Fermilab scientist: Why I search for dark matter
Two weeks after my first date with Brooke, I found myself Skyping with her at 11 p.m. on a Tuesday from 6,800 feet below the surface of the earth while trying to explain dark matter.
Using the camera on my computer, I showed her the detector that we were distilling refrigerant into (slowly), the weird whitewashed walls of SNOLAB (the underground lab in Sudbury, Ontario, that was the site of the experiment that won the 2015 Nobel Prize in physics), and the very attractive blue jumpsuit and hair net I was wearing because the entire laboratory is a clean space and all workers have to shower and change just to get in the door.
In retrospect, the fact that she didn't hang up immediately was a good sign.
I am a physicist at Fermilab in Batavia, and my job is to understand what dark matter is. That sounds pretty great, and it sounded good to me as a young graduate student when I first got into it, but eventually I came to realize that detecting dark matter particles is hard. They really don't want to be found. A dark matter particle could pass through more than 10 quintillion miles worth of lead without hitting anything.
Today, the physics community generally accepts that 95 percent of the universe is made up of stuff that we've never seen before and do not understand. We think about a quarter of this unknown density is in the form of dark matter. Although the phrase "dark matter" sounds sci-fi and romantic, it's actually a simple description of what we think it is -- matter that does not interact in the usual way with light, unlike the "ordinary matter" that makes up the Earth, people, the Sun, the periodic table and everything else that we can see in the sky at night.
Despite the fact that we don't actually know what dark matter is, many physicists think the circumstantial evidence for its existence is quite strong. Probably the simplest piece of that evidence comes from observations of rotating galaxies, many of them made by the late Vera Rubin in the 1970s (observations for which many people think she should have been recognized with a Nobel Prize).
Just as the Earth orbits the Sun, the stars of a galaxy orbit about the center, and we've all seen beautiful pictures of spiral galaxies in rotation. Astronomers like Rubin measured the speed at which those galaxies are spinning. She could then predict how much mass is required to keep stars moving at that speed bound in orbit. It turns out that there's not enough visible mass in the center of the galaxy to keep the galaxy together. This is evidence for some invisible matter that serves as an extra binding force.
Observations of much larger galaxy clusters find a similar result: there must be more matter than is visible. The best model physicists now have for our universe requires a large amount of dark matter. In this model, dark matter acts as the gravitational glue that allowed small pieces of matter to merge together into larger and larger ones, eventually forming the beautiful night sky that we see today. Without it, the Milky Way wouldn't exist -- nor would our solar system, nor us.
So, we think dark matter exists, but we don't know what it is. This is where I come in. The goal of my research is to directly detect a dark matter particle and to understand where all this extra mass is really coming from. The most popular candidate, and what I spend most of my life seeking, is called a WIMP, a cute acronym that turns out to be a quite literal description of what we're looking for: a Weakly Interacting Massive Particle.
A heavy particle that interacts weakly, or a WIMP, is exactly the kind of thing that could be the dark matter. We simply wouldn't have observed it before because the weak interaction is so rare.
The goal of my research is to build a very sensitive radiation detector and directly spot a WIMP (by observing the energy released on that rare occasion when a WIMP does interact with something in the detector).
This turns out to be hard. Given current limits on dark matter, we expect to see maybe a handful of events per year in our detector. That means we could run our detector continuously for an entire year, and we might see a single event that we could point to and say that it was a WIMP.
If that were the only requirement, building such a detector wouldn't be so hard. But problematically for our experiment, radiation from natural sources (i.e. not dark matter) is flying all over the place, all the time. Our detector is sensitive to all of this as well. Because of energetic particles called cosmic rays passing through the atmosphere and other ambient sources, a good radiation detector goes off about 100 times per second. Or 10 million times a day. Or 3.7 billion times a year. And we want to be sensitive to one event.
Imagine trying to hear what one particular person is saying in a room with half the people on Earth speaking at the same time.
Our first step to solving this puzzle is to go underground where the earth helps shield the detector from cosmic rays. This knocks the background down to, say, just the population of the U.S. The second trick is to use only very clean materials in our detector, which gets rid of as much ambient radioactivity as possible. Finally, we use the details of the signals we detect to figure out what kind of event caused it -- something we already know, or a dark matter interaction.
The plan, then, is to build a sensitive detector, eliminate all the backgrounds, and listen for that one interesting conversation.
Which brings me back to my Skype conversation on that Tuesday night in SNOLAB. The detector filling went fairly smoothly, so I was able to hear one particular person: the woman on my screen. While that run of the experiment did not detect dark matter, it did teach us more information about WIMPs than we have ever known before.
And the Skype conversation turned out to be just as interesting, as I recently got back from my honeymoon.