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Neutrinos Have Mass

Neutrinos Have Mass

(December 13, 2001)

The neutrino or the 'little neutral one' has been described as nothing, or almost nothing. Indeed, Nobel laureate Frederick Reines described the neutrino as 'the most tiny quantity of reality ever imagined by a human being.' The most common form of matter in the universe, neutrinos are uncharged and, until recently, considered to be massless.

But now, using data collected from 1993 to 1998 in the Liquid Scintillator Neutrino Detector (LSND) experiment conducted at Los Alamos National Laboratory, scientists provide further evidence of neutrino mass and oscillation. The LSND data suggest that 'muon anti-neutrinos' oscillate into 'electron anti-neutrinos,' and that neutrinos constitute approximately 1% of the universe's total mass. The results, stemming from a collaborative effort by the Los Alamos National Laboratory and several U.S. universities, are presented in the 1 December 2001 issue of Physical Review D. The University of California, Riverside Neutrino Physics group, a principal collaborator, has been an LSND member since 1988 and is represented by Prof. Gordon John VanDalen.

“The neutrino mass/oscillation work was principally a Los Alamos/UC Riverside effort,” says VanDalen, who, along with some of his postdoctoral researchers and graduate students, contributed to the experiment’s design, funding, construction, data collection and analysis. “Discovering the evidence for neutrino mass required developing a new detection technique and sorting through copious backgrounds,” he adds.

In spite of their ubiquitous presence, neutrinos are difficult to detect and exist in three known types or 'flavors': electron, muon and tau neutrinos. Neutrino oscillations, analogous to the musical 'beat' heard between two notes of slightly differing pitch, have been used to explain the apparent deficit of solar electron-neutrinos and atmospheric muon-neutrinos. Because scientists find it difficult to explain the solar, atmospheric and the LSND results using only the three flavors, the LSND results are controversial. Possibly, the incorporation of a fourth flavor -- the sterile neutrino -- can explain the LSND data, scientists say. Such a sterile neutrino would significantly impact the standard model of particle physics, however, and would also have wide-ranging implications for future research in at least nuclear physics, high-energy physics, and astrophysics.

“Hard experiments can be risky, but they can also be rewarding,” VanDalen says, noting that the LSND results, which are significant in the field, will likely interest other researchers. “LSND completed the data collection in 1998,” he says. “We are now part of a new experiment – MiniBooNE – at FermiLab, the MiniBooNE involving a much larger detector designed to confirm or refute the LSND results on neutrino mass.” Data collection for MiniBooNE will commence in summer, 2002.

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