Did scientists just find a dark matter particle? Maybe

[AUDIO LOGO] FLORA LICHTMAN: Hey, this is Flora, and you’re listening to Science Friday.

We’re talking about a flashy new find in the hunt for dark matter. That’s the stuff that makes up 85% of all matter in the universe, but we have no clue what it is. A mile under Lead, South Dakota, you’ll find a giant caldron filled with seven tons of liquid xenon, encased in water and surrounded by super sensitive photodetectors. Known as the LZ detector, its job is to look for flashes of light that come from particles bumping into xenon atoms. And over the course of seven months or so, the researchers say they saw one very weird flash, a bump that doesn’t look like it could have come from any particle we know of.

So as you might imagine, physicists are cautiously pumped about this finding, especially my next guest, Dr. Richard Gaitskell, a physicist at Brown University and the spokesperson for the LUX-ZEPLIN Dark Matter Experiment. Richard, welcome to Science Friday.

RICHARD GAITSKELL: Thank you. You’re very kind.

FLORA LICHTMAN: Is cautiously pumped how you’d describe the mood?

RICHARD GAITSKELL: There’re I’ll so many ways of expressing emotion in scientific experiments. Obviously, we always try to keep that a little bit aside. We have designed a very rigorous statistical rather dry process for looking at these events that we’re collecting deep underground at the lab in South Dakota.

FLORA LICHTMAN: Let your hair down. Tell us how you’re feeling. Come on.

RICHARD GAITSKELL: Well, after 40 years of being very repressed, I’m afraid that’s– you’d probably barely notice when I let my hair down. But, yeah, there were a team of 250 scientists who run our experiments deep underground looking for dark matter events and a new analysis that we have just released a paper for does, as you describe, have a fascinating a single event that looks consistent with dark matter particle.

FLORA LICHTMAN: I read this is not the interaction you were initially looking for.

RICHARD GAITSKELL: We’re looking for dark matter that is most of the matter in our galaxy. We– these particles– if the hypothesis is correct, these particles in hundreds of millions of them are passing through you every second.

FLORA LICHTMAN: Right now. They’re in the room with us.

RICHARD GAITSKELL: Right now, they’re in the room. They’re moving– they’re moving pretty fast. And we’re trying to look for the very occasional interaction of one of these dark matter particles with our detector. Usually we look at very, very low energies, but we decided that we were going to expand the energy range over which we would look for dark matter interactions. So we’ve decided to go to the next level up of possible interactions. And this new result, we actually found an event that looks very consistent with what we expect with such next to leading order if you like or slightly more complicated interactions.

FLORA LICHTMAN: If dark matter particles are all around us and coming for us every second, why are there so few interactions?

RICHARD GAITSKELL: So dark matter, we actually give dark matter another label for these particle searches. We call them weakly interacting massive particles. And that term weakly interacting is no small exaggeration. These particles are so weakly interacting that if we took a single particle and set it off on a journey through a stack of lead just continuously passing through lead, it could actually travel all the way from here to our closest star, well, after the sun.

Proxima Centauri, which is over four light years, and four light years of lead, the particle still wouldn’t on a 50/50 basis have interacted. It– this is crazy small probabilities of interacting, and the only reason it is even remotely feasible for us to say we’re going to try and measure such weakly interacting particles with an experiment here on Earth or if you like under the Earth is that we have a very large detector. It’s nearly 10 tons in scale and that we watch it for a very long period of time.

We’re actually watching it for years looking for occasional interactions. So it’s that combination that there are 100 million or huge flux of these particles traveling through us every second. So that combination means that even with this very, very predicted or hypothesis of a very weak interaction that it may be possible for us to see an occasional event.

FLORA LICHTMAN: I think it’s hard for me at least to imagine something with a lot of mass that’s invisible and undetectable. How do you mentally picture a weakly interacting massive particle?

RICHARD GAITSKELL: Well, in that sense as empirical scientists, the only way we can bring directness to it is by detecting it. We have done many experiments now over the last 50 years, which have all helped us build a standard model of particle physics, but one of the other things that period of building and testing has convinced us is that dark matter itself, this missing mass of the universe, the thing that we know is there gravitationally but we have yet to actually directly either make or interact with, those new models have to be described as something outside of the standard model. So one very significant hypothesis, which we’re still testing is supersymmetry, which is an extension of our standard model in a rather dramatic way in that every particle develops a partner, a superpartner.

And, of course, this is one hypothesis, and this is why we’re doing the experimental work we do because we’re looking for direct evidence that one of these hypotheses turns out to be the correct one. I think the one thing you always have to remember is nature rarely picks the easiest or most obvious solution, certainly as far as we’re concerned. Obviously, nature has its own agenda, so maybe it’s blindingly simple to nature. But we’ve found certainly in the 40 years I’ve been working looking for dark matter that all of the models that we’ve seen develop and turn out to be the case in cosmology have not necessarily– have often not been the simple– the most simple model you would have expected, and I think the same is also true of particle physics.

