Cryogenic dark matter detection

Whether this rumor turns out to be accurate or not, the technology used in the CDMS collaboration's dark matter search is quite interesting.  Working down the hall from these folks in graduate school definitely gave me an appreciation for the challenges they face, as well as teaching me some neat condensed matter physics and experimental knowledge.

The basic challenge in dark matter detection is that weakly interacting particles are, well, very weakly interacting.  We have all kinds of circumstantial evidence (rotation curves of galaxies; gravitational lensing measurements of mass distributions; particular angular anisotropies in the cosmic microwave background) that there is a lot of mass out there in the universe that is not ordinary baryonic matter (that is, made from protons and neutrons).  The dark matter hypothesis is that there are additional (neutral) particles out there that couple only very weakly to normal matter, certainly through gravity, and presumably through other particle physics interactions with very small cross-sections.  A reasonable approach to looking for these particles would involve watching for them to recoil off the nuclei of normal matter somehow.  These recoils would dump energy into the normal matter, but you'd need to distinguish between these events and all sorts of others.  For example, if any atoms in your detector undergo radioactive decay, that would also dump energy into the detector material's lattice.  Similarly, if a cosmic ray came in and banged around, that would deposit energy, too.  Those two possibilities also deposit charge into the detector, though, so the ability to identify and discount recoil events associated with charged particles would be essential.  Neutrons hitting the detector material would be much more annoying. 

The CDMS detectors consist of ~ cm-thick slabs of Si (ok) and Ge (better, because Ge is heavier and therefore has more nuclear material), each with an electrical ground plane (very thin low-Z metal film) on one side and an array of meandering tungsten micro-scale wires on the other side.  The tungsten meanders are "superconducting transition edge bolometers".  The specially deposited tungsten films have a superconducting transition somewhere near 75 mK.  By properly biasing them electrically (using "electrothermal feedback"), they sit right on the edge of their transition.  If any extra thermal energy gets dumped into the meander, a section of it is driven "normal".  This leads to a detectable voltage pulse.  At the same time, because that section now has higher resistance, current flow through there decreases, allowing the section to cool back down and go superconducting again.  By having very thin W lines, their heat capacity is very small, and this feedback process (recovery time) is fast.  A nuclear recoil produces a bunch of phonons which propagate in the crystal with slightly varying sound speeds depending on direction.  By having an array of such meanders and correlating their responses, it's possible to back out roughly where the recoil event took place.  (They had an image on the cover of Physics Today back in the 90s some time showing beautiful ballistic phonon propagation in Si with this technique.)  Moreover, there is a small DC voltage difference between the transition edge detectors and the ground plane.  That means that any charge dumped into the detector will drift.  By looking for current pulses, it is possible to determine which recoil events came along with charge deposition in the crystal.  The CDMS folks have a bunch of these slabs attached via a cold finger to a great big dilution refrigerator (something like 4 mW cooling power at 100 mK, for those cryo experts out there) up in an old salt mine in Minnesota, and they've been measuring for several years now, trying to get good statistics.  

To get a flavor for how challenging this stuff is, realize that they can't use ordinary Pb-Sn solder (which often comes pre-tinned on standard electronic components) anywhere near the detector.  There's too high an abundance of a radioisotope of Pb that is produced by cosmic rays.  They have to use special solder based on "galley lead", which gets its name because it comes from Roman galleys that have been sunk on the bottom of the Mediterranean for 2000 years (and thus not exposed to cosmic rays).  I remember as a grad student hearing an anecdote about how they deduced that someone had screwed up and used a commercial pre-tinned LED because they could use the detector itself to see clear as day the location of a local source of events.  I also remember watching the challenge of finding a wire-bonder that didn't blow up the meanders due to electrostatic discharge problems.  There are competing techniques out there now, of course.
Well, it'll be interesting to see what comes out of this excitement.  These are some really careful people.  If they claim there's something there, they're probably right.