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We are really interested in things that are unusual in the standard model.

What we are trying to do, which is different with respect to other experiments, is that we are trying to produce dark matter in our laboratory. We, as experimentalists, we have to be particularly cunning, let's say, and innovative in our such techniques to try and make sense of dark matter in the lab. We need to give this storyline to Marvel. Maybe we're going to have Spider-Man going to the dark universe. Dark matter is also not such a good name. Yeah. (Interposing Voices) We're the ones in the dark.

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Hello, everybody, and welcome to Early Morning Coffee at CERN. My name is Stephen Goldfarb. Hi, I'm Anna Paula de Casa. And I'm Batiste Rabinow. We have a great show for you. Today, we're going to be talking about what seems to be your favorite topic, dark matter. And of course, we're doing it because October 31 is dark matter day. I knew you knew that. Of course you knew that. So we have with us Anna Paula de Casa, an assistant professor at ETH Zurich, ETH, the Technical Institute in Zurich, and Batiste Rabinow, a research fellow now recently at CERN. Congratulations. It's not easy to get one of those fellowships.

And I want you guys to know that we held a poll after our last podcast. And in that poll, we asked people what topic do you want to hear about the most. And they thought about you guys. They said, we want to know about dark matter. We've heard about dark matter, but we don't know what it is. How do you go about looking for it, especially if it's something that you can't see? How do you look for it? So there's a lot of interest in the stuff that you're doing. So why don't we start with Anna Paula?

Maybe you can tell us a little bit about yourself. What got you interested in looking for something that you can't see here at CERN. OK. So I was at my first postdoc, so right after I finished my PhD. And was already here at CERN. And I had the possibility to work with tremendous colleagues.

And I started to work on the search for a particular kind of dark matter that we could wimp, weak interactive massive particle.

And this search was just amazing, I mean, for me. Because I mean, that matter has been-- as for many of you-- has been always quite a mystery. And this is especially something that we know that is there.

We have evidence for it coming from several sources, right? But still, I mean, we cannot explain it. So we don't know what is the nature of that matter. And after decades of looking for it, I mean, this is still a building, right? So this is what was what captured my attention. And then I started to think about completely different hypotheses about that matter. And just really leaving--

getting free my imagination on possible models and possible additional ways and possible tools, new tools, to catch that matter. And this is how I-- And you're doing these now on CMS. And I'm doing this now with the CMS experiment. Yes, exactly. OK. Baptiste, tell us about yourself. So I completely agree with what Annapalar said. I mean, it's really-- Dark Matter is really something that captures your imagination, especially as a young student where you're confronted to all of these big ideas. And then it's a vector of more creativity later on in your work. And for me, I started when I was a student working on some cosmological models, trying to explain maybe what Dark Matter could be, one of infinitely many models. And I thought, OK, this is very interesting, but now I would like to do something towards actually finding it. And that's what got me into experimental particle physics on the atlas experiment. And I started doing my PhD there. And then one idea led to the other. And I just took a very different path from what I would have imagined originally, but always driven by this goal of finding out what Dark Matter is. What is this stuff? I mean, it has a long history.

Dark Matter goes way back. I looked into this recently because I didn't realize it went back so far, 1880s or something.

Lord Kelvin was already talking about there must be other stars out there which are dark, which you can't see. And I think that it all came about from gravitational measurements. It seems something out there in space has mass. And it's attracting things. Or maybe it's a gravitational effect. We don't really know, right? But we can see this. What are the different ways that you can see this?

Well, there are actually really different ways that you can see it. Some of them are, for instance, looking at the way in which stars move in galaxies, especially in spiral galaxies, what they were expecting for the movement of the star was actually very different with respect to what they measured.

And this is something that even today we can explain only if we assume that there is another kind of matter, quite massive matter, that permeates these galaxies. And this is one of the evidence. We also have other evidence, for instance, from the study of what is called the cosmic microwave background, which is the radiation that comes directly from the past to us, from after a few seconds after the Big Bang. And we can actually study this today. And this is showing us how there were already different densities in the universe already at the time. And this difference can only be explained if we had dark matter playing a role. So if there's stuff out there that we can't see-- That we cannot see. That the visible light does not explain what we're seeing in these patterns that came from the cosmic microwave background, which is a long time ago-- before even I was a student--

something like 13.8 billion years minus 100,000 or something like that, when electrons started spinning around protons. And they gave us this background radiation. And you can map that. So there's maps of dark matter now.

