The XENON100 dark matter experiment now has a paper out reporting their first results, from a test run of 11 days. They claim a 90% confidence level exclusion of 50 GeV WIMPs with a spin-independent elastic cross-section above 3 x 10-44 cm2, which one can compare with the recent CDMS limit of 3.8 x 10-44 cm2 at similar mass (70 GeV). At the time of the CDMS result many seemed to believe that the two events seen were not background (see here), with Gordon Kane claiming “it is likely it is dark matter.” The New York Times has an article today, with Kane now commenting on XENON100 “if they see a signal, it will be unambiguous”, by which I presume he means that their full data will conclusively show whether the CDMS events could have been a signal. Given that they are seeing nothing at all now, at 10 times greater sensitivity later, they may very well see an ambiguous signal…
The XENON100 result also appears to rule out claims from the DAMA and CoGeNT experiments to have seen some sort of signal. For more on this, as always in dark matter issues, Resonaances has the best coverage of the story.
Update: Physics World reports on objections to the XENON100 claims from a CoGeNT physicist and others, see this arXiv preprint.
“It’s the strongest statement about dark matter today and it reads: we have looked here and there and over there but didn’t find nothing,” Rafael Lang of Columbia University, wrote…
I guess Columbia doesn’t teach remedial English.
Rafael Lang is a postdoctoral researcher. Columbia didn’t teach him anything, in that he never took a course there. He works there.
Oh, and he is German. Sorry that some people have less than perfect English… for it being at least his second language, I would say he does reasonably well…
Suppose XENON doesn’t see WIMPs even when it reaches the maximum sensitivity that people have planned for it. What would this tell us? What kinds of WIMPs would be ruled out as a plausible explanation of the amount of cold dark matter that we seem to see? And roughly when would this happen?
By definition, a WIMP interacts with nucleons via the weak nuclear force, implying an elastic cross section of the order of 10^-44 cm2. So already an order of magnitude improvement in the experimental limit would start to make things uncomfortable.
On the other hand, there’s so much uncertainty surrounding the properties of dark matter, and so much interest, that a sequence of much larger experiments beyond XENON100 (LUX 300 kg, XENON1T 1 ton, LZS 1.5 tons, LZD 20 tons) have been studied/ planned/proposed. So my guess is the search for WIMPs will go on for another 5 years at least.
WIMPs may have originally meant “weak nuclear”, but that is a strong version of the definition compared to the way the term is used today. Call it more “roughly weak nuclear” or even “sub-weak nuclear” more accurately.
Xenon100 will start to become sensitive to backgrounds at roughly one year of running. So end of this year, expect their big jump in improvement. It will take a bite out of, say, MSSM parameter space, but not one that will make SUSY practitioners uncomfortable.
Anonymouse, my understanding is that the observed abundance of dark matter particles as thermal relics of the Big Bang implies that their self-annihilation is of the order of the weak nuclear force. And hence they are usually considered to be WIMPs in the strict sense (the so-called ‘WIMP Miracle’) This conclusion holds regardless of whether they arise from supersymmetry or something else.
We sell the WIMP miracle as a pretty picture, but it is far from encompassing the spectrum of possibilities for dark matter or WIMPs. That said, I think it is likely to turn out the WIMP miracle does explain the relic abundance of dark matter.
However, when one gets precise, the thermal abundance actually requires “sub-Weak strength” interactions. Literally weak stength leads to a cross section which is too large, causing the WIMPs to stay in equilibrium longer, and they are depleted in the Universe by too much (e.g. there is too little dark matter to match observations). So in fact if we take the WIMP miracle seriously, defining WIMP to be “weakly interacting” in the strict sense is in fact excluded by measurements from WMAP and other experiments.
It’s true that none of this requires supersymmetry, but my hope is that my answer above provides context which is useful to someone like John Baez, who asked what Xenon100 can do and on what kind of time scale