Neutrinos have been shrouded in mystery ever since they were first suggested by Wolfgang Pauli in 1930. At the time physicists were puzzled because nuclear beta decay appeared to break the law of energy conservation. In beta decay, a neutron in an unstable nucleus transforms into a proton and emits an electron at the same time. After much confusion and debate, the energy of the radiated electron was found to follow a continuous spectrum. This came as a great surprise to many physicists because other types of radioactivity involved gamma rays and α-particles with discrete energies. The finding even led Niels Bohr to speculate that energy may not be conserved in the mysterious world of nuclei.
Pauli also struggled with this mystery. Unable to attend a physics meeting in December 1930, he instead sent a letter to the other "radioactive ladies and gentlemen" in which he proposed a "desperate remedy" to save the law of energy conservation. Pauli's remedy was to introduce a new neutral particle with intrinsic angular momentum or "spin" of (1/2)*h/2π, where h is Planck's constant. Dubbed the "neutron" by Pauli, the new particle would be emitted together with the electron in beta decay so that the total energy would be conserved.
Two years later, James Chadwick discovered what we now call the neutron, but it was clear that this particle was too heavy to be the "neutron" that Pauli had predicted. However, Pauli's particle played a crucial role in the first theory of nuclear beta decay formulated by Enrico Fermi in 1933 and which later became known as the weak force. Since Chadwick had taken the name "neutron" for something else, Fermi had to invent a new name. Being Italian, "neutrino" was the obvious choice: a little neutral one.
Because neutrinos interact so weakly with matter, Pauli bet a case of champagne that nobody would ever detect one. Indeed this was the case until 1956, when Clyde Cowan and Fred Reines detected antineutrinos emitted from a nuclear reactor at Savannah River in South Carolina, USA. When their result was announced, Pauli kept his promise.
Two years later, Maurice Goldhaber, Lee Grodzins and Andrew Sunyar measured the "handedness" of neutrinos in an ingenious experiment at the Brookhaven National Laboratory in the US. The handedness of a particle describes the direction of its spin along the direction of motion spin of a left-handed particle, for example, always points in the opposite direction to its momentum.
Goldhaber and co-workers studied what happened when a europium-152 nucleus captured an atomic electron. The europium-152 underwent inverse beta decay to produce an unstable samarium-152 nucleus and a neutrino. The samarium-152 nucleus then decayed by emitting a gamma ray. When the neutrino and the gamma ray were emitted back-to-back, the handedness of the two particles had to be the same in order to conserve angular momentum. By measuring the handedness of the gamma ray using a polarized filter made of iron, the Brookhaven team showed that neutrinos are always left-handed.
This important result implies that neutrinos have to be exactly massless. To see why this is, suppose that neutrinos do have mass and that they are always left-handed. According to special relativity, a massive particle can never travel at the speed of light. In principle, an observer moving at the speed of light could therefore overtake the spinning massive neutrino and would see it moving in the opposite direction. To the observer, the massive neutrino would therefore appear right-handed. Since right-handed neutrinos have never been detected, particle physicists concluded that neutrinos had to be massless.
This homepage is based on Feature Article "Origin of Neutrino mass" in Physics World, May 2002, by Hitoshi Murayama. The whole article can be download as a PDF file.