On the cosmic microwave background, it's very interesting, because that brings together general relativity, so the study of the universe as the infinitely large object that it is, together with particle physics. Because you have to know what kind of particles were there in the early universe and how they interacted or did not interact together. And that helps you map out the history of the universe. Whereas the first example that Annapara gave, just looking at galaxies, that's extremely simple. And that's why you could already do it in the late 1900s-- sorry, in the late 19th century.

Because you're just looking at galaxies. And you're doing two types of measurements. One, you're measuring how fast things are moving. And the second, you're measuring how bright things are. Because you can imagine that in a galaxy where you have a bunch of stars,

but most of the mass is actually in the stars. You just count the visible bright stuff to a very good approximation, you get the mass of the galaxy. And these two techniques give you very different answers. And that's why we think that if there's a mass that's there that we're not actually seeing is because it's not bright, because it's not interacting like normal matter would. Like normal matter. So it's only interacting through gravity. And I think it was first, there was the dark matter. It bothers me the name dark matter. I'm sorry.

It's really invisible. I think that came from way, way back. Fritz Zwicky was a Swiss physicist. And he came out with calling it dark matter. It's OK. Forgive him. But he was really one of the first to give that name and to have seen clusters of galaxies. And then later on in the 20th century, Vera Rubin did this final measurement of the spinning of stars and galaxies. And she gave a number to it. So it's not a small amount of matter. But how much of the matter in the universe is-- Well, this is about 25% of the entire universe energy content. But if you want to compare it with the ordinary matter, the matter that we know, the matter that composed stars or galaxies, even we as humans,

it's about five times. Which means that out there-- I mean, all the matter that is in the universe, we're just not 5%.

But then there is another kind of matter, which is at least five times more abundant with respect to what we know, which is really huge. And we don't know anything about its nature.

The point is that usually the way in which you perform and identify particles is trying to understand measuring what are the effects of its interaction with other particles. So how does a particle interact? And then we can perform and identify it. But the problem is that apparently that matter does not interact in another way as not gravity. So this is, for the moment, all the information that we have really come from the gravitational interaction of that matter, which is still quite limited. But we still have hope that there are other ways of interaction that we can study. Because gravity is rather hard. Gravity, compared to, say, electromagnetism, is something like, what, a million, million, million, million, million, million times weaker. So building an experiment to measure that effect, you need galaxies. You need clusters of galaxies like they did measuring it cosmologically. So we're here at the Large Hadron Collider at CERN, where we have two largest, most beautiful experiments in the Atlas and CMS.

How on earth can you expect to actually see dark matter in those experiments?

OK, so I would say that our plan is very ambitious.

What we are trying to do, which is different with respect to other experiments, is that we are trying to produce dark matter in our laboratory.

And this is amazing because if we manage to produce dark matter in our laboratory, this means that we can also study it. I mean, we can keep studying it for years and years to come and really try to understand all the properties of these particles. If it's just one particle, if there are more particles,

we can study all of this.

And there are several ways in which we can do it.

And this depends also, of course, on the nature of dark matter. So if dark matter is just composed of one particle that interacts very weakly, or if it's composed by several particles, and then, I mean, depending on which kind of dark matter we are looking for, we adapt our search.

And define new tools.

And I think it's very important to point out that we didn't build the LHC or any of the experiments to actually look for things that are not there, right? Like we actually need something to measure in our detectors, things to interact with.

So definitely, as experimentalists, we have to be particularly cunning, let's say, and innovative in our search techniques to try and make sense of dark matter in the lab.

If you'd rather open-minded. So my understanding is, if it's not going to interact, right, we don't see it interacting. We don't have light coming from it, so there's no electromagnetic radiation or anything coming from it. We don't necessarily see that there's no strong interaction. You say weakly interacting massive particles, maybe there's a weak interaction, we don't know, but then that might affect things a different way. So if we're not gonna see it in the detector, is there a way to still know that it's there?

