Thursday, January 05, 2017 ... Deutsch/Español/Related posts from blogosphere

XENON100 rejects DAMA/LIBRA dark matter modulation at 5.7 sigma

DAMA/LIBRA is an Italian dark matter experiment that most colleagues apparently don't take too seriously. In recent years, it has claimed to detect some clear signatures of dark matter, especially through the dark matter seasonal modulation. Some effects are different in the summer and in the winter, and so on. This is what you would expect from a dark matter counterpart of the "aether wind", if I dare to borrow to a debunked concept. ;-)

DAMA/LIBRA is some microwave oven with some sensitive pieces within 1 meter of concrete in all directions. A theorist must have a similar idea about it as I have about a laser printer, Canon LBP7018C, whose 3-of-4 cartridges I attempted to replace yesterday but the non-original compatible ones got stuck in it and the printer seems broken now. Probably not my fault but I can't be sure. ;-) The Rutgers+Harvard experience taught me that a dedicated professional is needed to maintain a laser printer.

The most recent DAMA/LIBRA paper is this 2013 update which says that the statistical significance of their observed – nominally discovered – dark matter modulation signal is 9.3 sigma. If true, it's a discovery on steroids.

When the pro-dark matter side seemed to be winning in the dark matter wars, there were other reasons to think that this modulation exists. CoGeNT has confirmed some modulation in 2011, too.

The atmosphere is different now and the anti-dark matter side seems to be on an offensive. It's particularly clear from the today's new preprint by the XENON collaboration:

Search for Electronic Recoil Event Rate Modulation with 4 Years of XENON100 Data
XENON, one of the most formidable dark matter detectors in the world (I think that LUX and XENON are upgrading and fighting for the leadership), investigated the annual modulation as well. Using the 2010-2014 data, they have evaluated the theory with dark matter modulation and parameters suggested by the claimed DAMA/LIBRA signal. And they have excluded this theory. Experimenters normally tend to disprove theories with newly proposed effects at 2 sigma or 95% confidence level – because the "null hypothesis" to which they return to isn't extraordinary and doesn't need extraordinary evidence.

But in this case, they could take the DAMA/LIBRA claim to be a "natural hypothesis" and XENON excluded it at 5.7 sigma – because it's more than five, you may say that XENON has made a discovery that DAMA/LIBRA is wrong.

Now, an alternative explanation is, of course, that XENON is wrong and DAMA/LIBRA (and perhaps CoGeNT) are still right and the dark modulation exists. If you want to defend this viewpoint, you may point out that the number 9.3 (the significance level of DAMA/LIBRA) is greater than the number 5.7 (XENON's negative significance level). Not too many people in the field will take this attitude – but needless to say, they may be wrong and their attitude may mainly reflect the group think.

At any rate, if there's some group think in the discipline, it's complex and softly self-contradictory. The researchers in that field significantly favor dark matter. But they also favor the idea that the Italians are too messy to discover it. ;-) (Just to be sure, I know that XENON is Italian, too.)

I don't know who is right. At least with some significant certainty. It would be painful if XENON missed this discovery despite their vastly more expensive experiment. But it may happen. For example, I still do tend to think that the positive claim about the discoversable primordial gravitational waves by BICEP* has a reasonable chance (20%?) to be right, despite the mission of their competitors Planck to kill anything of the sort.

The situation of XENON and DAMA/LIBRA has one more subtlety. In the new paper, XENON does see some modulation but it is weak – 1.9 sigma – and its period is 431 plus minus 15 days, rather safely away from the expected 365 days. So everyone is gonna assume that this weak "signal" is just noise.

Yesterday in The New York Times, Lisa Randall argued that Why Vera Rubin Deserved a Nobel. (Vera Rubin died on Christmas Day.) Well, maybe, yes, no. I think that Lisa's article is highly incomplete and tendentious.

You would think that an article titled "Why Vera Rubin Deserved a Nobel" (because of her contributions to the research of dark matter) would contain at least most of the basic data about the discoverers of dark matter. I don't think it does. Lisa writes that Rubin is "most often attributed" with establishing dark matter's existence. The NYT article doesn't even mention Fritz Zwicky who deduced dark matter (dunkle Materie) in 1933, using the virial theorem. I think that Zwicky is actually the scientist most often attributed with the discovery of dark matter, and rightfully so, and Randall's claim to the contrary is a part of the feminist propaganda.

XENON100 seems larger than DAMA/LIBRA.

Well, people may equally say that my perspective is biased because Zwicky was also a male, like me, who was also surrounded by many spherical bastards (they are equally bastards from any direction) and whose mother was Františka Vrček, an ethnic Czech – so Zwicky was Czech to the same extent as Ivanka Trump. But I think that it's ludicrously obvious that the priority would have to belong to Zwicky.

Another reason is that if dark matter were really proven, Zwicky was more certain of its existence while Rubin was hesitating, see this comment of mine about the (somewhat overstated) article "Vera Rubin Didn't Discover Dark Matter" by Richard Panek, a big Zwicky fan.

But these Zwicky-vs-Rubin disputes aren't too relevant for one reason: We are not terribly certain that dark matter is the right explanation of the anomalies. Given the not quite negligible "risk" that the right explanation is completely different, something like MOND, it could be very strange to give the Nobel prize for "it". What "it" even means? Look at the list of the Nobel prize winners. No one has ever received the Nobel prize for the discovery of "something" that no one knew what it actually was – a new particle? Black holes everywhere? A new term in Newton's gravitational law? The normal contribution rewarded by Nobel prizes is a clearcut theory that was experimentally proven, or the experimental proof of a clear theory. Even though most cosmologists and particle physicists etc. tend to assume dark matter, dark matter-suggesting observations aren't really belonging to this class yet.

And I think that this is the actual main reason why Vera Rubin hasn't gotten the prize for dark matter – and no one else has received it, either.

Add to Digg this Add to reddit

snail feedback (0) :

(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){ (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o), m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m) })(window,document,'script','//','ga'); ga('create', 'UA-1828728-1', 'auto'); ga('send', 'pageview');