On a Collision Course

Introduction to particle physics

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1:42Sometimes to get the big picture, you need to think very small and move at phenomenal speed. So the reason you accelerate these protons is when they collide, their kinetic energy is converted into new forms of matter. So that could be the Higgs boson, as it was in 2012, or it could be dark matter particles or other strange things that we might hope to discover. Surprising new discoveries and experiments around elementary particles are helping to reveal the fundamental nature of the universe. Welcome to Big Picture Science. I'm Molly Bentley. A panoply of particles, from neutrinos to dark

2:17matter to bottom quarks, puts this episode on a collision course.

The Higgs boson and cosmic neutrinos

2:31When scientists detected the tiny elementary particle, the Higgs boson, at the Large Hadron Collider in 2012, it was a big deal. It confirmed a long-standing theoretical prediction about its existence and the role it played in giving other particles mass. And it earned two scientists the Nobel Prize in Physics. I now ask you to step forward to receive your Nobel Prize from the hands of His Majesty the King. More recently, with the Higgs hoopla behind it, CERN's Large Hadron Collider has begun a long

3:11shutdown for an upgrade. It won't run again until 2030, so you could be forgiven for thinking that all is quiet on the high-energy particle front, but the field is as fast-moving and inspiring as ever. One of the the field's most ambitious experimenters has, once again, caught the attention of Stockholm. The Royal Swedish Academy of Sciences has decided to award the 2026 Nobel Prize in Physics to Francis Halson

3:42for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin. Neutrinos hold another key to understanding the fundamental structure of the universe. Produced by some of its most energetic events, such as supernovae, they escape intact to become cosmic messengers carrying information about otherwise opaque phenomena. Now mass is a theme here too, because these ghost particles are

4:18nearly massless. Neutrinos are also shapeshifters, changing their identity as they zip around. Dr. Halson's Nobel Prize acknowledged the study of cosmic neutrinos, but his isn't the only neutrino experiment in town. Another group doesn't plan to wait for the universe to churn up these high energy particles. They will be producing their own.

The DUNE neutrino experiment

4:40My name is Dr. Kate Shaw. I'm an experimental particle physicist working on the deep underground neutrino experiment, DUNE, and I work at the University of Sussex. Like many experiments in particle physics, the deep underground neutrino experiment, or DUNE, is a massive operation, in this case spanning three states. The neutrinos will be made and fired from Fermilab in Illinois, at a detector more than 800 miles away at Sanford Underground Research Facility, or SURF, a group of enormous

5:14underground caverns in South Dakota where trillions of neutrinos will plunge into tanks of liquid argon. They travel about 1,300 kilometers. And again, what's wonderful about neutrinos is they go straight through the Earth. We don't need to dig a big tunnel like you need for some big particle experiments. They go straight through the Earth's crust and they pop out at South Dakota at the Sanford lab. And it's there that we built our particle detectors, which are a mile underground at a very old mine.

5:46By observing what happens when neutrinos hit argon atoms, Dr. Shaw and her team hope to answer fundamental questions about the universe. We're really trying to understand why any of us are here at all. Neutrinos are fascinating particles. Billions are going through you right now, but they don't interact with anything, but they are the most abundant particle in the universe. And we think they may hold the key to why we live in a matter dominated world. So why we have, you know, stars, suns, black holes, you, me, trees, you know, the Earth planets, why any of this is here at all.

6:24The standard model that we have a particle physics, which means the picture that we have at the moment describes all the different particles and interactions very precisely. But the area that it does not describe precisely is with the neutrinos. And we therefore see it as a way of really going deeper into physics. In other words, if you're going to answer these questions about the nature of the universe, you need to understand the whole picture. And right now the picture is incomplete. And one of the reasons it's incomplete is we don't understand the behavior or the nature or we still have questions around these particles.

Neutrino oscillations and mass

7:02Exactly. So one of the mysterious things they do is a phenomena called oscillation. So neutrinos, when they travel from point A to point B, do not stay as the same, we call it flavor. They change into one another. So we have an electron neutrino, muon neutrino and tau neutrino inspired by some of the Greeks, which is bizarre. It doesn't make sense. Our description of particle physics doesn't describe or understand the mechanism why that should happen. Someone presented this metaphor for me, and let's see if this works, that if it's as though you were observing a race or one person was running, and they started with one jersey from one country.

