In the last post, I said that dark matter could be a new type of particle that only interacts weakly, which is why we've never seen it before. The goal of my research is to build a very sensitive radiation detector and directly detect a WIMP (by observing the energy released on that rare occasion when a WIMP does interact with something in the detector). This is hard. Given our current limits on dark matter, we expect to see maybe a handful of events per year in our detector. That means we would run our detector continuously for an entire year, and we might see a single event that we could point to and say that it was a WIMP.
If that was the only requirement, building such a detector wouldn't be so hard. The difficulty arises in the fact that there is radiation flying all over the place all the time that is not associated with dark matter, and our detector is sensitive to that as well. This is known as background. It's as if you were trying to have a conversation with someone at a loud party who refused to raise his voice. Because of all the background conversations, it would be very hard to understand what that person was saying. A dark matter detector has a similar problem. For example, because of energetic particles passing through the atmosphere called cosmic rays and other ambient sources of radiation, a standard radiation detector (a Geiger counter is shown in the picture) goes off about 100 times per second. Or 10 million times a day. Or 3.7 billion times a year. And we want to be sensitive to 1 event. Imagine trying to hear what one particular person was saying when half of all the people on Earth were speaking at the same time and that's what dark matter experimentalists are trying to do.
How do we plan to do this? First, we will put our detector underground (in an active nickel mine in Sudbury, ON). This has been done with great success by neutrino experiments in the past – if the detector is underground, the earth helps shield the detector from cosmic rays, knocking the background down to say just the population of the USA. Second, we want to use very clean materials in our detector – if you can purify and clean everything very well, you can get rid of many sources of background that are always just lying around. Specifically, we plan to use liquid argon or liquid neon as our detector materials. These elements are very easily purified, so that we can remove anything that might produce radiation before filling our detector. Argon and neon have the great property that when exposed to radiation, they will “scintillate” or produce light. That will be our signal, in that we will look for flashes of light produced by a WIMP interacting in the liquid. In addition, the size of an argon or neon detector can be quite large, helping increase the size of our dark matter “target.”
Finally, we hope to reduce the majority of our backgrounds by using the timing of the light produced by an interaction. Most backgrounds in our detector are caused by radiation scattering off of electrons – these are called “electronic recoils.” A dark matter event would occur from a WIMP scattering off a nucleus, or a “nuclear recoil.” These two types of events have different time signatures in the scintillation light, and we can use the timing to tell them apart.
Our plan is to build a sensitive detector, eliminate all the backgrounds, and listen for that one interesting conversation.