Explaining The 2015 Nobel Prize in Physics: Neutrinos

Neutrinos are sneaky little fellows. Their name means “little neutral ones”, but the truth is: the Nobel committee isn’t neutral about them. Neutrinos already have 4 Nobel prizes in their name, so it’s needless to say they are quite interesting. Where can we find these neutral particles you ask? Well, everywhere. You may not feel or see them, but there are literally billions of neutrinos passing at the speed of light through your body in this very second. They come from supernovas, nuclear power plants, other radioactive reactions and even from the Big Bang. Even bananas emit about 1 million neutrinos/day from decays of radioactive potassium atoms they contain.

Neutrinos don’t have electric charge, they are massless and they don’t interact with the strong nuclear force. Gravity interacts very very weakly with them. Only the weak nuclear force seems to interact with neutrinos. That’s why they are so difficult to detect. For example, when Supernova 1987A exploded, it released 100 times the neutrinos the Sun will emit in its whole lifetime (more or less 10 billion years), but how many neutrinos did we detect on Earth? Only 11! They are the second most numerous particles in the universe, only after photons, but we still can’t seem to detect them. That’s why scientists had to build very large underground spheres and fill them with water to be able to detect them.

In the 60s, we figured out how many neutrinos that were created from the Sun we were supposed to detect here on Earth, but most of them weren’t being detected in the neutrino observatories. The Standard Model of particle physics predicts there should be three types (or flavors) of neutrinos: the electron-neutrino, which is stable; and the muon-neutrino and tau-neutrino which are heavier and short-lived. The Sun can only produce electron-neutrinos, but if on the way here they were converted to their ephemeral cousins, that would easily explain why we weren’t seeing them all in the neutrino observatories.

Super-Kamiokande and the Sudbury Neutrino Observatory were both critical in providing proof that neutrinos really do come in three flavors and not just one. That’s why the lead scientists of those two observatories (Takaaki Kajita and Arthur McDonald respectively) won this year’s Nobel prize. Neutrinos can only change flavors – or as physicists say: oscillate – if they have mass, but the problem is that the Standard Model requires these tiny particles to be completely massless. Thus, we found a crack in our current understanding of particle physics and are entering the Era Beyond The Standard Model.

Congratulations to Takaaki Kajita and Arthur McDonald, but also to the researchers who dedicated countless hours working in the Super-Kamiokande and in the Sudbury Neutrino Observatory. It is by challenging our current knowledge of the universe that we progress forward. As John F. Kennedy once said: ‘The greater our knowledge increases the more our ignorance unfolds’.


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