Friday, December 18, 2009

Did they see it?

Has dark matter been detected (rumours were abound)? The CDMS (Cryogenic Dark Matter Search) experiment has detected some potentially tantalising evidence for the first direct detection of WIMPs - it saw 2 events! (I think the paper will be on the arXiv here.) However, they make no claim for detection, as they say:
In this new data set there are indeed 2 events seen with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made, in essence that the ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be less than one chance in a thousand of the signal being due to background. In this case, a signal of about 5 events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.

so these results can't be heralded as a significant for a while (with the events having a one in four chance of being background you're not going to convince anyone), but there is maybe room for some excitement - and setting upper limits is still fun! Right! It's nice to have definitive results in science, but in new discovery fields like this (and probably as will happen in my own field of gravitational wave detection) you need to take your time.

What's particularly ironic (from a UK perspective) is that yesterday's STFC funding announcement dropped the Dark Matter experiments at Boulby mine!