Oxford Sparks

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Emily Elias: There is a whole world of particles out there that we just don’t know about. We think that they exist, but proving them requires a lot of science and so on this episode of the Oxford Sparks Big Questions podcast, we are asking how do you find a theoretical particle?

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Hello, I’m Emily Elias and this is the show where we seek out the brightest minds at the University of Oxford, and we ask them the big questions. And for this one, we have found a researcher who loves thinking in the abstract.

Lizzie Bloomfield: I’m Lizzie Bloomfield. I’m currently a first year PhD student at the University of Oxford, working in the particle physics department, and I study dark matter.

More specifically, my degree title is that I am looking at spin dependent interactions in DarkSide-20k and QUEST-DMC. But what this means is I’m trying to directly detect dark matter as a particle.

Emily: Lizzie, you’ve said a lot of words, and I don’t necessarily know what they mean, but it sounds very, very smart. So can you just, like, back this up and walk me through what exactly is dark matter?

Lizzie: Sure. So dark matter is essentially a form of hypothetical matter that we believe exists in the universe, and we believe it exists due to a plethora of observational evidence that we have seen on a cosmological scale when looking at outer space, when looking at galaxies, when looking at spiral clusters. And we have cause to believe that there is matter that exists in the universe that is non-luminous. And so we call that dark matter.

Emily: So it’s stuff that is doing something, but you don’t know exactly what it is that’s doing the doing.

Lizzie: Yeah. So what it is, is we have seen many different pieces of evidence in the universe looking at galaxy clusters and spiral galaxies like our Milky Way, where we have seen a discrepancy between the amount of visible matter in a galaxy and the amount of matter that is actually there for it to create the gravity that the galaxy has.

And this is true for lots and lots of different cosmological features that we have measured in the universe. So from these conclusions, we can essentially conclude that in these large cosmological structures, there is matter that exists there that is invisible or non-luminous. It doesn’t interact with light, but it does interact with gravity. So from that, we have concluded that there is a non-luminous matter that exists in the universe, but we have never seen it experimentally. And that’s what we are trying to do, is to directly see something that is non-luminous or invisible.

Emily: So if you know that something is there but you don’t know what it is, how do you even start with trying to find out what that thing is, what that dark matter is.

Lizzie: So that’s where the theorists come into play. They come up with a whole load of different theories as to what this theoretical matter could be. So we’ve got quite a lot of different theories as to what dark matter could be.

There’s some that are more popular than others, uh, one of which is called the Weakly Interacting Massive Particle. Or we shorten that name to WIMP for short, uh, which is quite a fun name. And this is essentially a hypothetical particle that exists throughout spiral galaxies and throughout the universe and by, uh, extrapolation, throughout our Earth itself. And it is essentially a very small subatomic particle that doesn’t interact with light, that passes through the universe the same way radiation or cosmic rays might.

Emily: And so you in a lab in Oxford, not in the cosmos. How do you go about trying to find it?

Lizzie: So actually in Oxford, we don’t have a detector to find dark matter. In Oxford, what we do is we make the components to build a detector that will find dark matter. So the detector that I’m currently building is actually located in Italy under a mountain, which is quite fun. And essentially what it is, is it’s something called a dual phase time projection chamber. But in simple terms, it’s a very, very big tank of liquid gas that is incredibly cold. That is essentially a target. And we’re hoping that these dark matter particles, as they pass through the Earth on their way through the Milky Way and through the solar system, will pass through this tank, this target of liquid gas, and we will be able to see them.

Emily: Now, do you know what specific thing you’re looking for? Or is it kind of a bit of a scattergun approach that you’re hoping that the detector will pick up something?

Lizzie: So that’s actually a really good point. We don’t know the properties of the WIMP specifically. We have an idea of what they might be, but we’re not sure. And so that’s where, uh, this experiment comes in. It’s an experimental process where you’re continuously finding new parameters and narrowing down your field of vision to work out what you’re actually looking for.

So currently, there are a couple of TPCs, time projection chambers, that have been running across the world for years, one of which being LUX-ZEPLIN, which operates in South Dakota in the US. They’ve just finished a five year run in which they have been looking for the same particle with a similar technology to ours. And so essentially what this does is with these long runs, we have yet to actively see a new particle signal in these detectors. But what we can do is we can narrow down the window in which we look in. So these particles should have a mass that corresponds to an energy that they will deposit in a detector. So as we adjust the energy range that we look in, in these detectors, we can narrow down the field or the scope of where we’re looking. And hopefully one day essentially zero in on a particle itself. So we don’t know actually what these particles, the features of them, are yet.

