Neutrino

A neutrino is a neutral particle with very low mass, possibly zero. It has spin 1/2 and so is a fermion. It does not interact with the strong force or the electromagnetic force, but does interact with the weak force (and with gravity if it turns out to have mass).

Because the neutrino only interacts with the weak nuclear force the cross section is extremely low. It would take a light year of lead to block half the neutrinos flowing through it.

It comes in three varieties, the electron neutrino ν_e, the muon neutrino ν_μ, and the tau neutrino ν_τ.

See also solar neutrino problem, particle physics.