As I mentioned earlier today, the rumor is that in the next day the LIBRA experiment will release its first follow-on results to the DAMA experiment. Why is this such news? Let’s review.
In 2000, the DAMA experiment published an observation of an annual modulation signal which they interpreted as evidence for direct detection of dark matter. Once can interpret that signal in terms of a dark matter mass and dark matter coupling to matter. In such a plot of coupling vs. mass, DAMA appears as the cyan blob at the top of the plot to the left.
Just a few years later, the Cryogenic Dark Matter Search released their first results from a run in the Soudan Mine in Minnesota [1]. They excluded the DAMA result completely. If interpreted as a dark matter signature with certain couplings to matter, DAMA would seem unlikely. In fact, looking at the above plot we see that it’s unlikely by about an order or magnitude.
In the period after CDMS, there was a flurry of activity by some members of the community trying to explain and resolve the paradox. Many ideas for how a natural radioactive source could fake the DAMA signal were proposed, and DAMA claimed that none were compelling enough to explain their signal. Meanwhile, some member of DAMA claimed it was unfair to interpret their result along side CDMS since the interpretation of CDMS’s results requires assumptions about the spin structure of the dark matter interaction that might not apply to DAMA.
Other experiments were getting underway. The KIMS experiment [2] in South Korea would try to also measure a modulating signal, but with an independent collaboration and a different detector technology [2]. They published results in 2007, ruling out DAMA using the same annual modulation measurement technique.
New technologies for direct detection of dark matter have entered the scene. The XENON experiment, with its liquid noble detector, have competitive results with CDMS but are facing a serious challenge in controlling their background (CMDS is still background-free). In addition, an experiment is ongoing at the University of Chicago and Fermilab called COUPP [5], a bubble-chamber based dark matter detector that has presented preliminary results from their prototype and are eagerly scaling the detector and tackling their backgrounds. COUPP has also ruled out the DAMA signal in a different interaction framework.
Most recently, CDMS have released their latest run with the full CDMS detector, and now have the strongest limits on the field on the dark matter coupling vs. mass (see figure above). They have exceeded XENON’s efforts and it seems a race is on between a background-free solid-state technology (CDMS) and a liquid-noble detector which has dreams of getting bigger and bigger, but needs to address its backgrounds. This is all very exciting to watch, and I get to have a front-row seat!
If there is some exciting announcement from LIBRA, let’s not forget that there are half a dozen searches for dark matter going on and currently only one claims an observation that cannot be confirmed by any other technology. Will LIBRA balk in the face of conflicting evidence and stick to their one-horse race, or once revealed help to chart a path forward for the search for dark matter? What will it mean is LIBRA sees a signal but KIMS has not?
[1] http://cdms.berkeley.edu/conference_talks_links/APS_Apr07_Qiu.pdf
[2] 2007 Results of the KIMS Experiment, (PRL 99, 091301)
[3] http://www-coupp.fnal.gov/
[4] http://cdms.berkeley.edu/conference_talks_links/CDMS_5Tpreprint_080222.pdf
