Through a (Noble) Gas, Darkly

There's a minor kerfuffle at the moment over the XENON experiment's early data (arxiv paper) which did not detect any dark matter in 11 days of data acquisition. This conflicts with earlier claims by the DAMA experiment and recent maybe-kinda-sorta detections by the CoGeNT and CDMA experiments.

As a result, a couple of members of other collaborations have posted a response on the arxiv saying, basically, that they don't believe the sensitivity claimed for the XENON detector in the energy range in question, and that their result can't really be said to rule out the possibility of dark matter in that area as well as they claim. This has generated a little buzz, and some back-and-forth in the news story.

So is this the catfight of the century? Not really. This is more or less the way things are done, as far as I can tell-- people make claims to have seen things all the time, and those claims are questioned in very strong terms. That's how science works.

Is there any merit to the questions being raised about XENON? Almost certainly-- these types of detectors are fearsomely complicated devices, and the data analysis of such devices almost always include some steps that verge on Black Art-- simulations used to estimate sensitivity, complicated algorithms to decide what data to keep and what to discard, etc. There will almost always be differences of opinion on these issues, especially with a detector as new as the XENON project's.

The one thing that's a little different here is that this is being done via dueling arxiv preprints, rather than an exchange of informal letters, private discussions within a collaboration, or even testy questions at conferences. And, of course, these days we have science-themed media outlets who watch the arxiv for signs of this sort of thing, making it more of a story than it might otherwise be. As the Physics World piece says, "The real test of the XENON100 collaboration's analysis will be its peer review." What's going on here is just the first steps of that process of peer review.

It does seem a little odd to have the debate conducted through strongly worded pre-prints, but that probably just indicates that other channels failed for one reason or another. Many of the scientists involved are members of more than one collaboration, and some of these issues may have been raised during pre-posting discussions within the collaboration, and not dealt with to their satisfaction. It's tough to say without more knowledge of the people and politics than I have.

(Full disclosure: I co-authored a paper with Dan McKinsey at Yale, one of the authors of the response pre-print, and a person quoted in the Physics World story, and I got endorsements of my NSF proposal a few years back from both McKinsey and Elena Aprile of XENON. My interactions with both of them were (and are) very positive, but that's about where my knowledge of the issues involved end.)

(Also, technically, the detector involved uses liquid xenon, not gaseous xenon. I couldn't resist, though.)

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I don't know much about CoGeNT, but please don't refer to the CDMS result as even a "maybe-kinda-sorta" detection. After having gone to a few CDMS talks and talked with CDMS people, even THEY aren't claiming anything like that. They just don't have the statistics to make any sort of claim. They expect a background of 0.7 events and they see 2 -- rudimentary Poisson statistics (though probably not exactly correct in this case) tells you that 2 events is well within the realm of probability for an expected background of 0.7.

It does seem a little odd to have the debate conducted through strongly worded pre-prints, but that probably just indicates that other channels failed for one reason or another.

"Other channels" have included entertainingly heated exchanges at several recent workshops. I never thought of physics as a spectator sport before, but this has been great fun.

I agree that the "voicing objections through the archive" is a bit unusual, but what should one do if:

1) One thinks there are problems with with the low-energy-tail calibration of the XENON detector as used in the analysis.

2) The problems were raised to the XENON collaboration. (This is speculation on my part, but given everything else I can't see this not being the case.)

3) The XENON collaboration is putting out press releases before peer review (

In that context, publishing a detailed analysis of the perceived shortcomings of XENON's data analysis/calibration on the arXiv seems like the most appropriate course of action.

And I certainly agree with Chad's overall analysis: I'm sure this will all get sorted out; this is how science works.

By Anonymous Coward (not verified) on 07 May 2010 #permalink

This may be a lot more than simple bias on the side of XENON100. The footnote in the "Comments" begs to be carefully read.

By Anonymous (not verified) on 07 May 2010 #permalink

Well where is the dark matter? How many different ways are there to look for it. Don't you think it is somewhat peculiar that despite all the different ways scientists have sought to look for it, they have not yet found it? A high lot of high profile scientists believed in the ether and it was never found. Scientists think there is no decent believable theory that is out there to explain the need for the dark matter. There is one, however, and its a simple, believable theory. The trouble is it comes from an amateur, me. I have five experiments which show that when a test mass is placed between a hot source and a cold source its weight will change by 1.9%, -4.9%, 8.9%, 9.6% and 16%. They can be found at . They suggest that it is the earth's heat that attracts the moon and not its mass or that it is the sun's heat and not its mass that attracts the planets.