It is a strange headline – No WIMPS in the Xenon, but then Dark Matter is strange. It supposedly must exist, in fact, it makes up 25% of the material in the universe. However, it has never been seen. Not seen even by the latest super high sensitivity detector project called XENON100. Located at the Gran Sasso National Laboratory in Italy, XENON100 is looking for the signature of dark matter – a WIMP, a Weakly Interacting Massive Particle. Put another way, Dark Matter is supposed to be a (relatively) massive elementary particle, which (unfortunately for science) doesn’t interact much (or at all) with normal everyday matter. That makes it (very) difficult to detect. In fact, this latest and most sensitive test by XENON100 has not detected any WIMPs in a year’s testing.
However the motivation to detect a WIMP is great. If the existence of dark matter can be demonstrated by finding WIMPs, then an array of models and theories in physics, related to what is referred to as the supersymmetry model (SUSY) become at least partially validated. What’s at stake (besides a lot of jobs and reputations in theoretical physics) is the ability to fill in gaps of explanation for the way in which the cosmos (universe) formed, and further explanation for the behavior of fundamental particles at the sub-atomic scale.
The work at the Gran Sasso Laboratory, a large-scale collaboration between 14 different institutions from the United States, China, France, Germany, Israel, Italy, the Netherlands, Portugal and Switzerland has used a device containing 100 pounds (actually, 62 kg) of liquid xenon (in gas form, used for automobile headlights, for example) to interact with any WIMPs passing through it. Xenon has an unusually large nucleus, which makes it a better target for passing WIMPs. When (and if) they do strike a xenon nucleus, the xenon will emit a blue glow, which can be captured by photosensitive devices located in the device. In order to avoid false detection and reduce the amount of ‘noise’ (non-WIMP radiation), the device is located more than a mile underground and specially shielded with water, lead and copper.
Now, after 100 days of operating experiment during 2010, nothing has been detected within the limits of sensitivity for this device. You will see reference to this as: Dark Matter experiment shows nothing OR Dark Matter experiment moves one step closer to detection. It is remindful of the expression: For optimists the glass is half full. For pessimists the glass is half empty. For an engineer, the damn glass is twice as big as it needs to be. Finding just the right size experiment could be crucial to locating dark matter – or not. So the next step, called XENON 1T, will use one ton of Xenon (actually 2500 kg) and a more sophisticated detection setup. XENON 1T is scheduled to happen within this decade.
It should be mentioned that the idea of WIMPs and SUSY (supersymmetry) has been controversial from the outset and the continuing lack of solid evidence increases the level of skepticism. Experiments at this scale of physics are expensive, in a world where allocation of resources to scientific projects is always under pressure. Naturally, scientists with competing theories want SUSY experiments to ‘taper off’ (putting it finely). People who believe in the existence of WIMPs, also naturally, feel that they are closing in on evidence. Some time probably in this decade, between the XENON 1T project and the results of the Large Hadron Collider (LHC), there may be a line drawn for lack of evidence. Otherwise it could be very much like the expression, “Now that we know there are no unicorns in New York City, we are much closer to finding them.”
More Information: National Science Foundation, Press Release