So I’ve been putting off on posting what I do for a living for a while now.
Firstly because it takes a while for me to compress this stuff down and have a reason to do it. Secondly because of the same problem of when I make small talk with people (”oh what do I do? I work on experimental physics” followed by either the person feeling inadequate (which their not), in awe or mildy intrigue which leaves me trying to explain what I do. It always ends up with me being even more awkward). But I guess it needs to be done now.

So I work on a collaboration called CDMS, or Cryogenic Dark Matter Search (I’m a lowly lab monkey (i.e. research assistant) and have run calibration runs of detectors and worked on the next generation of electronics for the project.) There are about 20 different institutions involved in this collaboration (I’ve worked at Case Western Reserve University in Cleveland and University of Colorado Denver) We had an announcement today that we maybe saw dark matter, mostly likely, but not certainly.

So what is dark matter? Basically it stems from an astronomical observation over 70 years old. That Galaxies, based on all the matter that we can see shining in the nights sky, look less massive than what we observe their gravitational effects to be. This is not by a small amount, rather dark matter makes up 90% of all matter in the universe. It’s dark because it doesn’t interacted via the electro-magnetic force, nuclear strong or nuclear week force. It only interacts via gravity or it directly bumping into something. We know something must be there but what? The leading theory is for a particle we call a wimp (Weakly Interacting Massive Particle)

So here is how the experiment works: We cool down a small puck of germanium (1cm thick, ~5cm across) to 40 miliKelvin and watch what particles hit it. A lot of different types of particles permeate most matter and hit the detector. The detector measures 2 things, phonons and charge. Phonons, a quanta of vibrational energy, is essentially heat. By cutting the background heat down to almost nothing we can use superconducting filaments of tungsten (called Transitions Edge Sensors, or TESs) in arrays called QETs(Quasiparticle-trap-assisted electrothermal-feedback Transition-edge sensors) on the crystal of germanium to detect the heat of these vibrations. When a particle hits an atomic nucleus in the crystal the detector has a phonon rippling through it and we can detect it. The charge sensors detect any electrons being hit by a particle that float up to a collection layer on the back end of the crystal. This allows us to detect when something hits the detector that isn’t electrically neutral so we know it isn’t dark matter.

So we had about 3 kilos of these detectors running for about 2 years inside the Soudan mine in Minnesota (we put it underground to cut out cosmic rays to lower our background). The runs ended back in 2007. Applying the constraints I listed above plus a few others in the last 2 years of analysis of the data we saw 2 events that fit the dark matter theory. (I should note that we used blind analysis, so we weren’t making the points we saw fit the criteria, rather they were in a “black box” and we only looked at the results about a month ago).

So, we saw events, but that doesn’t mean we detected dark matter? The chance that a random even met our criteria twice in this run is about 20%. So we can say that there is an 80% chance that this is dark matter.This is not good enough for science. What we need is about 5 events to say “yes, this is dark matter”

So why meh? it’s potently big news right? But this is expected. When you first see dark matter, in a growing experiment like CDMS you expect the first time you see the particle to be one or two times. The next stage of CDMS (this was technically CDMS II) is SuperCDMS where we effectively will increase the mass five fold. So after this next run the possibility is that we see ~10 events, but the results are 3-4 years out. It’s not our of the ordinary, it’s not a definitive “yes”, it’s a maybe. So meh!

NYTimes Article on the subject
Sensationalistic Guardian article. Who says the brits know science.