FLORA LICHTMAN: One event feels like a small data set. How many more do you feel like you need to see to feel confident that this theory that you’re testing is on the right track?

RICHARD GAITSKELL: No, spot on. So with these rare event searches, firstly one event is it cannot be conclusive in any direction as you correctly identify. You want to try to accumulate more events. Now from that point of view, this data that we have just announced the result from was actually taken back in 2023 and 2024. And our experiment, the LUX-ZEPLIN experiment, has continued to run since then.

And I must add that with this one event we have, while the probability seems very low that this is a background event, it’s science. So we always have to report and discuss on the basis of having done an assessment, which right now is about a level of one in 200 that this event could be just simply due to a background. Now you might say, Rick, one in 200, that’s extremely low probability. Why would you think that that was in any way significant and imply that it was background?

But it’s a bit like going into the casino where you said there’s a one in 200 chance of winning a game. You’d say on that one casino event that’s not a high probability, but one of the beauties as you were suggesting of having more data and looking further for events is as you accumulate more events, that will allow you to test those events against the hypothesis that they are looking like dark matter or alternatively that they look like something peculiar, which is– turns out to be indicative of a background in your experiment.

FLORA LICHTMAN: How’s the field responding, and are the theorists going wild?

RICHARD GAITSKELL: So the theorists already go wild. That’s their job. They’re very important to this process.

So even back in the what– oh, gosh, it’s 16 or so– or more years ago, people, they were thinking about how dark matter particles might indeed interact with you and I, with conventional nuclei. And now that we have one event, they can actually now go back and look at what they’ve done or do new work, which is inspired by the particular energy at which this event is occurring. Now obviously they like everybody else, I think the theorists are very keen to see us get more events because that’s what really would begin to allow us to narrow the type of particle interaction that would be causing it.

But it could also be that the events do not have a signature that you associate with dark matter but actually have a different kind of signature, and then your hypothesis is that it’s a background of a more mundane variety. So we’re always going through that. That’s very much part of the scientific process, but it’s a very exciting one. That’s I think people– the young students, postdocs working on the experiment, this is fantastic opportunity for them to be living science live.

FLORA LICHTMAN: Oh, that’s so exciting. So I know this paper’s pretty peer reviewed, so it’s going to go through peer review. If the event is replicated many times over, where do you go from here? What’s the implication?

RICHARD GAITSKELL: Well, so we’ve been looking for direct detection of dark matter for the best part of 40 years now, and we’ve had no results verified that actually we’re a positive identification of dark matter to date. What we have had is over– we’ve managed to improve the performance of the detectors by over a factor of a million in the last 40 years. And what that’s allowed us to do is to eliminate a large number of potential models of dark matter.

So the process there was using negative data, and most science– most research is negative. We often have to have that conversation with students and say it’s not– nature’s not singling you out for mistreatment. It’s just that most research results are negative. That’s how it works.

That did allow us to eliminate many– some of the models were brilliant. They were beautiful models. I’d love for them to have been true.

FLORA LICHTMAN: Oh, that hurts. Yeah.

RICHARD GAITSKELL: Well, that’s, again, as I say, nature’s like that. But it’s very early days obviously. We’re at one, but you have to go through one before you get to many. So this is all exciting.

FLORA LICHTMAN: Yeah. There you go. You’ve been doing this 40 years. I’m glad you get to live the science, too.

RICHARD GAITSKELL: Well, it’s– science– if you look back in the history books sometimes in the history of science, you get this sense that, oh, things happen incredibly rapidly, but that’s, of course, because if you and I are writing a book, we would tend to pick–

FLORA LICHTMAN: You skip the boring parts.

RICHARD GAITSKELL: Exactly. But if you actually pick apart your science history book, you’ll realize how spaced out discoveries often are and how many avenues were explored that never went anywhere. And you have to be a very particular kind of historian to be willing to write about the things that failed. And because the other thing is as you’re writing them, you’ll go how could people be so crazy to have thought that because science from 100 years ago, 200 years ago, some of the ideas that were being sensibly and quite legitimately expressed to us, you and I today, would seem crazy, but that’s only because we’ve had the benefit of so many more results that have taught us about the way in which nature was structured. So this a– this is the same process, only it’s happening here today.

FLORA LICHTMAN: That’s really exciting. Dr. Richard Gaitskell is a physicist at Brown University and the spokesperson for the LUX-ZEPLIN Dark Matter Experiment. Thanks for joining us today.

RICHARD GAITSKELL: My pleasure. Fantastic talking to you.

FLORA LICHTMAN: This podcast was produced by Dee Peterschmidt and Rasha Aridi, and I wanted to shout out listener B Small 6 who said SciFri consistently interviews the most interesting people in science. Listen, if you love science, wonder, and awe. Thank you, B Small. I totally agree. Catch you next time. I’m Flora Lichtman.

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