Scientists Find Best Evidence Yet For Dark Matter

0:00Sierra has all the best active and outdoor brands you need. From athletic stuff, like a full-court pickup game, swish, to athletic stuff, like a half-mile stroll. Get those steps in! And from morning hikes up the mountain trail, good pace, to nighttime ghost stories from the camping chair. What a twist! Whatever level of active, Sierra loves it all. Head to Sierra or Sierra.com for the brands you want at the prices that let you do it all. From athletic to athletic-ish, Sierra's got it. Huge excitement in the world of physics on this episode as news comes in that the first signs of dark matter particles may have finally been spotted.

0:39Now, if this result is confirmed, it could be one of the most monumental discoveries in the history of physics. And I will say immediately that, you know, there is only one data point. I did say if and may. There's a lot of work to do. But just the idea that after decades of trying, we may have finally observed this object that makes up the majority of the universe. That is why physicists and I am getting excited. From New Scientist, this is The World, The Universe, and Us. I'm Dr. Rowan Hooper. And to discuss this, I'm joined by reporter Leia Crane. Leia, let's start with a recap on dark matter and how we know it exists if we've never actually directly observed it.

1:19All right, so the fast version is that dark matter makes up a huge proportion of the universe, 85 % of the mass. But for decades, physicists have been unable to determine what it's made of. We know it's there by observing its gravitational effect in the cosmos and how it affects the rotation of galaxies and a few other things. But we've never actually seen a dark matter particle. The leading idea has always been that it is made of particles because everything else is made of particles. But as you built - Yeah, what else could it be? It could be sort of strangely behaving gravity. It could be a new force.

1:57Basically, it could be relativity is wrong or it could be a particle are the two big theories. Right. And we've been looking for particles, which could be all different sorts, but the leading idea has always been the weakly interacting massive particle or WIMP. And we've been building more and more powerful detectors for decades to search for WIMPs, and we keep not finding them. Tell us about how you detect something, or even go about detecting something that evades detection so well. Yeah, so by definition, WIMPs are weakly interacting, so they don't interact with regular matter very much. But weakly interacting is not the same as never interacting.

2:40So we basically have to look for these very, very rare interactions between dark matter particles and regular mass. And to do that, you need a really big detector. The detector that's just made this big potential discovery is called Lux Zeppelin or LZ. And it is a huge seven ton tank of liquid xenon. And then outside that, it's got another tank of water and then another big layer of shielding, and it's buried a kilometer down under the Earth. So it's really trying to keep out other non-WIMP type of particles. But when you get a WIMP into the detector, or sometimes other particles, it hits a xenon atom once in a while.

3:25And that creates a tiny burst of light that's measured by sensitive detectors that are around that xenon tank. And we can then use the light to sort of reconstruct the path of the incoming particle and the energy level of the interaction. Okay. And maybe this is not relevant, but I've heard of other detectors that use ultra pure water and maybe helium. Is there any reason we use xenon in this thing? Yeah. So there's all kinds of detectors, but the biggest ones tend to use xenon and it's simple. It's just because it is a really big nucleus. And if your WIMP is going to hit a nucleus, it's more likely to hit one that's really big.

4:06So tell us about where they found this WIMP, right? Because physicists have been looking for it in a certain energy range. And can you explain what that means and where they found this potential one? Yeah. So in the search for WIMPs, we usually take all this data from the detector and we look for events with energies below about 30 kilo electron volts. And that assumes a really simple sort of interaction where the WIMP comes into the detector, bounces off an individual nucleon in the nucleus. So all of our previous searches for this detector have been in that energy range. We haven't found anything.

4:43So the team decided to reanalyze this first 220 days worth of data, looking for events with higher energies. So it's not necessarily brand new data, but it's a new search for higher energy, different, less simple interactions. And that's where they found a glimmer of an event at this higher energy. So does that mean that the WIMP is strong? Not as WIMP-y as we thought, right? It's got more energy, maybe. Not really. The WIMP basically is still in about the energy range where we thought we would find it. It just means it's having a more energetic interaction with the xenon. So instead of hitting a single nucleon, it's probably having, if it's there, it's probably having some sort of interaction with the whole nucleus, which would give it a mass around 200 times the mass of a proton, which is why we call these massive.

5:41They're pretty big compared to other particles. Yeah, and so weird that they're undetectable, such a massive particle. So weird. I want to get onto that in a minute. But where do we go from here? Because obviously this is just one glimmer of a potential observation, right? So what's next? Yeah. So right now we have one event in this energy range, and you can't really do statistics with one. Well, they have, right? It's got a sigma value on it, right? Yeah. Tell us about that. In particle physics, the statistical threshold for going from a hint to officially a discovery is five sigma, which means there's about a one in three and a half million chance that a similar signal would show up as a fluke.

6:26So this detection right now is at about 2.6 sigma, which is about a one in 200 chance it could appear as a fluke. And that sounds great, but with the amount of experiments we do and the amount of searching we do, you're going to come across something with a one in 200 chance. So what we really need is a bunch more. Yeah. I mean, over the years, Leah, we've talked about things with even higher sigma and got excited and it turns out to be nothing, right? So we can't get too excited. But the reason people are is that we literally have never had even anything like this before. Yeah. I mean, there have been candidates and they've disappeared.

