Neutrino physics gains weight

26 September 1999

Neutrinos are always compelling physics. The neutrino sessions at this year’s International Lepton-Photon Symposium at Stanford gave an up-to-date snapshot.


Neutrinos, which interact only via the weak force, may be the purest form of lepton and deserved to be in the spotlight at the International Lepton­Photon Symposium at Stanford. Neutrino physics questions are frequently unresolved and are even controversial, like Pauli’s prediction of a particle that hardly interacts at all. Neutrino physics has lived up to this reputation ever since.

Neutrinos can “oscillate” from one kind of neutrino to another 

Last year, data on neutrinos generated by cosmic-ray collisions in the atmosphere ushered out the old orthodox view that neutrinos are massless particles. Endowed with mass, the three kinds of neutrinos ­ electron, muon and tau ­ are not immutable and can “oscillate” from one kind of neutrino to another. This year the neutrino results presented at the International Lepton­Photon Symposium showed that this view has now firmly taken root.

The 1998 paradigm shift was the outcome of the release of initial data from the 50 000 tonne Super-Kamiokande underground neutrino detector in Japan. The effect ­ a deficit of muon neutrinos ­ measured by the Super-Kamiokande detector had been known since the early 1980s from smaller experiments. However, the Super-K signal inspired physicists with new 50 000-tonne confidence. One year down the line this confidence seems complete.

Atmospheric neutrinos are not alone in oscillating. The dearth of neutrinos from the Sun ­ an effect known for 30 years ­ was initially attributed to the innate difficulties of neutrino experiments and of estimating the radiative neutrino power of the Sun rather than the neutrinos themselves.

In his presentation, Yoichiro Suzuki of Tokyo asked why solar neutrino experiments, which were the first to detect neutrino deficits, had not claimed the discovery of neutrino oscillations. The reasons, observed Suzuki, were the uncertainty of gauging the solar neutrino flux and the difficulty in providing a unique oscillation solution.

Flux-independent indicators, such as a systematic difference between the daytime and night-time signal and a spectrum revealing the way that neutrino behaviour changes with energy, would provide a more reliable indication. However, these were not forthcoming from the pioneer experiments.

Neutrino oscillation looks like it is here to stay

Super-K has amassed 825 days of logged solar neutrino data, and the day­night rates differ by 6.5% (albeit with an error of almost 50%) ­ a possible indication of solar neutrino effects owing to neutrino transitions occurring while they pass through the Earth. The energy spectrum does not look flat.

The Sudbury Neutrino Observatory in Canada will soon provide valuable new data on additional reactions of solar neutrinos via the neutral as well as the charged current of weak interactions.

At Stanford, Tony Mann of Tufts covered the atmospheric neutrino sector ­ centre stage since the Super-K results of 1998. As well as the Super-K water Cherenkov detector, the Soudan II sampling calorimeter in the US and the Macro muon detector in the Italian Gran Sasso laboratory are also adding their weight.

The angular distribution of Super-K signals, originally displayed in only five angular bins ­ now extended to ten ­ clearly show the sharp deficit of muon neutrinos travelling upwards, after having passed through the Earth before hitting the detector.

Neutrino oscillation looks likes it’s here to stay, but the nature and parameters of the oscillations have yet to be determined. Studies using neutrinos from reactors and from accelerators are playing a key role. At Stanford, Luigi Di Lella of CERN was the reviewer.

The Chooz (pronounced “Shaw”) reactor experiment in France provided key results that limited the possibilities for the disappearance of electron-type neutrinos over a 1 km flight path from the reactor to the detector. The Chooz experiment has now completed its mission but another, at Palo Verde in the US, continues to take data.

As for neutrino experiments at accelerators, a long-standing feature has been results from the LSND study at Los Alamos and the Karmen detector at the UK Rutherford Appleton Laboratory on the appearance, or otherwise, of (anti)electrons from a beam of muon (anti)neutrinos. The former “sees” a signal, the latter does not, but the two results are not entirely incompatible ­ the regions accessed by the two experiments (and others) do not coincide totally. Thus, the oscillation signal suggested by LSND cannot be ruled out. Making this effect consistent with the rest of the neutrino data is highly constraining, according to Hamish Robertson of Washington in his subsequent review of neutrino mass and oscillations.

A feature of current neutrino dogma is that the bizarre “sterile” neutrinos, which do not react, could sap the power of other neutrino beams as their non-sterile particles oscillate out of sight. Some physicists are openly sceptical of the LSND result. Di Lella dutifully surveyed the experiment’s track record but found no cracks.

Chorus and Nomad at CERN had set out to see the production of tau neutrinos from an accelerator-produced beam mainly composed of muon neutrinos. Both studies have completed data taking, although Chorus has not yet analysed all of the interaction triggers in its emulsion target. With no tau production yet seen, the contours showing the limits on neutrino mass difference and mixing parameters can be extended substantially.

A major new player on the neutrino scene will be the MiniBoone experiment, which uses a beam derived from the Fermilab booster. This should definitively confirm or refute the LSND result.

To explore the remaining allowed oscillation territory, the emphasis also turns to “long baseline” experiments in which neutrino beams from accelerators travel over several hundred kilometres before reaching the main neutrino target.

The K2K experiment using beams from the Japanese KEK laboratory and the Super-K detector has now started, while the MINOS study will cover a longer baseline and use higher-energy particles. The KAMLAND experiment in Japan (CERN Courier April) will look for interactions from neutrinos emitted by nuclear reactors more than 100 km away.

With questions unanswered and several projects under way, with more at proposal stage, and the fact that the implications of the results are of importance for our understanding of the universe, neutrino physics will maintain interest well into the next century.

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