New limit on putative dark-matter particle

New limit on putative dark-matter particle. Several lines of evidence suggest that dark matter, the mysterious substance that makes up 83% of the mass of the universe, consists of subatomic particles known generically as WIMPs (weakly interacting massive particles). On cosmic scales, the presumed influence of WIMPs is easy to spot; the particles’ collective gravity controls the distribution and motion of stars in galaxies. Detecting single WIMPs is far harder. After running continuously for 100 days, the XENON100 experiment has now yielded the most stringent limits on WIMP properties. Situated in a cavern in Italy’s Apennine mountains, XENON100 consists of a bank of photomultiplier tubes that monitor 100 kg of liquid xenon. If a WIMP—or any other energetic particle—were to collide with one of the xenon atoms, the resulting scintillation would be detected. The overlying rock prevents cosmic muons from reaching the xenon, but the rock also contains radioactive elements, whose gamma-ray emission can beget scintillations. Although XENON100 incorporates ways to discriminate between the two sources of scintillations, some residual, unremovable background remains. In a paper submitted for publication, the XENON100 team report detecting three events that survived all the discrimination criteria. Given that the expected background was 1.8 events, the three events don’t constitute a firm detection, but they do lower the limits on the WIMP mass and the WIMP–nucleon cross section. Supersymmetry, an extension to the standard model of particle physics, predicts the existence of WIMPs of order 100 GeV/c², which is both consistent with the XENON100 results and within reach of the Large Hadron Collider. (E. Aprile et al., http://arxiv.org/abs/1104.2549 .) —