wimping out

JuicedupjenlucLike many denizens of teh Internets, I'm a huge fan of Mr. Deity, the online satirical series of shorts (usually around 5 minutes long) about a silver-haired middle-aged deity –reimagined as the ultimate Hollywood producer — and his two sidekicks, Jesus and Larry (not to mention his on-again, off-again relationship with Lucy, a.k.a., Lucifer). It's clever, well-written, and filled with amusing one-offs. Take this tossed-off bit from "Mr. Deity and the Really Big Favor," when Mr. Deity receives a call from Larry, who's currently in the 13th dimension overseeing the construction of the nascent universe. Larry and the construction guys are wondering "how dark do you want to go with the dark matter?" Mr. Deity responds, "Take it all the way to void and then back off a scosch…"

Dark matter was the topic du jour over the weekend at the APS April Meeting in St. Louis, which features several sessions on experimental searches for this elusive stuff. Quick recap: the current model for the actual "stuff" in the universe calls for only about 4% regular matter. That would be every visible bit in the universe, from galaxies and stars to quarks and leptons, and everything in between. The rest of the universe is comprised of the mysterious dark energy (73%), which is causing the expansion of the cosmos to accelerate, and dark matter (about 23%). Oh, and neutrinos might be 0.001% of all the stuff in the universe.

There's pretty strong indirect evidence of dark matter's existence already, most dramatically seen in the now-famous (among science geeks) Bullet Cluster imageDarkmatter2 (see the Spousal Unit's excellent description of what's going in the image at right here). But nobody really knows what this stuff actually is. The two leading contenders are massive astrophysical compact halo objects (MACHOs) and weakly interacting massive particles (WIMPs). The former would be things like black holes, neutron stars, and brown dwarfs, plus any other similar objects that emit little or no radiation and therefore escape detection by our instruments. MACHOs are still technically "normal" matter. WIMPs would be something else entirely, a new type of matter that pretty much never (or almost never) interacts with regular matter — and therefore is even harder to detect than MACHOs. They only interact through the gravitational and weak nuclear forces. Mr. Deity made the dark matter very dark indeed!

WIMPs seem to share certain qualities with neutrinos, often popularly referred to as "ghost particles" (a term that makes at least one neutrino researcher I know grit his teeth in distaste) because billions of them pass through our bodies every day unnoticed. (John Updike even wrote a famous poem about them back in the 1950s, called "Cosmic Gall.") Like WIMPs, neutrinos interact only rarely with other subatomic particles, although they're definitely a lot "chattier" than WIMPs, and scientists have gotten pretty good at finding them — using gigantic vats of liquid and really big, sensitive detectors buried deep underground, for instance. Perhaps that explains why many facilities and detectors currently studying neutrinos can also be used to search for WIMPs. It's dual-use technology for the cosmology crowd!

The big rumor buzzing around our St. Louis hotel is an expected announcement sometime in the coming week that the DAMA-LIBRA collaboration, housed in an underground laboratory in Gran Sasso, Italy, will report confirmation of an earlier, highly controversial 2000 experiment in which they claimed to have detected dark matter. I missed all the hullabaloo eight years ago, since cosmological matters weren't my usual "beat," but it's not hard to find quite a bit of information on the controversy via Google. I won't go into too much detail here, except to say that physicists were not convinced that the original DAMA experiment had detected a clear signal for dark matter. Instead, they thought it was probably a systematic error stemming from the high degree of background noise associated with DAMA's particular experimental approach: looking for a tiny signal variation in a sodium iodide detector over the course of a year, supposedly due to the motion of the Earth through the cosmic dark matter background.

The DAMA team vehemently disagreed, and a major battle ensued in the pages of the physics journals and associated trade press — "major" for physics, anyway. Judging from the collaboration's current home page, some of them are still a little angry about the reception their finding received: the page bears a quote from "If," by Rudyard Kipling: "If you can bear to hear the truth you've spoken/twisted by knaves to make a trap for fools,… you'll be a Man my son." (Three guesses who the "knaves" might be!) In 2002, a French underground experiment known as EDELWEISS (or, as Jen-Luc Piquant phrases it, "Experience pour DEtecter Les Wimps En Site Souterrain") failed to confirm the result; so did the Cryogenic Dark Matter Search (CDMS) experiment (based in the Soudan Mine), which announced its null result in May 2004. DAMA-LIBRA is a re-running of the original experiment using the same basic set-up and technology — in an attempt to repeat that first result, reproducibility being the heart of the scientific method.