Emily: And I can’t imagine that the amount of energy you would need is a small amount. Like, we’re not talking, like, enough energy to run a kettle here, right? Like, how much energy does it take to try and find an experimental theoretical particle?

Lizzie: Yes. You’re right, it’s definitely more than than to boil a kettle. So to give you a sense of scale as to what these TPCs are like, the one that we’re currently building in Italy is about thirty by thirty metre tank. It has fifty tonnes of liquid argon. So argon is a noble gas. At room temperature, it’s just a gas. It exists in the universe. But what we have is we have fifty tonnes of liquid argon. So this is incredibly, incredibly cold. And we cool it down to sort of very, very low temperatures like minus two hundred and something degrees C. And we have to keep it in that state for a very long time. And then we also have to keep the surrounding detector in a stable state as well. So yes, it takes a lot of energy to do this, especially because we are aiming to run these detectors for years at a time.

Emily: Okay. And Lizzie, like this obviously sounds like a massively huge, huge project. What’s the timeline on getting something like this to the place where you’re like, Eureka, we have discovered a new particle.

Lizzie: Yeah. So the timeline for such a big experiment is incredibly long. These large scale particle physics experiments, be it dark matter, neutrinos, the Higgs boson, these things will take years and years and years to go from proof of concept, or even just first ideas and theories to an actual discovery.

So, to put it in perspective of the timeline of dark matter, the first piece of observational evidence for dark matter was by a Swiss astronomer in nineteen thirty three. Since then, we have been finding more and more evidence, and since the millennium we have started to build these large scale direct detection experiments. DarkSide-50, which was our predecessor, was a smaller TPC that was built, commissioned and finished and was running in 2013 and 2014. And since then, we have now been essentially upgrading and upscaling to make DarkSide-20k. It’s now currently in some of the final mockup phases, and the aim is, is that we will have commission and first runs within end of 2026, start of 2027.

Emily: Does that mean you get to go to a mountain in Italy to check out a giant tank that could potentially have the secrets of the universe inside it?

Lizzie: Well, I don’t know about secrets of the universe, but yes, I will be spending part of my PhD in Italy at the lab itself. So the reason that we have it so far away is essentially because with these experiments, we are looking for the smallest particles in our known universe, which means that these detectors are incredibly sensitive. And when you have a very sensitive detector that is also massive, you get a lot of background noise because you’re getting radiation, you’re getting cosmic rays. You’re getting a whole load of noise from all of the other particles in the universe that are also passing through. So there are lots of ways that we can shield against this, but sometimes the simplest way is the best. And one of the easiest ways to shield against radiation and cosmic rays and the Earth’s atmosphere is to just go underground. So the LUX-ZEPLIN project in the USA is in an old gold mine, and this one is just under a mountain.

So yes, I get to go to the mountains of Italy for hopefully almost up to a year, which I’m very excited about.

Emily: Yeah, but it’s like the opposite of skiing because you’re going underground. Is there like anything to do there aside from, like, hanging out with me?

Lizzie: There is skiing. Quite a lot of the, uh, the scientists who work at the laboratory do go skiing during the winter times, but it’s also a beautiful national park and an amazing area. It’s got brown bears, wolves, mountain goats. It’s got a whole load of things to do on the weekends to keep you entertained, happy and fit and healthy. Uh, so I’m very much looking forward to going out.

Emily: Good. I’m glad for your mental health that you have something else to do.

Lizzie: Yes.

Emily: Is the hope that, like, you will be able to sort of find a particle and that it will be like in the annals of history that this is Lizzie’s particle.

Lizzie: Well, it’s definitely not my particle. There are hundreds of scientists who work on this across the globe, so it will definitely be a joint effort. But in terms of a particle that we can put in the history books, yes. In fact, for physics, we have our sort of equivalent to a periodic table. We have what’s called the Standard Model of elementary particles. And this is essentially a list, a table, a model of the most elementary fundamental particles that exist in our universe, the smallest, the most basic building blocks for matter across the universe. And currently, when we look at the Standard Model, there are gaps. And one of the main gaps is the gap in which dark matter should exist. So theoretically, if we do discover dark matter, yes, we’ll be adding a particle to the Standard Model of elementary particles, and that will be incredibly exciting. The last particle that got added onto it was the Higgs boson in 2012, and that was an absolutely huge step for particle physics and fundamental physics as a whole. If we could possibly add another particle to the Standard Model, it would be amazing.

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Emily: This podcast was brought to you by Oxford Sparks from the University of Oxford, with music by John Lyons, and a special thanks to Lizzie Bloomfield.

Tell us what you think about this podcast. We are on the internet at Oxford Sparks. We have a website, Oxford Sparks.ox.ac.uk. I’ve said it too many times.

I’m Emily Elias. Bye for now.

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