'Sterile' neutrinos may solve cosmic conundrums

By Aussiegirl

According to a brief Wikipedia article, a sterile neutrino is "a neutrino that does not interact via any of the fundamental interactions of the Standard Model" -- another example of physics humor, like the names of the six quarks: up, down, charm, strange, top, bottom -- as well as the name "quark" itself.

New Scientist Breaking News - 'Sterile' neutrinos may solve cosmic conundrums

'Sterile' neutrinos may solve cosmic conundrums
NewScientist.com news service
Maggie McKee

An as-yet undetected type of neutrino could explain a host of astrophysical conundrums, from the nature of dark matter to the ignition of the first stars, a new study suggests. But verifying the particle's existence could prove difficult.

Neutrinos are elementary particles produced in the nuclear furnaces inside stars and in supernova explosions. They come in three known types – called electron, muon and tau. Experiments within the last decade have proven that neutrinos oscillate from one type to another, something which is only possible if they have some mass.

Their mass is still not known, but the fact that they have mass implies there must be a fourth type of neutrino, says Scott Dodelson at Fermilab in Batavia, Illinois, US. That is because all other particles come by their mass through the union of two components with opposing quantum characteristics, called spin.

All three known types of neutrinos have left-handed spin, so researchers argue there must be another type with right-handed spin. Called "sterile" neutrinos, they are thought to interact with normal matter only through gravity and to be more massive than ordinary neutrinos.

If sterile neutrinos had masses trillions of times greater than their normal left-handed cousins, they would have decayed into these lighter neutrinos within the first second after the big bang. But if they are within 100,000 times or so the mass of normal neutrinos – or a few thousand electron volts – most should still exist, with some occasionally decaying into lighter neutrinos and X-ray photons.

Dodelson and Lawrence Widrow of Queen's University in Kingston, Canada, suggested in 1994 that such relatively low-mass sterile neutrinos could make up the dark matter that appears to outweigh normal matter in the universe by a factor of six.

Then, researchers led by Alexander Kusenko at the University of California in Los Angeles, US, calculated that sterile neutrinos produced in supernova explosions could "kick" the neutron stars created in the supernovae to speeds of 1000 kilometres per second – a phenomenon that had previously been unexplained.

Early universe
Now, Kusenko and Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn, Germany, say sterile neutrinos could also have helped the first stars form. Observations released in 2003 from NASA's Wilkinson Microwave Anisotropy Probe (WMAP) suggested that the first stars started ionising gas after only 200 million years. That puzzled astronomers because it was apparently too soon for gas to clump into stars in the first place.

But new WMAP results, released just last Thursday, suggest the ionisation did not happen for 400 million years, loosening the tight constraints on the formation of the first stars. But Kusenko says the timing is still several hundred million years earlier than expected, based on standard star formation theories.

He says sterile neutrinos could close the gap by spurring the formation of molecular hydrogen in the early universe. Molecular hydrogen cools gas clouds, helping them contract and form stars.

The molecule can form when two hydrogen atoms bond. But Kusenko says the reaction takes place much faster if one of the atoms is ionised. And he says this ionisation could have been caused by the X-rays produced when sterile neutrinos decayed in the early universe.

"So sterile neutrino decays ionised just enough gas to produce molecular hydrogen, and that accelerated the production of the first stars," Kusenko told New Scientist.

He says the fact that sterile neutrinos could account for such a wide variety of astrophysical puzzles is a "highly non-trivial coincidence" and a "strong indication this may be right".

Dodelson says the beauty of the work lies in the fact that it does not introduce totally exotic particles. "Everyone believes these things exist," Dodelson told New Scientist. "That's the main thing they have going for them – it's an elegant, minimalist way to solve some problems in the universe."

"I don't think there's any particle experiment that has ruled out the existence of what they're talking about," agrees Gary Feldman of Harvard University in Cambridge, Massachusetts, US. "But it's only one of many possibilities for dark matter."

Confirmation of this "dark matter" sterile neutrino could come from observations of clusters of galaxies – which are thought to contain dark matter a million billion times the Sun's mass. These neutrinos might be detected by the X-rays produced when they decay. But Kusenko says the observation will be difficult to interpret: "The problem is to distinguish this [spectral] line from all sorts of ordinary lines produced by gas in the cluster."

He says researchers may have a better chance of finding different types of sterile neutrinos that are less massive than those believed to be responsible for dark matter. Several years ago, an experiment at Los Alamos National Laboratory in New Mexico, US, turned up evidence of what appeared to be a sterile neutrino with a mass of about 1 electron volt.

Fermilab is currently running an experiment called MiniBooNE to verify that result and is expected to announce its findings in 2006. "It may be a brother of this [dark matter] neutrino," says Kusenko, adding that its discovery would show that "other sterile neutrinos could exist in the world".