I don't know if the rumors are true about DAMA-LIBRA making a big announcement this week, and I'm hardly qualified to pronounce any technical judgment even if they do. But it became pretty clear to me after the April meeting press conference on searching for dark matter that DAMA-LIBRA will have a hard sell facing them if such an announcement does transpire: physicists are still inclined to be skeptical. In fact, the general consensus among the featured speakers — Tom Shutt of Case Western University, Jody Cooley of the ongoing CDMS collaboration, and Juan Collar of the University of Chicago — is that any new result from DAMA-LIBRA will have to be confirmed by other dark matter experiments, using different (but complementary) approaches, before anyone can definitively conclude that we have directly detected dark matter — whether it be WIMPs or something else entirely. To that end, they're using liquid noble gases, solid state devices using germanium and silicon crystals cooled to cryogenic temperatures, and even resurrecting bubble chambers and pressing them into the service of the search for dark matter. And they're going deep underground for their searches, to filter out all the "background noise" from radiation emitted by other particles — you know, like cosmic rays.

The DAMA experiments, among others, use a "scintillating material" to detect atoms being kicked around by a WIMP, which would generate very faint light pulses that would then be detected. Noble gases are excellent materials for this purpose. (If any physicists would care to weigh in on why this is the case, feel free to do so. I think it has something to do with the fact that noble gases naturally block the passage of many radioactive particles, which could interfere with detecting dark matter signals.) Shutt's Large Underground Xenon (LUX) experiment will use xenon. Xenon is the heaviest noble gas and turns to a liquid at -100 degrees Celsius. The detector will have both a large pool of liquid xenon, and a layer of the gaseous version just above it.

Whenever a WIMP strikes a xenon atom, it will emit a flash of light, which will be recorded by photosensitive detectors. That's step one. But since electrons get bumped off the atom at the time of impact, they will be pulled through an electric field out of the liquid and into the gaseous layer, emitting a second flash of light when they encounter the gaseous xenon atoms. Something about the specific ratio between those two flashes of light will comprise a telltale "signal" for a collision between a xenon atom and a WIMP (as opposed to a neutrino or other type of particle, like those pesky cosmic rays). The signal will be different in part because a WIMP should strike the nucleus of an atom, whereas cosmic rays or neutrinos would strike the electrons on the nucleus' surface. This will change the "recoil" behavior (similar to what happens during the first break in a game of pool) of the atom post-impact — ergo, it is a unique "signature."

Interesting historical side note: LUX is housed in the abandoned Homestake gold mine in Lead, South Dakota, home to the very same cavern where physicist Ray Davis built an experiment that gave rise to the solar neutrino problem in the 1950s. (Neutrinos were first detected by Frederick Reines and Clyde Cowan. The photo below shows Davis taking a dip in the pool of water that surrounded his neutrino detector: 100,000 gallons of a chlorine solution housed 4850 feet below the ground. Jen-Luc opines that this is probably more of Ray Davis than anyone cared to see, but I think the photo's kinda charming.)Danc05davisswiml

Meanwhile, in another region deep in America's Heartland, CDMS has moved its experimental headquarters to the Soudan Underground Laboratory, in an abandoned iron mine some 700 meters below ground in Eli, Minnesota. The site also houses the MINOS (Main Injector Neutrino Oscillation Search) facility, which is investigating the mystery of neutrino oscillations. But Cooley and cohorts are more interested in the dark matter.

As cold as it gets in Minnesota during the winter, joked Cooley, it's still not cold enough for the cryogenics of their experiment. The germanium and silicon crystals they use are about the size of hockey pucks, according to Cooley, and are then cooled down to about 50 milliKelvins (i.e., "really, really cold"). Cooley suggested we try to envision the crystal lattice structure as being held together by springs (rather than the traditional model of rigid posts) connecting the individual atoms. When a WIMP passes through a crystal, it supposedly sets off tiny vibrations whenever it bump into an atom, which can be detected via a layer of tungsten-aluminum metal. The metal is superconducting at very cold temperatures, but the vibrations in the crystal resulting from the WIMP-atom collision.

Of course, they also sense vibrations from other sources, such as cosmic rays, hence the location of the experiment deep underground to use the Earth as a kind of shield. CDMS  also uses lead and copper for additional shielding to cut down on the problem of background noise (which plagued the DAMA collaboration). Because, in Cooley's words, WIMPs aren't all that chatty (unlike cosmic rays), even quieter than neutrinos, and are thus hard to "hear" over the usual noisy subatomic "hubbub." CDMS is pretty darn sensitive.

Last month, Cooley's team announced  some intriguing new results that apparently set an upper limit on certain key parameters, thereby excluding several of the numerous theoretical models that have been proposed for where the dark matter would likely be seen. It's the best upper limit achieved thus far; any model that predicts values above that can be safely excluded "because we would have seen it." And they didn't. (The actual numbers, for those who care, are something like a mass of 60 GeV/c<2>, with a size of 4.4 x 10<-44> cm<2>. The size is important because that's what determines the level of "chattiness," i.e., how much/often the WIMPs interact with ordinary matter.) The detectors are now being upgraded for even more sensitive experimental measurements in 2009.

