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.
Large Hadron Collider
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.
But WIMPs could show up this year. The souped-up LHC will smash together protons with combined energies of 13 teraelectronvolts — versus the 8 teraelectronvolts of the previous run: the extra energy will be capable of creating particles that were not possible to make before. Also, this summer, a WIMP detector called the XENON1T experiment, based under Gran Sasso in central Italy is scheduled to go live. It will have a sensitivity 50 times greater than LUX’s, says XENON1T spokesperson Rafael Lang, a particle physicist at Columbia University in New York.
SOURCES – Nature
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