(Image: Enrico Sacchetti)

NESTLED deep under an Italian mountain range, the bug-eyed light detectors point inwards, poised to join the global hunt for the universe’s missing matter.

Observations of how stars and galaxies move show that about 80 per cent of the matter in our cosmos is invisible. This dark matter doesn’t glow and, except through gravity, scarcely interacts with ordinary matter. Just occasionally, though, it might, so physicists build sensitive detectors in the hope of catching a strike from a WIMP, or weakly interacting massive particle – a possible form that dark matter could take.

Around the world, WIMP detectors are piling up, but this newcomer, DarkSide-50, could still make a contribution. The 4-metre-wide, photodetector-lined steel sphere shown here will house a canister filled with 50 kilograms of liquid argon. If a WIMP hits an argon nucleus, the nucleus recoils and produces a telltale flash of light.


Housed under the Apennine mountains, in the Gran Sasso National Laboratory, DarkSide-50 has 1400 metres of rock protecting it from cosmic rays from space, which can mimic WIMP hits. For further protection, the sphere is filled with a liquid scintillator, which glows when charged particles hit. That in turn is immersed in a tank of pure water. The water blocks charged particles from the rock, and the scintillator reveals if any get through. Who said chasing darkness was easy?

This article appeared in print under the headline “Power of the dark side”