the neutrino

The neutrino is the ultimate wallflower of the particle world, electrically neutral, almost massless, and so reluctant to interact that a neutrino could pass through a light-year of solid lead with even odds of coming out the other side. Right now, tens of trillions of neutrinos from the Sun are streaming through you every second, day and night, and virtually none of them notice you are there. They are the second most common particle in the universe after photons, and for decades they were the hardest to catch.

A neutrino is a neutral lepton of spin 1/2 that feels only the weak interaction and gravity, no electromagnetism, no strong force. There are three flavors, one for each generation: the electron, muon and tau neutrino. Wolfgang Pauli postulated the neutrino in 1930 as a 'desperate remedy' to rescue energy and momentum conservation in beta decay, where the emitted electron came out with a continuous range of energies rather than a single value; the missing energy, he reasoned, was carried off by an unseen neutral particle. It took until 1956 for Reines and Cowan to detect it directly, using the intense neutrino flux from a nuclear reactor.

In the original Standard Model neutrinos were exactly massless and only ever left-handed, but the discovery of neutrino oscillation proved they have tiny nonzero masses, the first solid evidence that the Standard Model is incomplete. Their feeble interaction makes them a unique messenger: neutrinos escape directly from the Sun's core and from a collapsing supernova, carrying information no light could bring, which is why neutrino astronomy exists. The abiding misconception is that neutrinos are hard to detect because they are small or fast; the real reason is simply that the weak force is weak, so their interaction cross-section is minuscule.