The neutrino is perhaps one of the most annoying kinds of particles in all of physics. The physicist Wolfgang Pauli first proposed the existence of the neutrino to explain why the nuclear beta decay reaction appeared to violate conservation of energy and momentum. He thought that a tiny, invisible particle may carry off the extra energy and momentum.
Later physicists would verify the existence of the neutrino, and Enrico Fermi named it from the Italian for “little neutral one.”
The neutrino has such low mass that for decades physicists thought they were completely massless and traveled at the speed of light.
Now, physicists know that neutrinos have mass, but they aren’t sure exactly what that mass is. Compounding the issue is that there isn’t just one kind of neutrino, but three. There is the electron-neutrino, the muon-neutrino, and the tau-neutrino. Each kind, or “flavor”, of neutrino only appears in reactions that involve the associated flavor of electron (the regular electron, its heavier cousin the muon, and its even heavier cousin the tau).
It gets worse. Neutrinos have the ability to change flavor as they travel, unlike any other particle. So a reaction can produce an electron-neutrino, but by the time is reaches a detector it may be a muon-neutrino or a tau-neutrino.
It gets even worse. Neutrinos hardly ever interact with normal matter, only participating in reactions involving the weak nuclear force. So it takes incredibly large detectors to find them. For example, the IceCube experiment at the geographic South Pole uses an entire cubic kilometer of the Antarctic ice sheet to find neutrinos. Sometimes they will hit a bit of water in the ice, causing a flash of blue light that detectors drilled into the ice can detect.
The unknown masses of the three neutrino flavors is one of the biggest outstanding mysteries of modern physics, along with the question of whether additional, even more difficult to detect, flavors exist. The Standard Model of particle physics has no explanation for neutrino mass, and many physicists believe that only a radical update to our understanding of physics will solve the mystery.
