What is a neutrino?

Neutrinos are elementary particles. Travelling close to the speed of light, lacking electric charge and able to pass through ordinary matter almost undisturbed, their detection is extremely challenging. Neutrinos were once thought to have no mass, but they are now known to have a minuscule mass.

Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those in the sun, in nuclear reactors, or when cosmic rays hit molecules. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos; each type also has an antimatter partner, in this case antineutrinos. Electron neutrinos or antineutrinos are involved whenever neutrons change into protons or vice versa, in the two forms of beta decay.

Most neutrinos which pass through the Earth emanate from the sun and more than 50 trillion solar electron neutrinos pass through the human body every second.