The Sudbury neutrino observatory, which was instrumental in demonstrating neutrino oscillations and the massiveness of neutrinos. With additional results from atmospheric, solar, and terrestrial observatories and experiments, we may not be able to explain the full suite of what we’ve observed with only 3 Standard Model neutrinos. (A. B. MCDONALD (QUEEN’S UNIVERSITY) ET AL., THE SUDBURY NEUTRINO OBSERVATORY INSTITUTE)

Is There Really A Fourth Neutrino Out There In The Universe?

The Standard Model explains all the particles and interactions we see. But it can’t explain this.

Of all the particles that we know of, the elusive neutrino is by far the most difficult to explain. We know there are three types of neutrino: the electron neutrino (νe), the muon neutrino (νμ), and the tau neutrino (ντ), as well as their antimatter counterparts (anti-νe, anti-νμ, and anti-ντ). We know that they have extremely tiny but non-zero masses: the heaviest they can be means it would take over 4 million of them to add up to an electron, the next-lightest particle.

We know that they oscillate — or transform — from one type into another as they travel through space. We know that when we calculate the number of neutrinos produced by the Sun from nuclear fusion, only about a third of the expected number arrive on Earth. We know that they’re generated in the atmosphere from cosmic rays, and from accelerators and reactors when particles decay. According to the Standard Model, there should be only three.

But that story doesn’t add up.