Neutrinos among the least understood of the fundamental particles. Electrically neutral – hence the name neutrino – and believed to be massless, they interact virtually not at all with matter. Countless billions pass through the Eart every second, from one side to the other.
Neutrinos are affected only by a the weak force of much shorter range than electromagnetism. They are produced or absorbed via decay processes involving the weak interaction eg neutron decay, illustrated below.
If neutrinos have mass, they also interact gravitationally with other massive particles, but since gravity is by far the weakest of the four known forces, the effect of gravity on each neutrino is negligible, even though by dint of their massive numbers, being many times more numerous than photons, they may make a significant contribution to the density of the Universe.
Three types of neutrinos are known; there is strong evidence that no additional neutrinos exist, unless their properties are unexpectedly very different from the known types. Each type of neutrino is related to a charged particle (which gives the corresponding neutrino its name). Hence, the electron neutrinois associated with the electron, and two other neutrinos – the muon neutrinoand the tau neutrino- are associated with heavier versions of the electron called the muon and the tau.