Lately, Cecilia Levy’s contributions to LUX-ZEPLIN’s hunt for direct evidence of dark matter come in the form of computational physics — that is, the extraordinarily complex analysis of the data coming out of the dark matter detector. (As a postdoc, Levy also contributed to its assembly and commissioning.)

Cecilia's data analysis work begs a more fundamental question: How would we know dark matter if we found it? What would the data say about a thing we cannot see? 

We got deep into these cosmic weeds in a portion of our conversation that did not make the final edit but is nonetheless fascinating.

One programming note: You’re going to read a lot about xenon below. If you’ve heard of it before, you probably know that xenon is considered a noble gas — an element that does not chemically interact with much. It’s used inside LUX-ZEPLIN as a liquid, in part because it’s very dense and helps shield the inside of the detector from cosmic interference.

Here’s how Cecilia explained what happens next:

When you're looking at the data that comes out of the detector, what are you seeing that makes you say, “Aha! A WIMP just crashed into our xenon!”? What does that look like?

CL: The data analysis on something like this is absurdly difficult. Please understand that it's not just like, “Oh, boom, there was a little spike on an oscilloscope,” and we know we just detected the WIMP. It doesn't work that way at all. 

You cannot actually detect the WIMP itself. What you detect is its interaction with the xenon nucleus. Imagine a billiard ball collision. One ball is dark matter; the other is a xenon nucleus. So you're playing pool, and the dark matter interacts with the xenon and then goes on its merry way. It's gone. But because it's deposited some energy into the xenon, the xenon has recoiled. And with this recoil, you get light, and then you get a little bit of electrical charge. 

We look at this light, and then this charge, and we move it up into our xenon and make it interact again with other xenon atoms to get secondary light. What we're looking at is really the light that is being produced in our detectors. The xenon is what we call our target material. It's not the actual detector. The detector is all the light sensors around it. So the target material is what's going to say, “Hey, boom, there was an interaction.” And then all the light detectors say, “Okay, was there a flash of light in here? That's how we know.

Annoying question. 

CL: Go ahead.

Why is that direct evidence and not indirect evidence? Because you're detecting the light, right? Light is an indicator of an interaction in a place so quiet that it almost certainly had to be dark matter. But you didn’t see the dark matter. You saw the light produced by its crash. 

CL: Because you can’t see it!

Because it’s dark matter.

Right, you can’t. That's the whole conundrum here. If we could see it, we wouldn't have to do all this. And this is something that's really important to understand in physics. There are a lot of indirect things like that. Because very seldom do you actually see exactly what you’re looking for.

[gestures to her eyeglasses]

OK. Right now my glasses are on the table. You see them, you think immediately in your mind, “This is a direct detection of my glasses on the table,” correct?

Yes.

CL: Okay, well, I'm going to turn that argument against you. I'm going to say, “Actually, you do not see my glasses. What you see is the light from my glasses arriving into your eyes, which are the detector.”

Okay.

CL: Same thing. We call this direct [detection of dark matter] because what we are looking at is a direct interaction of a dark matter particle with our xenon nucleus. In our case, it's direct because there is a direct collision

And you can see the light.

CL: And I can see the light. The same way that you can look at my glasses because you see the light from my glasses.

Read about the latest results from UAlbany’s contributions to the LUX-ZEPLIN experiment

Check out this video from the Sanford Underground Research Facility to see what the LZ detector looks like and how they got it a mile deep into the mine. 

There are more photos here from the Department of Energy’s Lawrence Berkeley National Laboratory, which is leading the project.

Audio editing and production by Scott Freedman 
Photos by Patrick Dodson 
Written and hosted by Jordan Carleo-Evangelist 

The Short Version is produced by the Office of Communications and Marketing at the University at Albany, which is part of the State University of New York.

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