NEUTRINOS Neutrino physics gains

The Sudbury Neutrino Observatory in Canada has produced its first images of neutrino interactions in its 1000 tonnes of heavy water. The data from this new type of neutrino detector could weight help to pin down the properties of solar neutrinos.

neutrinos from the Sun - an effect known for 30 years - was initially attributed to the innate difficulties of neutrino experiments and of Neutrinos are always compelling physics, estimating the radiative neutrino power of the Sun rather than the The neutrino sessions at this year's neutrinos themselves. In his presentation, Yoichiro Suzuki of Tokyo asked why solar International Lepton-Photon Symposium at neutrino experiments, which were the first to detect neutrino deficits, had not claimed the discovery of neutrino oscillations.The reasons, Stanford gave an up-to-date snapshot. observed Suzuki, were the uncertainty of gauging the solar neutrino flux and the difficulty in providing a unique oscillation solution. Neutrinos, which interact only via the weak force, may be the purest Flux-independent indicators, such as a systematic difference form of lepton and deserved to be in the spotlight at the between the daytime and night-time signal and a spectrum revealing International Lepton-Photon Symposium at Stanford (p6). Neutrino the way that neutrino behaviour changes with energy, would provide physics questions are frequently unresolved and are even contro­ a more reliable indication. However, these were not forthcoming from versial, like Pauli's prediction of a particle that hardly interacts at the pioneer experiments. all. Neutrino physics has lived up to this reputation ever since. Super-K has amassed 825 days of logged solar neutrino data, Last year, data on neutrinos generated by cosmic-ray collisions in and the day-night rates differ by 6.5% (albeit with an error of almost the atmosphere ushered out the old orthodox view that neutrinos 50%) - a possible indication of solar neutrino effects owing to are massless particles. Endowed with mass, the three kinds of neutrino transitions occurring while they pass through the Earth.The neutrinos - electron, muon and tau - are not immutable and can energy spectrum does not look flat. "oscillate" from one kind of neutrino to another. This year the The Sudbury Neutrino Observatory in Canada will soon provide neutrino results presented at the International Lepton-Photon valuable new data on additional reactions of solar neutrinos via the Symposium showed that this view has now firmly taken root. neutral as well as the charged current of weak interactions. The 1998 paradigm shift was the outcome of the release of initial At Stanford,Tony Mann of Tufts covered the atmospheric neutrino data from the 50 000tonne Super-Kamiokande underground sector - centre stage since the Super-K results of 1998. As well as neutrino detector in Japan.The effect - a deficit of muon neutrinos - the Super-K water Cherenkov detector, the Soudan II sampling measured by the Super-Kamiokande detector had been known calorimeter in the US and the Macro muon detector in the Italian since the early 1980s from smaller experiments. However, the Gran Sasso laboratory are also adding their weight. Super-K signal inspired physicists with new 50 000-tonne confi­ The angular distribution of Super-K signals, originally displayed dence. One year down the line this confidence seems complete. in only five angular bins - now extended to ten - clearly show the Atmospheric neutrinos are not alone in oscillating.The dearth of sharp deficit of muon neutrinos travelling upwards, after having

CERN Courier October 1999 19 NEUTRINOS

passed through the Earth before hitting the detector. surveyed the experiment's track record but found no cracks. Neutrino oscillation looks likes it's here to stay, but the nature and Chorus and Nomad at CERN had set out to see the production of parameters of the oscillations have yet to be determined. Studies tau neutrinos from an accelerator-produced beam mainly com­ using neutrinos from reactors and from accelerators are playing a posed of muon neutrinos. Both studies have completed data taking, key role. At Stanford, Luigi Di Leila of CERN was the reviewer. although Chorus has not yet analysed all of the interaction triggers The Chooz (pronounced "Shaw") reactor experiment in France in its emulsion target. With no tau production yet seen, the contours provided key results that limited the possibilities for the disap­ showing the limits on neutrino mass difference and mixing pearance of electron-type neutrinos over a 1 km flight path from the parameters can be extended substantially. reactor to the detector.The Chooz experiment has now completed its A major new player on the neutrino scene will be the MiniBoone mission but another, at Palo Verde in the US, continues to take data. experiment, which uses a beam derived from the Fermilab booster. As for neutrino experiments at accelerators, a long-standing fea­ This should definitively confirm or refute the LSND result. ture has been results from the LSND study at Los Alamos and the To explore the remaining allowed oscillation territory, the emphasis Karmen detector at the UK Rutherford Appleton Laboratory on the also turns to "long baseline" experiments in which neutrino beams appearance, or otherwise, of (anti)electrons from a beam of muon from accelerators travel over several hundred kilometres before (anti)neutrinos.The former "sees" a signal, the latter does not, but reaching the main neutrino target. the two results are not entirely incompatible - the regions accessed The K2K experiment using beams from the Japanese KEK labora­ by the two experiments (and others) do not coincide totally.Thus, the tory and the Super-K detector has now started (p5), while the oscillation signal suggested by LSND cannot be ruled out. Making MINOS study (p6) will cover a longer baseline and use higher-energy this effect consistent with the rest of the neutrino data is highly con­ particles. The KAMLAND experiment in Japan (CERN COURIER April straining, according to Hamish Robertson of Washington in his sub­ p22) will look for interactions from neutrinos emitted by nuclear sequent review of neutrino mass and oscillations. reactors more than 100 km away. A feature of current neutrino dogma is that the bizarre "sterile" With questions unanswered and several projects underway, with neutrinos, which do not react, could sap the power of other neutrino more at proposal stage, and the fact that the implications of the beams as their non-sterile particles oscillate out of sight. Some results are of importance for our understanding of the universe, neu­ physicists are openly sceptical of the LSND result. Di Leila dutifully trino physics will maintain interest well into the next century. •

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