7:44And as they ran that lap, by the time they came through the finish line, they had changed their jerseys a couple times from maybe, you know, USA to UK to Spain. And you want to know how were they able to do that while running that lap? How were they able to change those, you know, identities? I absolutely love that. That explains it really well. Another aspect of that explains it well is when we say they oscillate, it doesn't mean they just turn from one to the other. They could sometimes turn, you know, change their jersey back and forward, become back what they had in the first place and change again.

8:19So there's a lot of oscillation that happens on the way. So I love that metaphor. And what we're interested in looking at is the difference between how that oscillation, how that changing of their jersey shirts differs between a neutrino beam and an anti neutrino beam. And we think there will be a difference between these two different types of beam that we're projecting across. Help us understand how a particle that has no mass can oscillate, can shapeshift in this way. What is it, what is it changing from if it's so elusive, but also if it doesn't have mass?

8:54That's an absolute question, Molly, because you've really hit on the exact issue that oscillation solved. So in our standard model of particle physics, our very precise description of the universe, we do think that neutrinos should have zero mass. That they oscillate, the evidence that they oscillate, tells us that the mass cannot be zero. And that was one of the key ways we understood that neutrinos were breaking our standard model. We don't know the mechanism by which they get their mass, and we have no explanation for why they have it.

9:28Do you understand a particle like this, then, by how it interacts with other particles? Is that the key to understanding the behavior and the properties of neutrinos? Yeah, exactly. In particle physics, if we want to study a particle, we have to shoot it into other particles. We have to collide them together and see what happens in that collision. But as you can imagine with neutrinos, if there's billions going straight through the Earth every second, and they're not interacting with anything, how on Earth do we actually manage to make that interaction happen to study it?

10:01As we're talking today, it's a good day for neutrinos because another neutrino experiment was just recognized by the Nobel Committee, and that is the Ice Cube experiment. Now, that experiment is looking at cosmic neutrinos, and they're looking at how they rain down on Antarctica. But the Dune experiment is different, and I wonder, Kate, if you could just distinguish between these two experiments, because you're actually creating the neutrinos yourself. Yes, exactly. So, Ice Cube is really famous for using Antarctic ice to detect the neutrinos that are coming from deep in the cosmos and allowing them new ways to observe the universe.

10:43Really incredible stuff. What we're doing at the Deep Underground Neutrino Experiment based in the US is to shoot a beam of neutrinos and antineutrinos through the Earth into some big vats of liquid so that we're actually emitting the neutrinos we are detecting. That said, we are also going to be sensitive to intergalactic neutrinos coming from outer space. Now, to better understand the difference between the cosmic neutrino and the neutrinos that you're creating, how are neutrinos created in the cosmos, in their natural habitat?

11:18So, the main producer of neutrinos are supernovae of stars. They produce an incredible amount of neutrinos. So, Kate, does that mean you have to recreate the conditions of a supernova there at Fermilab to produce these neutrinos, or do you have a shortcut or another method to produce them? That's an excellent question, and very useful for us. We don't have to go up to anywhere near those kind of energies. We wouldn't be able to if we tried, right?

11:48No, the way we make neutrinos is actually really simple. We fire a very small beam of protons. So, protons, of course, make up all of atoms. So, they're positively charged. So, using some magnets, it's very easy to extract protons. And we fire these protons at a target, which creates new particles, which then decay into neutrinos. So, they're relatively easy to make in a laboratory.

Matter and antimatter asymmetry

12:15I wonder if you could now describe the role of the anti-neutrinos and how, if you have neutrinos and anti-neutrinos, they might help us understand something about matter and antimatter? How so? Yes. So, when particles are created, they can be in their positive or their negative form. So, example, for an electron, you can be an electron which has negative electric charge, or you can be a positron, which is exactly the same as electron, but has positive electric charge. And so, for example, when the sun burns, it creates positrons. And so, positrons are created in the sun and they have positive electric charge, and they are associated with a given neutrino. So, you might have an anti-neutrino or a neutrino associated with your electron or your positron.