Well, yes, we are actually looking for something that we don't see, which might seems a bit weird. But in the end, what we do is that we expect our matter to behave in a very similar way to neutrinos, which means it does not interact with our detectors, right?

If we manage to produce that matter at the proton-proton collisions at the larger drone collider, what would happen is that it would simply escape

our detectors. But the point is that we know what is the energy of the, and the momentum of the incoming particles, the incoming protons, right? And we know that energy and momentum are conserved, so this is what we use in order to infer the presence of that matter. So if we have an imbalance in the energy that we measure, this means that there is something which is escaping our reconstruction, it is escaping our detectors.

And then I mean, we infer the production of that matter particles.

So you know, the first step is to become really, really good at measuring what can actually be seen, because you have to keep very good track of all of the energy that you actually produce in the form of visible particles. So you need to be extremely precise there. And then you can say, okay, if something's missing, then perhaps it's not a fault of the detector or one of the neutrinos, because this, and I've probably mentioned, we do have neutrinos in the standard model, particles that we know of, and that we also know can't be dark matter particles. This has been ruled out quite some time ago. Okay.

So it's not neutrinos? So it's not neutrinos, because dark matter also has a role on the cosmological scale to help galaxies form. So you need to clump a lot of dark matter to help attract the normal matter into form galaxies. This is our current understanding. And the problem is that if you have neutrinos as your dark matter particle, they're extremely light. They're going extremely fast, especially in the early universe, and they're not gonna want to clump together. They're just whizzing away. Zooming around. Yeah, so that's what we call warm dark matter, because it's very hot and energetic. It's just, it doesn't fit the observation that we have now of the universe that's settled down with a lot of galaxies and large galaxies.

A neutrino is this very, very light particle that only interacts through the weak nuclear interaction. And it's called weak for a reason. It's very, very weak relative to electromagnetism. And so in our detectors, we produce neutrinos that happens when we have the collisions. We're colliding protons. The constituents of the protons, the quarks or gluons interact with each other. Stuff comes out, lots of different tracks come out, and we build our detectors so that we can measure all these different types of tracks and all the different energy that comes out. Neutrinos slip out. We just don't catch them. We do have some dedicated detectors, which if there's a gazillion neutrinos, we'll catch a few of them. So that's how we know they exist. We've measured them, and they've done this in detectors all over the South Pole and in Japan, many places, and even here at CERN, at the ends of our detectors, right? Both for Atlas and CMS.

But they will slip through. We won't see it, but we'll see because we've surrounded the collision point entirely.

We have a hermetic seal, beautiful detectors. They're completely sealed, we hope. And that stuff came out because when you have conservation momentum, if you have a collision, two protons, then in the plane that's perpendicular to the beam, it should add up to zero, right? We learned this in school. You add up the vectors, it comes out to zero. So stuff is missing. Now, how do we differentiate between the stuff that's missing that's neutrinos that we expect and if it were dark matter?

Is there a way to differentiate?

Well, the answer to this question is it depends. It depends on which kind of dark matter you're looking for.

There are different kinds. As I was saying, for instance, if you expect dark matter to be similar to neutrino, but much heavier, right?

Then, I mean, what you expect is to see very huge imbalance in the energy that you reconstruct in your detector. So there is something that doesn't sum up to zero in the momentum that you reconstruct and the energy that you reconstruct. And then this would be not sufficient, right? Because the problem is that to define an imbalance, you need to have visible object, right, that you reconstruct.

Which means that you're looking for a kind of dark matter that is produced in the long, right? Usually, you expect two dark matter particles that are produced together with some other visible objects.

For instance, it can be some quarks, right? Or it can be some leptons.

And this is one way of doing it.

This is the most simple one. But then the point is that, in principle, dark matter can be almost anything. So you can imagine very complex models in which you have entire new sectors of particles, an entire new family of dark matter particles, right? So not just one, but several of them. And the way in which they would manifest in our detectors is for very weird signature. Oh, yeah. Define weird. What is weird? Well, it's something which is very different with respect to what we've built a detector for.

