Monday, September 27, 2010

Sometimes It's Just Hay

Last December I wrote about rumors that an experiment called CDMS had found evidence for direct detection of the Dark Matter. I called my post "Searching for Unusual Hay in a Haystack" because the "needle" they were looking for (the dark matter) is so close in appearance to the "hay" (background events) that it is really hard to tell them apart. At the time, I said that it was quite likely that the "signal" of two events was just some background events that happened to look a lot like the signal they were looking for---that they had just found normal hay that looked a little unusual. And I concluded, "So we await future experiments with more signal and less background".

Well, that data has just been published in the journal Physical Review Letters. Here is a nice writeup of the results. In brief, an experiment called XENON100, which is much more powerful than CDMS, was able to take enough data in just its first 11 days of running to basically rule out the CDMS signal (in the plot pictured above, the solid black XENON100 line is below the dotted CDMS line on the left half of the plot, where CDMS signal events were found). Another way to put that is this: if the CDMS signal were real (not just a background fluctuation), XENON100 would easily have seen it. But XENON100 saw nothing unusual.

This is often the pattern on the frontiers of science. There is a hint of a signal, and then it is either confirmed or it is ruled out by a more powerful experiment. Alas, this time it was ruled out. So it's back to waiting for a hint of a signal from somewhere else.

[the plot is taken from the journal article, which is available here]