Neutrinos rarely interact with ordinary matter, and following the detection of the electron neutrino in 1956 progress was very slow. Physicists used particle accelerators and built cumbersome apparatus in deep underground mines (as protection from cosmic rays). Experiments ran for years on end to record a handful of interactions. In the Standard Model of particle physics the fundamental particles are six quarks and six leptons, the leptons being the electron, muon and tau plus electron neutrino, muon neutrino and tau neutrino. Only the six leptons are directly accessible because free quarks do not exist (they are tightly bound within particles such as the proton). Hence the importance of neutrino physics: it is a direct probe of the fundamental particles, and a test of the Standard Model.

This period the Top Ten reflects the intellectual turmoil wreaked by neutrinos. In #6, John Bahcall, Institute for Advanced Study, Princeton, New Jersey, gives the roundup on solar neutrinos. Since the late 1960s, experiments to detect solar neutrinos have consistently observed only about half as many solar neutrinos as expected from theory. We know how the Sun works in great detail, so the solar neutrino deficit is very difficult to understand. It is almost impossible to devise a model of the Sun which can produce the solar neutrino deficit without major violence to accepted physics.

Papers #1, #8, and #9 deal out that violence in spades, from observations of atmospheric neutrinos, which are produced when energetic cosmic ray particles collide with the atoms in the upper atmosphere. As with the solar neutrino problem, theory and observation have been seriously out of line: theorists expected muon neutrinos to be twice as common as electron neutrinos but roughly equal numbers are observed. The Japanese experimenters have evidence that the neutrino "oscillates," which changes the physics.

Physicists have long assumed that the neutrino is massless. But if the neutrino has a very small mass, each neutrino flavor (electron, muon, tau) is a mixture of its three underlying mass states. Any neutrino will then act like a mixture of electron, muon and tau states. The mix of flavors we see in a cascade of atmospheric neutrinos or in a neutrino beam will depend on how far the neutrinos have traveled. If oscillations are occurring, then the intractable problems of neutrino physics are soluble, possibly at the cost of slight damage to the Standard Model, because neutrinos then carry mass.

The current generation of oscillation experiments uses controlled beams of laboratory neutrinos. Super-Kamiokande has recently staged another coup by linking up with KEK, the Japanese national high-energy physics laboratory. KEK's facilities include a proton synchrotron accelerator which can produce an intense beam of 12 GeV protons. By colliding this proton beam with aluminum, the researchers concoct a beam which starts as almost pure muon neutrinos. This is aimed 1 degree down into the Earth, where it traverses the 250 km from KEK to the Super-Kamiokande detector. In July 2000 the Japanese-US-Korean collaboration responsible for this experiment announced that in the first nine months of operation the beam had produced just 17 detections at Super-Kamiokande, barely half the rate expected if neutrinos do not change. Their result is consistent with neutrinos oscillating between electron, muon, and tau types. The high citation rate for #1 arises because of its pioneering place in the food chain feeding the frenzy surrounding the latest oscillation experiments.

The final brick is in place for the Standard Model with the first direct observation of the tau neutrino announced in July 2000 by Fermilab, in Batavia, Illinois. An international team (U.S., Japan, Korea, and Greece) used Fermilab’s Tevatron to produce an intense neutrino beam. This passed through a 14-m long detector where just one tau neutrino in 10¹² was predicted to interact by producing a tau lepton from interaction with an iron nucleus. In a classic needle- in-a-haystack experiment which took three years, they found four tau lepton decays consistent with tau neutrino interactions. Twenty-five years have elapsed since the discovery of the tau lepton, so finding the matching neutrino was long overdue.