Neutrinos are an ancient relic of the Big Bang, and millions of them fill every corner of the cosmos. But scientists have been puzzled by their true nature ever since the surprising discovery that fewer neutrinos came from the sun than expected from the nuclear fusion that produces the sun's energy. We now know that this is due to neutrino oscillations: neutrinos can change flavor on their way from the sun to the Earth. This means that neutrinos must have mass.
Initiated in early in 2004, the study's primary purpose was to help set priorities to answer some of the most vital open questions in neutrino physics. Since it is such a broad, interdisciplinary field, the study was divided into six working groups: solar and atmospheric neutrino experiments; reactor neutrino experiments; superbeam experiments; neutrino factory and beta beam experiments; neutrinoless double beta decay and direct searches for neutrino mass; and what cosmology and astrophysics and neutrino physics can teach each other.
"It's more than just arguing for some experiments," study co-chair Stuart Freedman (University of California, Berkeley) said of the neutrino study's purpose. "It's an attempt to provide background information to people who would be in a position to argue to the higher levels of government that this is important science. This study will help the people who do the funding to get a coherent view of the field, especially since there are so many facets, because the field is so cross-disciplinary."
According to Freedman, the study members framed their view of the future of neutrino physics along three overarching themes: the impact of neutrino research on the Standard Model; the fact that neutrino physics has been marked to date by anomalous unexpected results; and the potential of neutrinos to probe the cosmos, including the generation of the sun's energy.
The report's first recommendation calls for a phased program of sensitive searches for neutrinoless double beta decay, a rare process in which one atomic nucleus turns into another by emitting two electrons. This is the only way to determine if the neutrino is its own antiparticle, and is also critical to our understanding of the origin of mass.
A second high priority should be establishing a comprehensive US program to improve our understanding of neutrino mixing, as well as to determine the nature of the neutrino mass spectrum, and to search for CP violation among neutrinos. Such a program would include several experiments, including one located a few kilometers from a nuclear reactor and a beam of accelerator-generated neutrinos directed towards a detector several hundred kilometers away.
Future plans should also include a neutrino "superbeam" program using a megawatt proton accelerator. New technologies will also be needed, including massive new detectors capable of producing the largest and most precise samples of neutrino data yet recorded, and a new neutrino factory with very pure neutrino beams.
Less pressing, but still important, is the need to develop an experiment to make precise measurements of the low-energy neutrinos from the sun. Thus far, only solar neutrinos with relatively high energy have been studied in detail, and that's only a small fraction of the total. Being able to precisely measure lower- energy solar neutrinos could help us better understand how they change their flavor. It may also enable scientists to predict how bright the sun will be tens of thousands of years from now.
Freedman emphasized that these three primary recommendations assume continued strong support of the existing neutrino programs still in progress. And because neutrino interactions are extremely rare, many of the proposed experiments will need to be carried out in a deep underground laboratory facility. The study report also stresses the importance of international cooperation with the neutrino programs of other nations and geographical regions.
The full text of the joint unit neutrino study can be found at http://apsreactor.uchicago.edu/archive/0045.html. The full text of the reports from each of the six working groups can be found at http://www.interactions.org/cms/?pid=1010561.
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