Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. As of 1999, it's believed neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter u_/E. This doesn't significantly affect the experiments, because neutrinos involved are nearly always ultrarelativistic, and thus mixing amplitudes are vanishingly small (for e… (More on Neutrino)