· Fundamentals · 3 min read
The Three Neutrino Flavours Explained
Neutrinos come in three flavours: electron, muon, and tau. Here is what neutrino flavour means and how a neutrino changes flavour as it travels.
Neutrinos come in three flavours: the electron neutrino, the muon neutrino, and the tau neutrino. Each flavour is named after the charged particle it is born alongside, and together they form one of the three generations of matter in the Standard Model. A neutrino almost always belongs to one of these three types when it is created.
What does neutrino flavour mean?
Flavour is simply the label that ties a neutrino to its charged partner. The electron neutrino is the companion of the electron, the muon neutrino of the muon, and the tau neutrino of the tau. These three charged particles are leptons, and so are their neutrinos.
The pairing is not just a naming convention. When a neutrino interacts via the weak force and produces a charged lepton, it produces the matching one: an electron neutrino yields an electron, a muon neutrino yields a muon, and a tau neutrino yields a tau. This is how experiments tell the flavours apart, by watching which charged particle emerges from a collision. The three flavours are otherwise remarkably similar, sharing the same near-zero mass scale and the same vanishing electric charge.
How are the three flavours produced?
Different sources favour different flavours. The Sun produces almost exclusively electron neutrinos in its fusion reactions. Cosmic rays striking the upper atmosphere generate mostly muon neutrinos, along with some electron neutrinos. Tau neutrinos are rarely made directly, because the tau lepton is so heavy that the energies required are extreme. As a result, the tau flavour is the most elusive of the three. Reactors and supernovae add their own contributions, and particle accelerators can manufacture intense beams of muon neutrinos to order, which is why so much oscillation research begins with that flavour.
Can a neutrino change its flavour?
Yes, and this is the most surprising fact about neutrinos. As a neutrino travels, it can transform from one flavour into another, a quantum phenomenon called neutrino oscillation. An electron neutrino leaving the Sun may arrive at Earth as a muon or tau neutrino.
Oscillation is possible only because the three flavours are blends of three states with slightly different masses. This discovery, which earned the 2015 Nobel Prize in Physics, proved that neutrinos have mass, contradicting the original Standard Model. Our introductory series walks through the idea step by step.
Read more
References
- 1.S. Navas et al. (Particle Data Group), Review of Particle Physics (neutrino properties), Phys. Rev. D (2024). DOI: 10.1103/PhysRevD.110.030001.