Its own inventor doubted if anyone would ever see it. Two thirds of a century ago, physicist Wolfgang Pauli postulated a new particle to explain the apparent nonconservation of energy in radioactive decays. But the theoretical particle he described had properties that made it so elusive that even Pauli wondered whether anyone would ever observe it. And yet today, not only have scientists observed neutrinos, but researchers can carry out detailed experiments involving millions of neutrino events.
Past neutrino experiments have helped establish the validity of the Standard Model of particle physics, the theoretical framework that provides our best explanation of the basic properties of matter. Future neutrino experiments promise not only to tell us more about the nature of the neutrino itself, but also to illuminate the path toward new physics beyond the Standard Model.
In a letter to the attendees of a physics conference in Tübingen, Germany, Wolfgang Pauli proposes as a "desperate remedy" the existence of a new neutral particle to explain the apparent energy nonconservation in radioactive decays. During the next few years, scientists elaborate Pauli's theory and conclude that the new particle must be very weakly interacting and extremely light.
Enrico Fermi proposes "neutrino" as the name for Pauli's postulated particle. He formulates a quantitative theory of weak particle interactions in which the neutrino plays an integral part.
Two American scientists, Frederick Reines and Clyde Cowan, report the first evidence for neutrinos. They use a fission reactor as a source of neutrinos and a well-shielded scintillator detector nearby to detect them.
An Italian physicist, Bruno Pontecorvo, living in the USSR, formulates a theory of neutrino "oscillations." He shows that if different species of neutrinos exist, they might be able to oscillate back and forth between different species.
Maurice Goldhaber, Lee Grodzins, and Andrew Sunyar at Brookhaven National Laboratory demonstrate that the new neutrino has lefthanded helicity, meaning that it spins along the direction of its motion in the sense of a lefthanded screw. The experiment helps to distinguish among different forms of weak interactions.
A group of scientists from Columbia University and Brookhaven National Laboratory perform the first accelerator neutrino experiment and demonstrate the existence of two species of neutrinos, the electron neutrino, νe, and the muon neutrino, νμ. In 1987, Jack Steinberger, Leon Lederman, and Mel Schwartz win the Nobel Prize for this discovery.
An experiment deep underground in the Homestake mine in South Dakota makes the first observation of neutrinos from the sun. But experimenters see far fewer neutrinos than solar models had predicted.
An international team working at CERN, the European Laboratory for Particle Physics, in Geneva, Switzerland, uses a bubble chamber to observe the first example of a "neutral current" event. Observation of this new interaction lends strong support to a unified theory of weak and electromagnetic interactions proposed a few years earlier by Sheldon Glashow, Abdus Salam, and Steven Weinberg. Shortly afterward, scientists at Fermilab confirm the discovery.
A new lepton, tau, is discovered by a group led by physicist Martin Perl at the Stanford Linear Accelerator Center. Experiments performed shortly afterward provide strong evidence that there also exists a third species of neutrino, the tau neutrino, ντ. In 1995, Perl and Reines win the Nobel Prize for their discoveries.
Large underground water detectors in the Kamioka mine in Japan and in the Morton salt mine in the U.S. detect the first neutrinos from a supernova, SN1987A.
Experiments at CERN and at Stanford show that there can exist only three species of light (or massless) neutrinos. Thus νe, νμ and ντ must complete this class of particles. This direct measurement verifies strong suggestions previously deduced from the cosmological measurements.
At the Neutrino '98 conference in Japan, physicists from the Super-Kamiokande experiment present significant new data on the deficit in muon neutrinos produced in the Earth's atmosphere. The data suggest that the deficit varies depending on the distance the neutrinos travel - an indication that neutrinos oscillate and have mass.
The Main Injector at Fermilab begins operation. The combination of its high-intensity particle beam and an energy of 120 GeV allows a new generation of neutrino experiments that will continue to probe some of nature's most fundamental questions. View Aerial Photograph