The elusive particles, if they exist, could help solve some of the most pressing problems in astrophysics.
Neutrinos like to keep to themselves. These ghostly particles are so reluctant to interact with ordinary matter that billions zip harmlessly through each person every day, and it takes giant, specialized detectors to capture even a handful of them. Now astronomers are finding hints of an even more elusive type of neutrino, one so shy that it could never be detected directly: the sterile neutrino.
The question of sterile neutrinos is absolutely crucial for nuclear particle physics and astrophysics. ,
For more than a decade, this subatomic spectre has intrigued theorists and experimenters, but experimental efforts have had trouble catching them. Now, two observations in space — one in microwaves and the other in X-rays — are raising hopes again.
If sterile neutrinos could be identified, they would provide the first glimpse of a new realm of physics beyond the tried-and-tested standard model. They could also help to explain a host of astronomical puzzles, and perhaps even account for the invisible dark matter that is thought to make up 85% of the Universe's mass.
"The question of sterile neutrinos is absolutely crucial for nuclear particle physics and astrophysics," says William Louis of Los Alamos National Laboratory in New Mexico, who worked on a ground-based experiment in the mid-1990s that provided one of the first hints of sterile neutrinos.
The three types of ordinary neutrinos (see graphic) are hard enough to detect. They interact with matter through the weak nuclear force, which means they can pass through huge volumes of material yet collide with atomic nuclei only rarely. To catch them, physicists build detectors using tanks of mineral oil or heavy water, and they improve the odds by focusing on the torrents of neutrinos coming from nuclear reactors, particle accelerators and the Sun.
In 1998, results from an accelerator at CERN, Europe's particle-physics facility near Geneva, Switzerland, indicated that only the three known families, or flavours, of neutrinos could be affected by the weak force. Any beyond these three families would have to be 'sterile' — that is, immune to the weak nuclear force. And that would mean that they would pass straight through existing neutrino detectors.
Why would theorists invoke such an unsociable particle to begin with? Because of another surprising discovery made in 1998: that the three ordinary neutrinos have a small but definite mass.
The result came as a shock, and not simply because the standard model assumes that neutrinos have no mass. The masses of the three neutrinos, determined from the way they oscillate, or switch, from one flavour into another, turned out to be incredibly small: the heaviest ordinary neutrino is at least seven orders of magnitude lighter than the electron. Theorists reasoned that some other particle — perhaps a sterile neutrino — should sit in between.
Hints of such a missing link have emerged from the Wilkinson Microwave Anisotropy Probe (WMAP), a spacecraft that since 2001 has been mapping tiny fluctuations in the radiation left over from the Big Bang. The pattern of fluctuations holds clues to the stew of particles that existed shortly after the Big Bang.
Results from the past seven years, which were released in January1, suggest that the most likely number of neutrino families in the early Universe was four — implying that one more neutrino type awaits discovery. "To say that there's something else is a major deal," says principal investigator Charles Bennett of Johns Hopkins University in Baltimore, Maryland. But he cautions that the error bars are still wide enough that three might still be the maximum number. "I would call this intriguing, but I'm not losing any sleep right now."
A more recent clue has come from the orbiting Chandra X-ray Observatory. Sterile neutrinos, if they exist and if they are heavy enough, should emit faint pulses of X-rays as they decay into lighter neutrinos. Alexander Kusenko of the University of California, Los Angeles, has focused on regions of the sky thought to contain lots of dark matter but few stars or other light sources. Sure enough, he claims to have found2 the predicted X-ray signature in Willman 1, a dim dwarf galaxy that orbits the Milky Way. But, like Bennett, Kusenko says that it is still too early to be claiming a discovery. "Everything seems to fit together," he says, "and it's tantalizing."
Kusenko's neutrinos would be heavy and plentiful enough to be the dark-matter particles themselves. But the WMAP data point in another direction, towards a sterile neutrino at the lighter end of the spectrum of possible masses. George Fuller, director of the Center for Astrophysics and Space Science at the University of California, San Diego, in La Jolla, would find this lighter mass agreeable, because it would help him to solve a long-standing problem with supernovae. These massive star explosions cause neutrons to fuse together, creating elements heavier than iron that end up in other stars and in planets. But they also produce inordinate numbers of neutrinos, which should, in theory, inhibit fusion because they kill off neutrons by changing them into protons and electrons.
Sterile neutrinos could provide a way out. If some of the neutrinos created in supernovae morphed into sterile ones, they would fly off without interacting with neutrons, leaving many of them intact. "We're very interested in this, because the stakes are so high," says Fuller.
Sobering news for the sterile-neutrino hunters came this month3 from the Main Injector Neutrino Oscillation Search (MINOS), an experiment that shoots a beam of neutrinos from an accelerator at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, underground to a detector 735 kilometres away in Minnesota to observe neutrino oscillations. The MINOS data show that one family of neutrinos does not have a high propensity for turning into sterile ones, although spokesman Robert Plunkett says that the results do leave room for sterile neutrinos. "We're limiting the parameter space for them."
Another Fermilab experiment, built to follow up on the earliest clues to sterile neutrinos, potentially offers more encouragement. The original evidence came from an experiment at Los Alamos in which Louis took part. From 1993 to 1998, he and his colleagues used the Liquid Scintillator Neutrino Detector experiment to shoot antineutrinos — neutrinos' antimatter counterparts — into 167 tonnes of mineral oil. Their data on how antineutrinos switch from one species to another suggested four distinct flavours. A decade later, data from a follow-up test also at Fermilab — the Mini Booster Neutrino Experiment, or MiniBooNE — found no evidence of a fourth neutrino type. But the MiniBooNE had used a beam of neutrinos, making it difficult to compare its results with the antineutrino data from Los Alamos. Now, after collecting data for a year and a half with antineutrinos, the MiniBooNE experiment sees a different pattern (see 'Neutrino hunters'). When combined with other antineutrino experiments worldwide, says Louis, its data fit "beautifully" to a 3 + 1 model: three ordinary neutrinos plus a sterile one4.
Louis, who is also a part of the MiniBooNE collaboration, says that the next batch of data, to be released this summer, should give a better idea of whether this fleeting interloper has finally been caught.
Komatsu, E. et al. Preprint at http://arxiv.org/abs/1001.4538v1 (2010).
Lowenstein, M. & Kusenko, A. Preprint at http://arxiv.org/abs/0912.0552 (2009).
Adamson, P. et al. Phys. Rev. D 81, 052004 (2010).
Karagiorgi, G., Djurcic, Z., Conrad, J. M., Shaevitz, M. H. & Sorel, M. Phys. Rev. D 80, 073001 (2009).
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Hand, E. Hunt for the sterile neutrino heats up. Nature 464, 334–335 (2010). https://doi.org/10.1038/464334a