cosmic rays create background events that mask the critical events being searched for. It takes 2 miles of rock to absorb the most energetic of the muons created by cosmic-ray protons striking Earth’s atmosphere.

At such great depths, the only backgrounds are made by neutrinos (which easily penetrate the whole Earth but, by the same token, interact very seldom) and by local radioactivity in the rock itself. The latter can be shielded by the use of specially purified but otherwise ordinary materials, such as water. For instance, the Sudbury Neutrino Observatory (SNO) in Canada is built as a high-tech clean room 10 stories tall and more than a mile underground. Only in this laboratory could the collaboration achieve an experiment that is 10 billion times cleaner than our typical living room in terms of natural radioactivity. SNO is the most background-free environment ever achieved on Earth.

Some experiments do not require the greatest depths and can tolerate less stringent conditions either because the process being sought has a higher signal rate or because some special experimental tag can be used to identify the important events even in the presence of background. For other experiments, however, there is no option but depth and extreme cleanliness. Only in such an isolated environment can we hope to detect the faintest signals of our universe.

Scientists addressing issues of intense international interest—solar neutrinos, double beta decay, and dark matter—are poised to develop next-generation detectors that require low background, and they need an underground facility for technology development in the next few years. Once the neutrino mixing and mass parameters have been measured with some accuracy, a long-baseline experiment should be developed. The KamLAND, Borexino, MiniBooNE, and MINOS experiments are expected to lead—over the next 5 years—to the synthesis necessary for the long-baseline program. A long-baseline target detector is likely to also carry out a proton decay experiment and serve as a supernova neutrino telescope, as well as many other purposes.

NEUTRINO PROPERTIES

The neutrino has had a very rich history. As described in the science overview (Chapter 2), the neutrino was postulated to preserve important conservation principles in the decay of nuclei and, as a consequence, had to possess novel properties: zero charge, zero mass, spin 1/2, and very weak interactions with other particles. It took the advent of nuclear reactors, which were able to produce neutrinos in profusion, to clearly demonstrate that the neutrino indeed existed. Furthermore, not one but three distinct types of neutrinos exist: an electron, muon, and tau type of neutrino, each coupled to its respective electrically charged partner. After intensive efforts to directly measure neutrino masses, an upper limit of 1–3 eV has



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