Sixty-seven WIMPs spotted in the wild, maybe

Dark matter has slim evidence, cautious optimism

It’s not quite enough evidence to constitute a discovery, but scientists working on the CRESST experiment think they may have spotted Weakly Interacting Massive Particles (WIMPs).

WIMPs are one of they key theories to account for the “missing stuff” of the Universe. The amount of baryonic matter we observe with telescopes is too small to generate enough gravity to hold large structures like galaxies together, so astrophysicist have long hypothesized the nature of the matter and energy we can’t see.

The physicists’ prediction is that the WIMP would not carry either of the “strong” forces – the electromagnetic or strong nuclear force – and would only interact with gravity or the weak nuclear force. That makes it very difficult to detect, since the WIMP's interactions with other matter are uncommon.

CRESST’s (the Cyrogenic Rare Event Search with Superconducting Thermometers) search uses 300-gram crystals of calcium tungstenate – CaWO4 – cooled close to absolute zero, and located around 1,400 meters underground to minimize background noise from cosmic rays.

It’s also dark, which is handy because one of the very small signals the detector seeks is flashes of scintillation light given off after a particle collides with the crystal.

In this paper, CRESST says it has detected 67 particle interactions that can’t be attributed to noise in its observation run, which ran from June 2009 to April 2011. All of these were in the energy range below 40 keV (kilo-electron volts) that the experimenters consider the “acceptance range” for WIMPs.

Some of these, the authors state, can be associated with the decay of isotopes in the detector materials. They propose improving both their detector material (using crystals made from zinc tungstenate) and removing from the instrument possible sources of impurities that could have introduced decaying isotops (for example, bronze clamps).

Whether these are WIMPs or just random observations will probably have to wait for the next detector run. ®