I was at the conference in Takayama, near Kamioka, in 1998 when the SuperKamiokande collaboration announced the first evidence for neutrino mass. It was a moving moment. Uncharacteristically for a physics conference, people gave the speaker a standing ovation. I stood up too. Having survived every experimental challenge since the late 1970s, the Standard Model had finally fallen. The results showed that at the very least the theory is incomplete.
The SuperKamiokande collaboration looked for neutrinos that were produced when cosmic rays bombarded oxygen or nitrogen nuclei in the atmosphere. These "atmospheric neutrinos" are mostly muon neutrinos and interact very weakly with matter. Filled with 50 000 tonnes of water, however, the SuperKamiokande detector located deep in the Kamioka mine in Japan is so large that it can detect atmospheric neutrinos. These neutrinos interact with atomic nuclei in the water to produce electrons, muons or tau leptons that travel faster than the speed of light in water to produce a shock wave of light called Cerenkov radiation. This radiation can be detected by sensitive photomultiplier tubes surrounding the water tank.
From these signals, the SuperKamiokande team could also determine the directions from which the neutrinos came. Since the Earth is essentially transparent to neutrinos, those produced high in the atmosphere on the opposite side of the planet can reach the detector without any problems. The team discovered that about half of the atmospheric neutrinos from the other side of the Earth were lost, while those from above were not. The most likely interpretation of this result is that the muon neutrinos converted or "oscillated" to tau neutrinos as they passed through the Earth. SuperKamiokande is unable to identify tau neutrinos. The particles coming from the other side of the Earth have more opportunity to oscillate than those coming from above. Moreover, if neutrinos convert to something else by their own accord, we conclude that they must be travelling slower than the speed of light and therefore must have a mass.
SuperKamiokande was also used to monitor solar neutrinos. The fusion reactions that take place in the Sun only produce electron neutrinos, but these can subsequently oscillate into both muon and tau neutrinos. Though the experiment was able to detect the solar neutrinos, it was unable to distinguish between the different neutrino types. In contrast, the Sudbury Neutrino Observatory (SNO) in Canada can identify the electron neutrinos because it is filled with "heavy water", which contains hydrogen nuclei with an extra neutron. Small numbers of electron neutrinos react with the heavy-hydrogen nuclei to produce fast electrons that create Cerenkov radiation (figure 1).
By combining the data from SuperKamiokande and its own experiment, the SNO collaboration determined how many muon neutrinos or tau neutrinos were incident at the Japanese detector. The SNO results also provided further evidence for neutrino mass and confirmed that the total number of neutrinos from the Sun agreed with theoretical calculations.
The implications of neutrino mass are so great that it is not surprising that particle physicists had been searching for direct evidence of its existence for over four decades. In retrospect, it is easy to understand why these searches were unsuccessful (figure 3). Since neutrinos travel at relativistic speeds, the effect of their mass is so tiny that it cannot be determined kinematically. Rather than search for neutrino mass directly, experiments such as SuperKamiokande and SNO have searched for effects that depend on the difference in mass between one type of neutrino and another.
In some respects these experiments are analogous to interferometers, which are sensitive to tiny differences in frequency between two interfering waves. Since a quantum particle can be thought of as a wave with a frequency given by its energy divided by Planck's constant, interferometry can detect tiny mass differences because the energy and frequency of the particles depend on their mass.
Interferometry works in the case of neutrinos thanks to the fact that the neutrinos created in nuclear reactions are actually mixtures of two different "mass eigenstates". This means, for example, that electron neutrinos slowly transform into tau neutrinos and back again. The amount of this "mixing" is quantified by a mixing angle, θ. We can only detect interference between two eigenstates with small mass differences if the mixing angle is large enough. Although current experiments have been unable to pin down the mass difference and mixing angle, they have narrowed down the range of possibilities (figure 4).
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.