Science News - August 28, 2010

Nonetheless, says dark matter theorist Katherine Freese of the University of Michigan in Ann Arbor, “the low-mass region — that’s the one that everyone is excited about, up, down and sideways.” Hooper, who has been in the field for a decade, says it is hard to explain why XENON10 is not seeing a signal, but he still has hope that some of the hints of low-mass WIMPs will be credible.

“This is the first time in my career that I’m prepared to make bets with my colleagues that we’re actually seeing something,” he says.

New physics, hot debate

There’s a delicate balance in trying to make sense of the existence of a light WIMP, explains Weiner. On the one hand, a low-mass WIMP would have to interact with ordinary matter in an unusually weak way — otherwise, atom smashers would have already revealed hints of its existence. On the other hand, WIMPs that interact so weakly would tend to have too large an abundance in the universe today to fit with the WIMP miracle. They would have been less likely to be destroyed in the dense, early universe, when collisions between particles were more common.

One solution proposed by Weiner and
other researchers is that nature harbors
not only a low-mass WIMP but also a new
force in the dark sector. The low-mass
WIMP would account for the results
of the experiments, and the dark force
could have depleted enough WIMPs in
the early universe so that there would
not be too many around today.

WIMP wind the DaMa/LiBRa experiment has recorded seasonal variation in particle collisions. since ordinary matter collisions shouldn’t vary as the earth revolves around the sun, the team suspects that earth is traveling into a WiMP wind in the summer and away from it during the winter.