York physics Professor and neutrino researcher Sampa Bhadra offers an insider’s guide to neutrinos and the Nobel Prize for Physics:
This year’s Nobel Prize in Physics was awarded to Professors Takaaki Kajita in Japan and Arthur McDonald in Canada “for the discovery of neutrino oscillations, which show that neutrinos have mass.” What are neutrinos and why we do care if they have mass?
We are made largely of water whose molecules contain the simplest atom, hydrogen. If you had a hammer and could smash the hydrogen atom you would see that it’s made of a positively charged proton and a negatively charged electron. If you smashed even harder, you could see inside the proton and find there are particles we call quarks. Our real life atom smasher has a name – it’s called the “accelerator”. At accelerators around the world, as much as we have tried we haven’t been able to break the quarks and electrons any further, so we call them fundamental particles.
The “neutrino” is also one of the fundamental particles of nature, in fact one of the most abundant particles in the universe and yet its properties have remained elusive. For every proton there are about 700 million neutrinos in the universe, so it’s about time we understood this particle better. Neutrinos are everywhere – in the cosmos as a relic signature from the Big Bang, the sun, the atmosphere, nuclear reactors, and we can even make them at accelerators. Though “neutrino” sounds similar to the more familiar “neutron”, the latter is a companion to the proton within atoms, and emits a neutrino during radioactive decays. In fact, your breakfast banana, which contains radioactive potassium atoms, emits a huge number of neutrinos via this process.
To hone in on the properties of neutrinos scientists had naturally tried to measure the most obvious thing – their mass – but previous results had been consistent with neutrinos having zero or no mass. Indeed the theory of the building blocks of matter (such as quarks, electrons and neutrinos) is based on this assumption. This theory incorporates three types of neutrinos without mass (electron, muon and tau) and understanding their properties has become a global quest.
Neutrinos are extremely reluctant to interact with other matter making them hard to study. Consider that about 70 billion of them from the sun are going right through your fingernail every second, but you don’t feel a thing. And yet we could not exist without neutrinos! The process of fusing hydrogen atoms in the sun releases neutrinos and the heat energy we need for life on earth. Note that the sun produces only electron neutrinos and not the other two types. Physicists made important theoretical calculations on how the innards of the sun work, and based on the temperature of the sun they predicted how many electron neutrinos should reach earth. Early experiments made the startling discovery that only a fraction arrived! Could the theory be wrong? Were the experiments wrong? Or?
There is really no fundamental reason why neutrinos should have zero mass, so despite no evidence for massive neutrinos, some bold theorists worked out the implications of the three neutrinos having non-zero and different masses. Using quantum mechanics principles they showed that neutrinos in flight could change from one type to another, back and forth (i.e. oscillate). If we could actually observe this identity change in an experiment, that would resolve the solar neutrino puzzle, and we would have proof that neutrinos are massive! This would imply that our current theories are incomplete, but also be a big step in accounting for the mass of the universe since there are so many neutrinos.
In Japan, the Super-Kamiokande (SK) experiment in the Kamioka mine recorded the behavior of muon neutrinos from the atmosphere. These are created from cosmic rays uniformly bombarding the atmosphere of the earth from all sides. So it was expected that the number of neutrinos recorded would be independent of which direction the experimentalists were looking – upwards in the sky above, or below, where neutrinos are arriving at SK after travelling through the earth from the other side of the globe. This expectation turned out to be wrong! It was found that there were fewer muon neutrinos coming from below than above. It could only happen if muon neutrinos changed their identity to some other kind of neutrino as they travelled through the earth! Prof. Kajita led the analysis team that conclusively demonstrated this disappearance.
Let’s get back to the mysterious deficit of electron neutrinos from the sun. Could they be morphing to the other types as well? How could we prove this? The Sudbury Neutrino Observatory (SNO) team, led by McDonald, designed an ingenious experiment to be sensitive to all three types of neutrino. They showed that indeed there was a deficit of electron neutrinos as seen by the earlier experiments. By accounting for all three types, the results were in complete agreement with the theoretical prediction. Electron neutrinos had changed into other types on their way from the sun. The deficit problem was finally solved; the morphing of neutrinos among themselves was firmly established, requiring that neutrinos be massive.
There is much more to learn and perhaps more Nobel prizes to come for neutrino physics. York University is a collaborator with Kajita on an important experiment, T2K, which is studying the oscillatory behaviour of muon neutrinos produced by the JPARC accelerator laboratory in Japan and sent on a long journey towards the SK detector where the changed identity is recorded. Neutrinos could hold the key to the profound mystery of how a tiny fraction of matter remained after the annihilation of the equal amounts of matter and anti-matter created in the Big Bang. If this had not happened, there would be no stars, planets and us. We at T2K are searching for this effect, so stay tuned for exciting times ahead.
Read more in the award winning book Neutrino Hunters by Ray Jayawardhana, astrophysicist and dean of the Faculty of Science at York University.