neutrino
/ noo-TREE-noh /
A neutrino is a tiny particle of matter that is electrically neutral, almost without mass, and so reluctant to interact that trillions of them are streaming through your body right now without leaving a trace. They pour out of the Sun, out of nuclear reactors, and out of the Earth itself, yet most of them would sail straight through a wall of lead a light-year thick. People sometimes call them ghost particles, and the nickname is fair.
More precisely, a neutrino is one of the leptons — the same family as the electron — but it carries no electric charge and only an extraordinarily small mass. Because it has no charge it ignores electromagnetism, and because it is a lepton it ignores the strong force, so the only way it can touch ordinary matter is through the weak force, which acts only at the briefest of ranges. That is why a neutrino almost never bumps into anything: the door it must use is nearly always shut. There are three kinds, called flavors, paired with the electron, the muon, and the tau.
Neutrinos matter enormously out of all proportion to how faint they are. They were the first crack in the Standard Model: the original theory assumed they were perfectly massless, but the discovery that they change flavor as they travel proved they must carry a small mass, forcing physicists to extend the theory. They are also among the most abundant particles in the universe, they govern how stars explode, and because they pass through matter unscathed they let us see into places — the core of the Sun, the heart of a supernova — that light can never escape.
About 65 billion neutrinos from the Sun pass through every square centimetre of your skin each second — roughly the area of a fingernail. In an entire human lifetime, on average only a handful will ever actually interact with an atom in your body.
Neutrinos flood through us constantly yet almost never touch us.
A neutrino is not the same as a neutron: the neutron is a heavy, composite particle that lives in atomic nuclei, while the neutrino is a featherlight, truly elementary lepton. The similar names are an accident of history.