So it's very different, for instance, for an electron, from an electron or from a photon.

Or even just a simple quark, which actually we don't really see so easy way in our detector. Because we cannot see a simple quark stand alone. No, no, they don't like to stay alone, they just get with each other. And essentially what you have is that you start from one quark, which essentially keeps other quark together, right?

And then the result in the detector is a spray of particles. So you don't see one particle, but you see a spray that we call jet, jet of particle, because it really looks like a jet.

And you can have very weird ones that are very different from what we expect. Yes, you can have, for instance, jet of particles, which are not all visible. You can have visible ones inside. They can be a bit larger with respect to what you expect. Or they can be narrower. They can have a lot of constituents. They can have just few ones. But this really depends on the actual nature of the matter. So I mean, it's similar like seeing galaxies and seeing that they're not quite moving right. They're different. There's stuff inside there that you don't see. So we'd have a jet, which has a certain amount of energy, but that's what you're seeing is less than actually what's being produced, right? Because there's that sort of invisible stuff that's inside there. So in a way, what Annapara is describing is that we're really interested in things that are unusual in the standard model. Because dark matter is really a concept. It's not a theory of itself. Theories predict perhaps a dark matter candidate. There are as many theories as there are theorists, probably even more. So we can't possibly just test each of them one or the other. We have to come up with the interesting bits. And perhaps looking for weird types of jets of signatures in our detectors is one way to do it.

And another way to do it, I think, also very interested in is to really leverage all of the information we get from our detectors. So instead of just looking at specific bits, we look at the entire collision as we've recorded it in all of its gory details. And we pass that through algorithms that will tell, okay, this is an event that we think is pretty much standard model-like. And here's another one, and here's another one, and here's actually a billion standard model events. Learn what you come from that. And then we'll show you some real data.

And within this data, we hope that every billions, trillions, collision, whatever, there might be something that's anomalous enough, different enough from the rest of what we've seen so far that it might point us towards, hey, maybe this is something new. Maybe it's also a mistake of the detector. Maybe it's a temporary fault or something. Or maybe it's some phenomenon that is actually standard model-like that we hadn't really anticipated. In all of these cases are interesting to us anyway. Sure. As experimentalists. Sure. Yeah, that's the tricky part is your detector has to be working perfectly, right? If a little part of your detector is off,

then something looks like it slipped out, but it was just that it didn't get detected. But we have many, many tests, I think, both Atlas and CMS to make sure that our data are good.

We even have, I'm sure it's the same for CMS and Atlas. We have these good run lists that say that for this particular thing, for like a search for dark matter, everything was on, everything was working well. And so take a look and see if you see anomalies in this.

So I think that's, you know, so it's interesting. The difference between what we're doing here on the LHC is, as you said, you were trying to produce it. The LHC, we're hoping the LHC will produce it inside of our detectors.

And I suppose there's a good chance of that because what we have found 12 years ago now, was a Higgs boson.

And it must mean there's some chance if dark matter is an elementary particle, right? Because elementary particle with mass must interact with the Higgs boson, right? Must it?

(Laughing) Yeah, that's a very good question because as you described, we know that the Higgs is responsible for giving mass to the particles that we know of, what about particles that we don't know about. Some people would posit that indeed the Higgs must couple to these dark matter particles. And that's one way we can look for it at the LHC. And indeed, that's what we did on both Atlas and CMS is look for events that pretty much should have a Higgs in them, but then the Higgs doesn't decay to anything, or perhaps it decays to invisible particles. Invisible Higgs.

Invisible decays of the Higgs. And that didn't turn any positive results for dark matter. So, okay, possibly it's something else.

Do we have to do away with the Higgs to explain the mass of dark matter? Well, not necessarily, we could just have more Higgses. There could be one Higgs that we discovered that couples to the thermal particles. And that's why we discovered it because we looked at the thermal particles. There could be partners of the Higgs that couple either to thermal particles and dark matter particles or just to dark matter particles. So, dark Higgs? Yeah, dark sector Higgs, an extended Higgs sector. And that's why we're saying that dark matter is not a theory of its own, it's just a concept. Then you can invent as complicated a theory as you want to explain whatever other problems as well. Because if you start introducing new Higgs, then perhaps you solve other problems than just dark matter.

