What is Dark Matter?

The world as people experience it is made of atoms: protons, neutrons, and electrons. When atoms get close together, they may interact with one another; the electrons orbiting the protons may get shared between atoms or simply repel. These interactions govern the vast majority of daily life, from chemistry to simply being able to touch objects. 

Dark matter is not made of ordinary atoms. While it doesn't attract or repel atoms normally, it does have mass, so it can exert its presence with gravity. At a cosmic scale, this becomes evident by watching how galaxies spin and how light bends from distant sources.

What is SuperCDMS?

SuperCDMS is an experiment attempting to detect dark matter. Our detectors are made of pure silicon and germanium. When a particle hits the detector, it makes a little "sound" that we can measure. The sound is made up of what are effectively particles called phonons. The two sides of the detector have 6 pieces called "channels", and each one listens to the sound individually.

Along with the sound, particles hitting the detector also knock negatively-charged electrons off of atoms. This leaves a conspicuously vacant part in the crystal, which is effectively a positively-charged particle called a hole. We can use this to our advantage by applying a voltage gradient to the detector. This pulls the electrons to one side of the detector and the holes to the other, emitting more phonons and amplifying the signal. 

Our detectors don't just detect dark matter. Other particles can hit the detector and confound our results. We shield ourselves by doing our experiments 2 kilometers underground, but there is still some background.

Luckily, there is a pattern to some of the background; they tend to occur closer to the edge of the detector. The channels closer to the edge of the detector will hear a louder sound.

Visualization

All of this is rather abstract. I have made a rudimentary visual aid to see what happens in the detector. 

There are three colors for the phonons in motion: the green ones are from the original recoil, the blue ones from the electron-hole pairs, and the orange ones from "recombination" where the electron-hole pairs reintegrate into the lattice.

While the physics is a lot more complex than what the model would suggest, the basic ideas still come through.

Note that only 1/5 of the true number of blue phonons are being shown here to decrease visual clutter.

Take a look at these two videos. On the left is an event generated at the very center of the detector, and the right has one generated close to the edge. By the end of the videos, you can see how the absorbed phonons are collected closer to the edge where the recoil was also closer to the edge.

In the videos above, you can see a plot of the amplitude of the pulse in both the inner (red) and outer (blue) half of the detector, as well as the total amplitude. It's simple to see here how the delay of for the pulse to begin can inform where in the detector an event happened.

Let's take a look at two events from our real detectors! Knowing that the F-channel is at the center of the detector and the A-channel is at the edge, where in the detector do you think these detector hits happened?