The first deep space neutrinos to be detected since the 1980s may be the spawn of mystery dark matter. That would explain puzzling features of these particles – and suggest an unusual identity for dark matter.
Neutrinos, ghostly subatomic particles, are routinely produced by the sun and on Earth, but apart from those seen after a 1987 supernova explosion, none had been detected from beyond the solar system.
Then, earlier this year, the IceCube collaboration, which monitors a cubic kilometre of ice at the South PoleMovie Camera, reported two deep-space neutrinos, dubbed Bert and Ernie, each with a mass of about 1 petaelectronvolt (1015 electronvolts). These were quickly followed by reports of a bunch more, with masses of tens of teraelectronvolts (1012 eV), mass and energy being equivalent for particles.
Deep-space neutrinos are prized because they could allow “neutrino astronomy” – using neutrinos to investigate mysterious cosmic objects. Being chargeless, neutrinos zip from a source direct to Earth without being waylaid.
However, expected sources of such neutrinos, including energetic explosions called gamma-ray bursts or emissions from supermassive black holes called active galactic nuclei, should also produce neutrinos of energies different from those seen by IceCube so far.
Pasquale Serpico of the University of Savoy in Annecy-le-Vieux, France, and colleagues wondered if the lack of these other energies could be a sign of decaying dark matter – the invisible stuff thought to make up about 80 per cent of the universe’s matter.
They calculate that heavyweight dark matter particles of about 1 PeV would decay either directly into neutrinos of about 1 PeV, or into other particles and then into neutrinos with energies of tens of TeV. “It exactly reproduces the features that you see in IceCube,” says Serpico.
This comes hot on the heels of recent reports from several dark matter detectors, which have seen signs of much lighter particles, with masses of about 10 gigaelectronvolts.
Tom Weiler of Vanderbilt University in Nashville, Tennessee, says there is no theoretical reason why dark matter shouldn’t be heavy. The production of such particles would require more complicated mechanisms in the early universe, so theorists tend to prefer lighter particle candidates. “But Nature is the arbiter, not theorists,” says Weiler.
Or the mysterious stuff might come in flavours – both light and heavy. “In general, the physics community prefers to have a single dominant dark matter [type], but it doesn’t have to be so,” says Serpico.
Francis Halzen, of the University of Wisconsin-Madison, and the principal investigator of the IceCube collaboration, isn’t convinced by the new theory. The neutrinos seen by IceCube can still be explained by standard sources if the gap in neutrino energies goes away as the experiment collects more particles. “I do not think that anything in the data requires a more exotic explanation at this point,” he says.
Dan Hooper, a theoretical physicist at Fermilab in Batavia, Illinois, agrees: “My money is on an astrophysical origin for these neutrinos, rather than dark matter.”
Whether the neutrinos come from dark matter will become clearer as IceCube amasses more neutrinos and the gaps in energies either persist or vanish.
“If the hypothesis is correct, the birth of neutrino astronomy coincides with the discovery of dark matter,” says Weiler.