Over at the University of Chicago, Collar is taking a very different approach, resurrecting the relatively old technology of bubble chambers to search for dark matter. His project is called the Chicagoland Observatory for Underground Particle Physics (COUPP) experiment, located 350 feet underground in a tunnel on the Fermilab site. Bubbles chambers were nearly extinct in high-energy physics labs before Collar hit upon the notion of using them to search for dark matter. (They're great as neutrino detectors, too.) While the basic technology might be old, Collar insisted, "This is not your daddy's bubble chamber." Bubblechamb

COUPP's "detector" is basically a glass jar filled with a liter or so of a fire-extinguishing liquid (iodotrifluoromethane) — a simple bubble chamber, and a refreshingly inexpensive approach to this very fundamental problem in cosmology. When a WIMP hits a nucleus of one of those atoms, it triggers an evaporation of a small amount of that liquid, producing a tiny bubble. It's initially too tiny to see, but it grows, and that growth can be recorded with digital cameras. Once the bubbles reach about one millimeter in size, the COUPP scientists can study the images in earnest, looking for telltale statistical variations between photographs. Ideally, this enables them to distinguish whether a bubble resulted from background radiation, or from a dark matter particle.

Like the CDMS collaboration, Collar's group has succeeded in placing some fundamental limits on certain properties for WIMPs — and if you combine that with the findings of EDELWEISS and CDMS, it doesn't look good for DAMA. But time will tell. Next on the agenda for COUPP is to increase the detector's sensitivity by increasing the amount of liquid from one liter to around 30 liters. (One assumes this helps because there are that many more atoms with which to encounter the occasional WIMP.) Collar has also just installed a new compact neutrino detector (germanium-based) 330 feet below ground in the sewers of Chicago (renting this unusual lab space for the city — apparently Chi-Town has one of the longest systems of tunnels every built, in its case, to control flooding). The design has been modified to detect not neutrinos, but WIMPs.

So for all intents and purposes, bubble chambers are back, baby! Talk about a stunning comeback. Still, Collar and the others emphasized that "there is no perfect dark matter detector out there." Each approach has its own strengths and weaknesses. Which is why it's highly unlikely that one single experiment will conclusively "demonstrate" the first detection of dark matter. "One day lots of lines [of data] from all these different experiments will cross," explained Collar — and that will constitute what a criminal lawyer might call a preponderance of evidence verifying direct detection. "We all weigh in from different directions" and compare results, according to Shutt. That includes upcoming experiments at the Large Hadron Collider at CERN, which will look for "missing energy" in its collisions as a telltale signal for direct detection of the dark matter.

It's going to make for a one hell of a tough call by the Nobel Prize nominating committee one of these years — because one of the reasons the competition is so fierce, is that the first group to achieve that pivotal first detection is pretty much a shoo-in for an all-expenses-paid trip to Stockholm. I guess if all else fails. Mr. Deity could step in as arbitrator.

5 thoughts on “wimping out”

  1. I think the “lines [of data]” should be more like “lines [of research]” or maybe [of inquiry] or something. What they mean, I think, is that the approaches from different directions will all converge on one answer. Your “[of data]” seems like not the right editorial insertion.

  2. AFAIK, xenon (and argon too) are nice for this purpose because they’re dense liquids with large atomic numbers. This makes nice, big targets for particles to smack into. In addition, their “ionization potentials” are low, which means that it doesn’t take much energy to kick an electron away from the nucleus. This makes it easier for scintillations to happen, so you get a better scintillation yield. Argon is cheaper than xenon, but it has the problem that it’s likely to be contaminated with the radioactive isotope Ar-39, and radioactive decay causes interfering events of its own.

  3. Jeff Filippini

    To add to Blake’s comments, the use of liquid targets is also motivated by ideas of scalability – if you want a bigger target, you just make a bigger tank. Things aren’t quite that simple, of course – you need to ensure that the big tank is sufficiently low in radioactivity and has sufficient discrimination power to distinguish a dark matter signal from any radioactive backgrounds (e.g. from the glass of the light detectors which line the tank), but the collaborations involved are making excellent progress in these areas.
    Argon is much like xenon for these purposes, but requires a significantly larger detector (due to the smaller atomic mass of argon) and should have vastly more discrimination power against backgrounds. The question is the balance between the extra discrimination power and the added radioactivity of the argon itself (though less radioactive sources of argon are being identified).
    Very powerful background discrimination is the traditional strength of CDMS (the current world leader) and similar cryogenic detectors, but the construction of a large cryogenic detector array presents substantial challenges of its own.

  4. @Joshua:
    I think the idea is that one set of data will exclude a certain range of dark matter parameters, then another set will exclude a different range, and another set another range — and so on, until there is a very definite, small region of parameter space in which the dark matter particles must be living. Identifying that region will be seen as “detection” in the same way that determining the mass of the top quark counted as “detection.”

  5. The neutrino fraction should be 0.001 of the critical density, not 0.001 percent. This is assuming the sum of the three neutrino masses is about 0.05 eV/c^2.

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