Saturday, February 28, 2015
Crunch time for pet theory on dark matter
Thought to make up the Universe’s missing matter, WIMPs are running out of places to hide.
After decades of searching and multiple no-shows, it is crunch time for a leading theory of what comprises dark matter, the mysterious stuff thought to make up around 85% of the Universe’s matter.
The Large Hadron Collider (LHC) at CERN, Europe’s particle-physics lab near Geneva, Switzerland, is scheduled to restart in March after a major upgrade. It is widely seen as the last chance in a generation to create — and thus confirm — theoretical particles known as WIMPs, or weakly interacting massive particles. A super-sensitive ‘direct-detection’ experiment, which is designed to catch naturally occurring WIMPs streaming from the heavens, is also due to start this year.
At the same time, the failure so far to glimpse WIMPs at either the LHC or through direct-detection experiments, combined with surprise signals from others, is fuelling suggestions that dark matter is made of something else. A range of alternatives that were previously considered underdog candidates now look “less exotic”, says Kevork Abazajian, a theorist who studies particle cosmology at the University of California, Irvine.
Whatever dark matter is, most astronomers believe that it is real. The amount of ordinary, ‘visible’ matter does not produce enough gravity to explain the speed at which stars move inside galaxies, or at which galaxies move inside galaxy clusters. Dark matter would solve this mystery because it does not absorb or scatter light, making its presence known only by its gravitational pull on normal matter. Different theories posit different suggestions as to what kind of particle would have these properties.
WIMPs are theoreticians’ darlings. They are relatively heavy — somewhere between 1 gigaelectronvolt, or roughly the mass of one proton, and 1 teraelectronvolt — and thus would be relatively slow, or ‘cold’. These properties fit well with the current best models for the evolution of the Universe, in which haloes of cold dark matter are the prime movers in the formation of galaxies and galaxy clusters. WIMPs also fix problems in two separate branches of physics: particle physics and cosmology. The mass of the WIMP and the strength of its interactions with other particles would help to explain why the Higgs boson has the mass it does. But these figures also mean that WIMPs would have been synthesized at just the right rate in the early Universe for the creation of the abundances that theory requires today — a coincidence that is dubbed the ‘WIMP miracle’.
Yet despite being on physicists’ most-wanted list, WIMPs remain on the run. When the LHC shut down for maintenance in 2013, its WIMP searches had come up empty. And the most sensitive of the direct-detection searches, carried out by the Large Underground Xenon (LUX) experiment at the Sanford Underground Research Facility in Lead, South Dakota, found no WIMPs during its first major run in 2013
(see Nature http://doi.org/zh8; 2013)