Neutrino Physics

Birth of Neutrinos

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.


A view of the SNO detector located 2000 metres underground in the Creighton mine near Sudbury, Canada. The vessel is 12 metres across and is filled with 1000 tonnes of heavy water. A few of the neutrinos that pass through the detector interact to produce electrons that travel faster than the speed of light in the heavy water. These electrons create flashes of Cerenkov light that are detected by the 9600 photomultiplier tubes surrounding the vessel.

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.

  • Introduction
    Neutrinos are everywhere. Trillions of them are passing through your body every second,but they are so shy and we do not see or feel them. They are the least understood elementary particle we know that exist.
  • Birth of Neutrinos
    Existing of neutrinos was suggested as a "desperate remedy" to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei.
  • The Standard Model
    The Standard Model of particle physics can describe everything we know about elementary particles. It says that neutrinos do not have mass. Neutrinos do not have mass because they are all "left-handed" and do not bump on the mysterious "Higgs boson" that fills our entire Universe.
  • Evidence for neutrino mass
    In 1998, a convincing evidence was reported that neutrinos have mass. The Standard Model has fallen after decades of invicibility. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines.
  • Implications of neutrino mass
    Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lighest elementary particle: electron. How do we need to change the Standard Model to explain the neutrino mass? Some argue that our spacetime has unseen spatial dimensions, and we are stuck on three-dimensional "sheets". Other argue that we need to abandon the sacred distinction between matter and anti-matter.
  • Why do we exist?
    When Universe started with the "Big Bang", there were almost equal amount of matter and anti-matter. Most of matter was annihilated by anti-matter when Universe cooled. We are leftover of one part in ten billions. Why was there a small excess matter over anti-matter so that we can exist? Once we abandon the sacred distinction between matter and anti-matter, it provides a key to understand why we exist.
  • Outlook
    The mysteries about neutrinos are now being unraveled dramatically. We will learn much more in the coming years.
This page 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.