Imagine that you actually have an entire world parallel world done just with dark particles. Okay? Okay. It can be, it can compose your dark matter in the end. You can have, so we have electrons, muons, that we know and we see, but you could have some dark leptons. You can have some dark quarks. So, you could have really an entire new family of particles that is living with its own rules, which means with its own interactions. So, while in the world that we know, and there is the gravitational, there is the strong interaction, there is what we call the weak interaction, which is responsible for the activity and so on, or the strong interaction that keeps the quarks together, right, in the nuclei of the atoms, we could have something similar also in this parallel world. Just that these rules, these interactions act,

only in the, would play a role only with the amount of dark particles. But then we still need something right that we had two words communicating. Uh-huh, because-- And this is a big question mark. Yeah. And this is what it was mentioned, what it was mentioning, that for instance, an hypothesis could be having the X right. Uh-huh. That really-- They could take it from one to the other. Yes, exactly. This could be one of the hypothesis. Or you could have another particle, like one particle similar to the Z boson. And then you can even think about having dark kicks in this new dark sector, that would explain why this particle at mass.

So there is, and if you can go ahead with your imagination. Yeah, I was gonna say, we need to give this storyline to Marvel, or maybe we're gonna have Spider-Man going to the dark universe.

It is great. What a great field we have, that we can use our imagination like this and think about dark universes out there.

So before we end up, I did want to ask you a couple questions about the future. What are we gonna do in the future? So we have a lot of data now. We're in run three of the LHC, and we're gonna get a couple more years of data and run three, and then we're gonna shut down for a while, and we're gonna go to something called high luminosity LHC. So that's five years from now, we hope, something like that, and we're gonna have a lot more collisions per second. Is that gonna help, do you think, in these searches? I'm very excited by that, yes. Because I'm going at it with the point of view of data-driven science. So the more data I have, definitely the more science I have to do. And it's worth reminding ourselves, yeah, that we haven't even collected 10% of the data that we hope to have by the end of the high luminosity LHC. So definitely, yeah, there will be more room for very rare events, very anomalous-like signatures. And also if you just go about it looking for rare things and anomalous things, we did a metadata study to see actually how much of the ground are we covering with our existing measurements, and it's only also about 10, 15% of what is possible. It doesn't mean that we're lazy. It's just really, really hard to cover more. That's a lot of work. So what if, I know it won't happen, but what if we don't find a conclusive answer in high luminosity LHC?

What's our next step? You wanna dream? Well, yes, yes, why not? I mean, this is actually what is driving right now science, right? So we're dreaming of different things. And I think that right now this is a unique moment because it's true that we haven't found new physics yet. But now, I mean, we are realizing that actually, we just looked in one direction, forgetting all the others. And this is what we are doing right now. This is why the round-free, which is the exit moment in which now, I mean, the machine is operating, is so exceptional, and why the eye lumen is going to be exceptional. Now we are really thinking about beyond what we have imagined up so far,

and the technology is coming with us, which means, or I mean, we are going with technology, which means that we are exploiting the new tools that we haven't before, like for instance, machine learning, right? We are using artificial intelligence in a really massive way. We are pushing even the application of these techniques at the frontier, because I mean, the Large Adron Collider allows us to really handle a lot of data and quite complex data problems. And then, for the next future, and still I think that this is going to really be exciting for the next years, but then, I mean, behind that, behind the eye lumi, we are thinking about building an even larger collider, which we call the Future Collider,

where we plan to collide electrons. And this is going to be amazing, because it's very different with respect to what we are doing right now at the Large Adron Collider, where we are colliding protons. And the reason for that is that we will have collisions, which are very clean. This means that we won't have, we will have very good resolution, very low uncertainty on the way in which we reconstruct invisible particles, for instance, so that matter will be easier to be looked for with these machines.

We will understand much more about what we already know, because the point is that, yes, I mean, we think that everything works well, but maybe if you go and you look with higher precision, then you might realize that this is not actually the case, and this could be another indication of new physics. So there is, I think, I mean, we have plenty in front of us. We have plenty in front of us. Now, I understand that we're not, we should be fair, we're not the only game in town.

