Monday, December 14, 2009

Dark matters



The latest rumors about forthcoming hot results from the Cryogenic Dark Matter Search at the Soudan Mine in Minnesota only confirm what I've always suspected: Physicists! They're just like us! They love rumors!

Into this wild Abyss
The womb of Nature, and perhaps her grave—
Of neither sea, nor shore, nor air, nor fire,
But all these in their pregnant causes mixed
Confusedly, and which thus must ever fight,
Unless the Almighty Maker them ordain
His dark materials to create more worlds,—
Into this wild Abyss the wary Fiend
Stood on the brink of Hell and looked a while,
Pondering his voyage; for no narrow firth
He had to cross."
—John Milton, Paradise Lost
In many other ways, physicists are not just like us. To get to work, CDMS physicists descend 2,341 feet via a rickety metal cage elevator to the 27th level of an abandoned iron ore mine. Once there, they monitor frozen towers of hockey puck-sized germanium crystals for the incredibly gentle rattle of a passing dark matter particle. The dress code is bunny suits, hairnets and slippers, and the cement walls of the "office" are spattered with the dehydrated remains of dead bats.

But when rumors circulated last week that CDMS had a paper coming out in the December 18th issue of the journal Nature, the physics blogosphere went mad in a way that reminded me endearingly of teenage girls pouncing on the latest US Weekly photo of Robert Pattinson holding hands with someone.

Nature is an extremely selective journal and only publishes physics research if it's likely to impact the field in a big way. Thus, wrote Ted Bunn, a physicist/blogger at the University of Richmond, the paper must be discussing big results. And for CDMS, that could only mean one thing: detecting dark matter, that mysterious stuff that physicists reason must make up about 85 percent of the matter in our universe in order for the observed dynamics of stars and galaxies to make any sense within accepted theories of gravity. It's invisible to our telescopes, be they infrared or gamma-ray, since it neither emits nor interacts with electromagnetic radiation. But sensitive detectors should be able to feel the barely tangible rumble as it passes, provided nothing else is moving. Bunn wrote:

"Nature usually only publishes high-profile results. If CDMS had a non-detection to report (even if it set a new and interesting upper limit), Nature would be less likely to accept it. Nature articles are embargoed until publication, meaning that the collaboration can’t release the results or talk about them until December 18. Members of the collaboration have canceled seminars before that date and scheduled talks at a number of universities to take place on that date."
Adam Falkowski, a post-doc at Rutgers, wrote in his blog Resonaances that CDMS's last analysis, published in 2008 and covering data from 2006-2007, turned up no dark matter particles, or WIMPs (weakly interacting massive particles.) "By now CDMS must have acquired four times more data. The new data set was supposed to be unblinded some time last autumn, and the new improved limits should have been published by now. They were not," Falkowski wrote. He then pointed to the Nature rumor and the fact that seminars had been canceled, likely due to the embargo. New Scientist wrote a blog post on the buzz, and ScienceNOW's Adrian Cho pointed out that even if the rumor was true it would take much more than a few signals to make a case that dark matter had been detected.

In fact, in 1998 a group of physicists monitoring working deep inside Gran Sasso mountain in Italy claimed they'd seen the flash of passing WIMPS in a 220-odd-pounds sodium iodide cystals. Although they couldn't be sure that the flashes they detected were due WIMPs and not other particulate culprits, they saw the amount of flashes rise and fall over the course of the year, consistent with how the Earth moves through the vast cloud of dark matter permeating our galaxy. Wired UK reported in August:
The Sun moves round the Milky Way at about 485,000mph, dragging its orbiting clutch of planets along with it. The Earth has its own motion circling the Sun each year as well. The upshot is that the Earth's total motion through space varies over the year, being about 30 per cent faster in June than December.

They even saw the signal in an upgraded version of the experiment, now 550lbs of sodium iodide. But neither CDMS nor its arch rival, XENON100 (as their name suggests, they look for signals in vats of ultra-pure liquid xenon), saw so much as a whisper. What's more, writes mathematician Marcus du Sautoy in the Times, the team have not made it easy to reproduce their potentially groundbreaking results:

Science depends on being able to reproduce experiments, and currently no one has been able to repeat the Italian team’s claim. Not through want of trying but because, according to the journal Nature, the only company that makes pure enough sodium iodide crystals, Saint Gobain in Paris, has signed an intellectual property agreement with the Italian team and is therefore unable to supply the crystals to anyone else.
A close-up of one of CDMS's crystal germanium detectors. Credit: Fermilab.
And the latest is that CDMS is unlikely to release any game-changing data this week. The physical sciences editor of Nature wrote a rumor-quashing email to Falkowski, which he promptly posted on Resonaances:

I was alerted to your blog of yesterday (you certainly don't make contacting you easy). Your "fact" #1, that Nature is about to publish a CDMS paper on dark matter, is completely false. This would be instantly obvious to the most casual observer because the purported date of publication is a Friday, and Nature is published on Thursdays. Your "fact" therefore contains as much truth as the average Fox News story, and I would be grateful if you would correct it immediately.

Symmetry Breaking published this statement from CDMS that says yes, analysis is coming, but not the kind that would make it into Nature, necessarily:

The CDMS collaboration has completed the analysis of the final CDMS-II runs, which more than doubled the total data from all previous runs combined. The collaboration is working hard to complete the first scientific publication about these new results and plans to submit the manuscript to arXiv.org before the two primary CDMS talks scheduled for Thursday, Dec. 17, at Fermilab and at SLAC.

A wave of deflated sighs rippled throughout the blogosphere. But in the aforementioned WIRED UK article, folks from both CDMS and XENON100 expressed a strong belief that 2010 will be the year of the WIMP:

This year, both projects have scaled up their experiments to make them more sensitive to dark matter, which both hope to nail within 12 months. "Next year will be an exciting one," says Elena Aprile, team leader for XENON100. "Hopefully we'll have a first result by early 2010 - there's a lot of expectation as to what will happen."

So one of these days, hopefully soon, a rumor that dark matter has been found will actually prove true. Or we might have to reconsider everything we think we know about gravity.

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