Diving into Dark Matter (and Acronyms)!16th July 2018
Deciding on Dark Matter
In my first quarter at Stanford, I started working with physics professors Dan Akerib and Tom Shutt on the LZ dark matter experiment at SLAC National Accelerator Laboratory on the recommendation from one of my undergraduate mentors. I wasn’t particularly interested in dark matter, but my mentor had worked with Dan before and she told me that the group was great.
She turned out to be right! I loved my job. I loved going to work every day. I loved my colleagues. Never had I been a part of a team that felt so much like family. But working on dark matter wasn’t really what I had pictured when I came to grad school, so I knew I wanted to try working with some other groups before I made my final decision. I worked with two other, very different groups for the winter and spring quarters. Though I learned a lot from both of them, I realized that neither of those groups were really for me. So at the end of the spring quarter, I went and told Dan that I wanted to come back for good. Even though it wasn’t what I initially expected to be working on, I had found my new academic home with the LZ experiment.
But what does LZ stand for, you ask? Physicists are sometimes not great at naming things. The name LZ is a combination of the names of two previous experiments, LUX and ZEPLIN, both of which are also acronyms (I call it acronym-ception). These two different experimental groups have come together to build the world-leading experiment LZ, whose name comes from its predecessors.
The LZ experiment is looking for dark matter, which makes up about a quarter of the cosmos. Even though we can’t “see” dark matter the way we see other things (by bouncing particles of light off of it), we know that dark matter exists because we can see the effect it has on other things. For example, it causes stars to rotate around the centers of their galaxies more quickly than we would expect considering only the visible matter. It also causes light to bend around galaxy clusters more than it should if you only account for the visible galaxies. According to simulations, if dark matter wasn’t there, the stars and galaxies would simply fly apart! These things, among other evidence, leads us to the conclusion that there must be some invisible, ‘dark’ matter there as well!
There are many different theories about what dark matter could be. One popular and well-motivated theory says that dark matter consists of WIMPs, or Weakly Interacting Massive Particles. “Weakly interacting” means that it doesn’t hit regular matter very often, so it usually passes straight through the earth without hitting other particles. It’s probably passing through you right now! But the probability of dark matter hitting regular matter is small - not zero. If you have a detector that is very large and dense, it has a higher probability of intercepting a dark matter particle. That brings us to the LZ experiment.
The LZ experiment will contain 10 tons of liquid xenon, and will be located nearly a mile underground in an old gold mine in South Dakota. Xenon is ideal for this experiment because, among other reasons, it is very heavy and dense. The idea behind the experiment is that a dark matter particle passes into the detector and hits an atom of xenon. This collision creates a burst of light which is picked up by sensors on the top and the bottom of the detector. The collision also liberates some electrons, which are drifted to the top of the liquid using an electric field applied across the detector. When they get to the top of the liquid and cross into the gas, they create another burst of light. Using the patterns in the sensors, the relative size of the signals, as well as the time between the two light signals, we can essentially tell what kind of particle interacted in our detector. Even though our detector is big, we only expect to see 5-10 dark matter particles over the course of three years!
The biggest obstacle to detecting dark matter this way is the large number of background particles (particles that aren’t dark matter) that also hit your detector and create signals. There are lots of other particles, like muons (the heavier cousin of an electron) raining down from space in the form of cosmic rays, that are around in abundance and would completely swamp any dark matter signal that might be there. Unlike dark matter, these particles are not weakly interacting, so we go a mile underground in order to put a mile of rock above our heads and shield our experiment from most of the incoming particles. Many materials also emit other types of particles, so all of our equipment and materials have to be extremely pure and clean. We also put the detector inside a big tank of water in order to further shield it, this time against particles coming from the rock of the cavern walls.
When it is completed in 2020, LZ will be the most sensitive dark matter detector in the world. Will we actually discover dark matter? Who knows! All we can do is build the best detector we can and hope that Nature cooperates with us. If not, we’ll go back to the drawing board.
LZ is an international collaboration of over 250 scientists and engineers from around the world, and SLAC is one of the major players. At SLAC, we have several main roles. First, we have built a small-scale version of LZ here which we call the “System Test”. Why? Because it tests systems. I told you we were bad at names. This prototype tests many of the critical systems of LZ, including things like xenon circulation and the high voltages needed to create the electric fields. We have a number of post-docs, graduate students, undergraduate students, engineers, and other research scientists who are responsible for building, maintaining, and running this detector. We then analyze the data that comes out and transfer this knowledge to inform the design of LZ. We make sure to solve any problems ahead of time, so they don’t slow us down when we want to turn our full-scale detector on for the first time.
Our second role is that of xenon purification. In order to eliminate as many backgrounds as possible, the xenon that goes into LZ has to be very pure. Since xenon is a noble element, it is actually rather easy to purify. The biggest problem is krypton - another noble gas. That means typical purification techniques won’t work on it. And the xenon can only have fifteen parts per quadrillion of krypton. That’s a million times higher than in the xenon we buy! At SLAC we are building a system that will clean all of the xenon using one ton of charcoal. When we push the xenon and krypton through the charcoal, the xenon travels through more slowly than the krypton. Over time, they separate and we can get rid of the krypton. All 10 tons of LZ xenon will first come to SLAC to be purified before being shipped off to South Dakota.
Finally, we are responsible for creating the LZ “grids”. These are giant metal meshes that are used to create the electric fields for the detector. There was nothing commercially available that fit the bill, so we designed and built a literal loom in order to weave these grids. It actually quite amazing - you can find a nice video here.
Two years after deciding to join LZ, I still love my job. I still love going to work every day, and even though some have left and some have joined, our group still feels like a family. Doing physics in our group requires a number of different tasks and skills. Going to work each day I face new challenges and learn new things. We design things, build things, break things, fix things. Analyze data, run simulations to make predictions. Use the physics we have learned in school to determine design drivers, and make decisions on the best design. We work independently to drive projects forward, and also contribute to team efforts. Some of my days are spent at my computer trying to figure out what our latest data is trying to tell us. But my very favorite days are when I get to work with my small team, crawl around in the lab, and work on building the detector. It’s like adult Legos!