experiments led by Kajita and McDonald not only showed us a whole new side to neutrinos, but also broke the Standard Model — playing a key role in lighting the way towards new physics.