'Sterile neutrinos' laid to rest – for now

12:57 12 April 2007
NewScientist.com news service
Hazel Muir

Do ghostly, imperceptible particles called "sterile neutrinos" wander the universe? The question has given physicists sleepless nights since evidence for the particles emerged a decade ago.

But now a new experiment has poured cold water on the idea, reassuring many scientists that their ideas are on the right track.

"Our results are the culmination of many years of very careful and thorough analysis – scientists everywhere have been eagerly waiting for our results," says Janet Conrad, a spokeswoman for the experiment at Fermilab, near Chicago in Illinois, US. She announced the result at a Fermilab meeting on Wednesday.

Neutrinos are lightweight particles that whiz around the universe, barely interacting with matter. They stream out from nuclear reactions in the Sun and continually flood straight through the Earth.

Flipping out

The particles come in three different types, or "flavours", dubbed electron, muon and tau. And several experiments have proved that neutrinos and their antiparticle counterparts can flip from one flavour to another, or "oscillate", as they travel.

One of these experiments was the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico, US, which gathered data from 1993 to 1998. The experiment suggested some muon antineutrinos had flipped into electron antineutrinos after travelling about 30 metres.

However, the results of this experiment did not mesh with other experiments unless at least one extra, fourth neutrino existed, with roughly one-millionth of the mass of the electron. This fourth neutrino would be "sterile", meaning it would not interact with matter at all, except through gravity.

But sterile neutrinos had no place in the standard picture of particle physics, so they would force physicists to radically overhaul their theories. Sterile neutrinos of this mass also conflicted with cosmology, because they would have interfered with the growth of galaxies in the universe.

Butts on the line

"The implications were staggering," says Scott Dodelson at Fermilab. "Cosmologically, we decided there should not be a sterile neutrino, so to some extent, our butts were on the line."

Physicists were therefore keen to double-check the LSND result, so they dismantled the experiment and used the parts to build a more sensitive experiment at Fermilab called MiniBooNE, the first phase of a project called BooNE (Booster Neutrino Experiment).

Now, after analysing data from MiniBooNE gathered between 2002 and 2005, the team say they have resolved the issue, without the need for exotic sterile neutrinos.

MiniBooNE fired a beam of muon neutrinos into a detector 500 m away. None of them flipped into electron neutrinos. This result is consistent with other experiments and the standard three-neutrino picture.

"This kind of confirms what we were saying," says Dodelson. However, he adds that there might be some exotic, convoluted reason why both LSND and MiniBooNE are correct and can be reconciled with new physics – something physicists intend to explore.
[Read the NY Times article here.]