The Most Unstably Stable "Neutrino"

The smallest things in the universe

Atoms, despite the Greek name (“cannot be cut”), are not elementary particles, meaning they can be disassembled. An atom is a diffuse cloud of electrons surrounding a tiny, dense nucleus composed of protons and neutrons, which can be broken into up and down quarks.

Particle collider, which accelerate particles to near the speed of light and smash them together, help us discover new elementary particles. First, because of E = mc2, the energy in the collision can be converted into the mass of particles. Second, the higher the accelerator’s beam energy, the more finely we can resolve composite structures, just as we can see smaller things with X-rays than with visible light.

We haven’t been able to take apart electrons or quarks. These are elementary particles, forming the basic constituents of ordinary matter: the Lego bricks of the universe. Interestingly, there are many heavy cousins of familiar particles that exist only for fractions of a second, and thus are not part of ordinary matter. For example, for electrons these are the muon and tauon.

Elementary particles, of which neutrinos are one kind.

What’s a neutrino?

How is this elementary particle – the neutrino – different from all other elementary particles? It’s unique in that it’s both almost mass-less and almost non-interacting. Those features are different, though often conflated, hence we can call it introvert.

It’s a mystery why neutrinos are almost, but not quite, mass-less. We do know why they’re almost non-interacting, though: They don’t feel the electromagnetic or strong forces that bind nuclei and atoms, only the aptly named weak force (and gravity, but barely, because their masses are small).

Though neutrinos are not constituents of ordinary matter, they are everywhere around us – a trillion from the sun pass through your eyes every second. There are hundreds per every cubic centimeter left over from the Big Bang. Because they so rarely interact, it’s almost impossible to observe them, and you certainly don’t feel them.

Neutrinos have other weird aspects. They come in three types, called flavors – electron, muon and tauon neutrinos, corresponding to the three charged particles they pair with – and all of these seem to be stable, unlike the heavy cousins of the electron.

Because the three flavors of neutrinos are almost identical, there is the theoretical possibility that they could change into each other, which is another unusual aspect of these particles, one that can reveal new physics. This transformation requires three things: that neutrino masses are nonzero, are different for different types, and that neutrinos of definite flavor are quantum combinations of neutrinos of definite mass (this is called “neutrino mixing”).

For decades, it was generally expected that none of these conditions would be met. Not by neutrino physicists, though – we held out hope.