It's crunch time for dark matter if WIMPs don't show

If dark matter isn't made of WIMPs, could neutrinos or axions fit the bill? What if it's not a particle at all but a strange modification of gravity?

ROADS may soon diverge in the dark matter wood, and some physicists want to take the ones less travelled.

The most promising candidate for a dark matter particle could be about to show itself at last, as it is running out of places to hide. But should the hunters fail to bag one of these WIMPs, or weakly interacting massive particles, the search for dark matter could be thrown into crisis.

At a meeting in Cambridge, Massachusetts, last week, researchers debated the best paths forward into the wilder landscape of less-favoured candidates, from alternate particles to changes to our theory of gravity.

"It's really refreshing," says Lisa Randall at Harvard University. "For years I went to conferences where people said, 'We know what dark matter is and we're just cutting out the parameter space'. I thought that was strange, because we really don't know what dark matter is."

So far we have only sensed dark matter's presence through its gravitational effects. But theory says that WIMPs should also brush shoulders with normal atoms occasionally, producing signals we can detect. WIMP champions are pinning their hopes on more sensitive underground detectors that are running or under construction.

"This is a golden decade for dark matter because of detector sensitivity," says Kathryn Zurek at the University of Michigan in Ann Arbor.

The trouble is that background noise can prevent usnoticing the impact of a WIMP. Beyond a certain sensitivity limit, the signal would be swamped by neutrinos, nearly massless particles that are constantly streaming from the sun and from particle collisions in our atmosphere. After just a few more upgrades, WIMP hunters will hit this limit and the desired particles may no longer be detectable.

Indirect methods for spotting WIMPs offer the best chance of a sighting. When WIMPs collide they should annihilate, shattering into other particles. This includes gamma rays, and an excess of these high-energy photons spotted in the centre of our galaxy seems to fit nicely with the simplest models for WIMPs. But one criticism is that the rays could just as easily come from fast-spinning dead stars called pulsars.

So if not WIMPs then what? Some theories modify the classic particle, changing its properties and offering new places to look. Others focus more on runner-up particles, such as axions or sterile neutrinos. And still others say dark matter might not exist at all, and we just need to modify the laws of gravity (see right).

"It's always possible WIMPs are just around the corner," says Avi Loeb at Harvard University. "But when there is no evidence, you have to be careful. We're looking for a black cat in a dark room."

reference : http://www.newscientist.com/article/mg22229712.600-its-crunch-time-for-dark-matter-if-wimps-dont-show.html#.U4rEXnaqJR4