There are others who are looking for dark matter in different ways, right, in different places, from deep down in gold mines to up in space. In fact, someone I used to work for, when I first came over here a long, long time ago, Professor Sam Ting has a wonderful experiment that's up there on the International Space Station called AMS, and there's a lot of other ways that people are looking for dark matter, but they aren't going to produce it though, right? They're trying to detect it. Yeah, that's a big difference, that those experiments rely on there being dark matter in the universe, streaming through the galaxy, and hopefully once in a while, visiting our detectors on Earth, or as you said, up in space, and perhaps knocking an atom a little bit off, creating a signal that otherwise wouldn't happen.

So definitely that's the big advantage of our experiment here at CERN, is that if and when we actually find it, we can then tune our machines to make more of it, to study it, and to study its interactions with all of the other particles. Okay, okay, so someone is pretty complimentary, I guess. All of these approaches are complimentary, I think, because we're still very much in the dark about dark matter, that's why dark matter is also not such a good name, to be some form of invisible matter, we're the ones in the dark, right? We definitely need all of the tools that are this puzzle to actually look for it. Yeah, I was just imagining, if I went to the bank, and I asked the banker, I'd like to get my money, and he says, "Well, I only know we're about 10 to 20% "of it is, missing the other 80 to 90%,

"then I wouldn't be too happy." And so we owe it to the public, we're sorry, we're gonna look for it, and we're gonna find it, and in fact, we have seen it, it's actually not, if you think about it, we need to discover it, what we need to do is to identify what it is, because we know it's out there, we've seen what's out there. We're dealing here with quantum physics, and quantum physics is really weird. Yeah. You think we're looking at quantum physics is weird, and even if we don't see the particle at the LHC or at the (Speaking In Foreign Language)

just because it's a particle that has some quantum properties, it must have a field associated with it. Much like the Higgs boson is a particle that's extracted from a Higgs field, a permeacy universe, the electron is associated to an electric electron field.

So just because this dark field, whatever it is, we postulate that it's there, it should be wobbling around with the other field that we're exciting at the LHC. So even though we can't perhaps put our finger on it or even have the right amount of energy to produce to extract a particle of that matter from that field, it will modify the interactions of everything else around it. Good point. And so we could still see hints of it. So much like the Higgs field was giving masked particles long before we discovered the Higgs boson. So there we have quantum field theory lesson number 17.

(Laughing) Every particle, there is a field. Excellent. Well, I wanna thank you guys very much for coming here. Tell us, enlightening us about dark matter on dark matter day. So Anna Pola-Dakosa is assistant professor at ETH Zurich and Baptist Ravina is a CERN research fellow. Thank you very much for coming here. Thank you. Thank you. It has been absolute pleasure. This has been Early Morning Coffee at CERN, a podcast by the scientists at CERN about the science of CERN. You can find all of our episodes, well, for now, both of them, on the CERN YouTube channel or wherever you get your podcasts. And I mean anywhere, Spotify, Apple podcast, Castbox, these are things that I've never even heard of. Be sure to go there. Be sure to like us, love us, whatever, follow us. Give us a review we'd like to hear from you. Our editor and producer sitting back there behind the table is Chetna Krishna. Joni Pham also back there is our social correspondent. Our executive producer is Jacques Fiché. Ron, the third person sitting back there, hi Ron. Ron Sucrebuch is our technical lead. Our studio manager is Max Brice, sound design by Piotr Trexic. Our original theme comes from the Kness Blues Band and I promised him I would mention him. The piano bit is done by Voit, play it Nini Kekryevsky. Many thanks to Paolo Cattapano, Matthew Chalmers and Arno Marcellier for all of their advice and their strategic planning. And a big thanks to the entire education, communication and outreach team here at CERN for providing us with access to the wonderful Wire Chamber Studios here at CERN and all the help that comes with it. The opinions expressed here are our own and do not necessarily reflect those of CERN or our colleagues, even though we think they ought to. My name is Steven Goldfarb. This has been Early Morning Coffee at CERN and we'll see you again next month.

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