7:06And right now, this is looking like the best one we've got. The other great thing is that, But I said they analyzed the first 220 days of data from LZ to find this. That's about a third of the data that we have in hand already. And that's just from LZ. There are other detectors as well. So we can go back. We can analyze all this other data and look for more events. And we're continuing to collect more data in the future. So if this is really dark matter, there is a chance that we will know that, you know, on the year side rather than the decades or centuries scale. Well, that's amazing. And, you know, like we hear discussions about we need bigger particle colliders, bigger than the Large Hadron Collider.

7:54And similarly, there's discussions like, well, we need a bigger xenon tank to basically increase our chances of capturing this collision. So maybe this will give those plans a boost. Yeah, I think, you know, as I've been reporting on this and reporting on dark matter and detectors over the years, what I've heard is that LZ and its next successor experiment don't find anything. We're sort of starting to reach the limits of searching for wimps, at least. There are many other dark matter ideas. So the fact that LZ has pretty quickly after turning on found a candidate really bears those out that we maybe have our detectors at the right size now.

8:42Sierra has all the best active and outdoor brands you need from athletic stuff like a full court pickup game. Swish! To athletic stuff like a half mile stroll. Get those steps in. and for morning hikes up the mountain trail good pace it's a nighttime ghost choice from the camping chair what a twist whatever level of active sierra loves it all head to sierra or sierra.com for the brands you want at the prices that let you do it all from athletic to athletic sierra's got it and so like you're saying that the the wimp this weekly interacting massive particle 200 times bigger than a proton i'm thinking of the of like the particle zoo of of particles in the standard model of physics.

9:25And at the moment, that's kind of complete, right? Because the Higgs boson was the last one we hadn't found. So the zoo is full, and yet we know it can't be full because of the problem of dark matter. So what happens if this, let's say if again, that this is confirmed and we suddenly have to, what happens to this zoo when we have another particle to fit in? I would build a new cage.

9:52Dark Matter has always been a place where we knew that the standard model was incomplete. Which part of the standard model was incomplete has been always up for debate. But we've always known that it wasn't 100 % finished. So this does mean we have to add on to it and that could have downstream effects. But we've always known that we would have to do that. So it's not necessarily a horrifying shakeup of our understanding of physics. Well, I mean, it's a good thing. Many physicists have been wanting desperately to try and shake it up. When the Higgs was discovered, some people were a little bit like, it doesn't really shake it up enough.

10:34You know, it could have potentially really, really smashed up the cage, but it didn't. So we really need this shakeup. Yeah, it's great. And, you know, over the years since the Higgs, as we've continued to not find any more new particles, there's been, I think, a sort of growing concern of is particle physics dead? Are we going to be able to keep getting funding for it if it appears dead? And I think this will be a sort of shot in the arm for particle physics in general. Had physicists generally and you, I don't know, been starting to get a bit impatient with the idea of a wimp, like waiting all this time for it and thinking maybe it is something else, even more exotic?

11:16I think so. That is sort of the vibe I've been getting from a lot of people. Obviously, not everyone. And most people do try to remain sort of impartial. There was still space for it to be a wimp. So it still very well could have been and could be. But I do think that in the last couple of years, there has been a growing amount of research into axions, which are another type of potential dark matter particle and other ideas, because, you know, we've just not found WIMPs and we've looked for them, not everywhere, but an awful lot of places. So at the start, I said, you know, I made a big song and dance about this could be one of the biggest discoveries in physics ever.

11:54I mean, thinking about, well, what were the biggest discoveries in physics? And one just pops into my mind would be the discovery of the electron. And then you think, well, look at all the things that happened after that, after the electron was discovered. You know, let's speculate at the end of this podcast and say, you know, would it be comparable? Like, can you even start to imagine what might happen if we have a WIMP confirmed? I kind of can't, but I think that's kind of the point. Before the discovery of the electron, we could have never imagined all the stuff that would come with and after that discovery.

12:31My suspicion is that a WIMP will be much less practically useful. They're harder to control than electrons, right? Yeah, a lot harder to control than electrons and a lot fewer predicted useful properties. Maybe there are unpredicted ones. And I think it would be more of a big deal for scientists than for everyday life. But an enormous deal for scientists because of just the pure amount of the universe that this stuff is. And how deeply we don't understand it. I think getting even a tiny bit of insight into that would be an enormous achievement. Yeah, because I still have to remind myself, you know, The amount of stuff we've learned about matter, particles, and that's still only 15 % of the universe or something, right?

13:2415 % of the matter, 5 % of the universe. It's embarrassing, isn't it? Between dark matter and dark energy, which we also know absolutely nothing about. Yeah. You got 95 % of the universe that is, you know, totally inaccessible to us. And making it accessible would just be amazing. It's hard to think of another word for it. It would be amazing. Yeah. Well, that's awesome. We'll leave it there. Thanks, Leia. Thanks to our guest, Leia Crane. Thanks to you for listening. I'm Rowan Hooper. Do subscribe, follow wherever you get your podcasts. Bye for now. Bye.

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