13:04And so, this is the case for all particles. Every particle has this kind of partner. And in general, they act exactly the same. They are equal. And what we're trying to see with neutrinos is, are they equal? And we suspect there may be a difference between neutrinos and anti-neutrinos. But would that then explain why there's matter in the universe and not, I mean, maybe there is also antimatter still, but the matter seems to have won out because we have stars and galaxies and the earth and you and I are talking to each other now. Is that one of the puzzles is why it is that the universe was filled with matter and not antimatter?

13:42You're exactly right. At the beginning of the universe, these positive and negative particles were created equally. And then, as the universe evolved, many of them came together and cancelled each other out. But we have so much matter remaining, right? And so, we realised that for every, say, billion particles of antimatter and billion particles of matter, when they annihilated, there was always one left over. So, there would always be one particle left over for every billions that cancelled each other out.

14:16And that's what you and me and the earth and the stars and the suns are all made out of. It's kind of this, you know, every single one of our particles are that one billion and one special particle that made it through in the early universe. So, we should all be thankful for that asymmetry. Exactly. Yeah. So, what we're looking at is to see if our neutrino beams are going to be acting differently, if the oscillation will be acting differently. And we think that if we see some asymmetry between the two neutrino beam and anti-neutrino beam, then it might help us understand how neutrinos played a part in the beginning of the universe.

14:58And is that a matter of, this is a really naive question, of counting the anti-neutrinos and the neutrinos? Does it come down to a numbers game that way? I mean, fundamentally, yes. When it comes to particle physics, yes, we're dealing with some of the most complex theories in the universe. But when it comes down to it, a lot of the science is relatively simple, right? We are going to be doing counting experiments. We're also going to be extracting more complex parameters that will be going into our theories that will help us understand the biggest picture.

15:31But one way to think about looking at this asymmetry is through a counting experiment. So, Kate, let's say you do answer this question and you do identify this asymmetry through the behavior of these neutrinos and you have a better understanding of how we all came to be. How does that change our lives or affect our lives? How does it change the way that we interact in the world? And in some ways, I'm looking at the role of these, some could say, esoteric questions and how they impact, you know, what it means to be human for those of us who are not doing the experiment.

16:07Yes. So the experiment we're doing at June, I mean, we'd be discussing the matter-antimatter asymmetry. But actually, I mean, what's fascinating about neutrinos is because they are the one part of our known area of the standard model that we understand so well, that is not behaving as we want it to behave. It can really open doors to understanding much more about the universe than just answering these questions. And we see that again and again in science. And I mean, why do we do this work in the first place?

16:40I mean, it's really a continuous cosmic and journey, right? We are all interested in where we came from, how the universe works. So I think fundamental science is a really important part of humanity, of our journey to understanding everything that we're made out of and to help inform and develop, you know, new technologies, new ideas, but always to inspire the next generation of scientists. Well, Kate, best of luck with the DUNE experiment. And thank you so much for this conversation.

17:11Thank you. Kate Shaw is an experimental particle physicist at the University of Sussex who works on the Deep Underground Neutrino Experiment, or DUNE. Well, Roland Pease, we turn to you now. Your background is physics. You've covered particle physics for years. I didn't think you had a favorite particle, but I understand that you do. And we just talked about it. Yes, it is my favorite particle because it's not just that it's elusive and hard to find and sort of mysterious.

17:43It's a real keystone in physics in all kinds of ways. So the neutrino really gets to the heart of lots of processes, the important processes of the universe in a way that, for me, some of the other things like the electrons and the quarks and things don't do. And those peaks that the neutrinos give us into these fundamental forces help us imagine what the early universe was like, maybe even the birth of the universe. You know, when the Big Bang happened, the universe was made up of light x-rays.

18:18It was made up of quarks and electrons and also neutrinos, all in equal number. So they were all created at the same time. So in a sense, if you're not studying the physics of the neutrinos, then you're missing out on part of the whole picture. And that's, I think, why what Kate was saying about if we could understand the neutrinos, maybe we couldn't understand why we are surrounded by matter but not antimatter. It's because these are all things that were going on at the Big Bang, and we don't have the full picture of it yet.

Hunting for dark matter

18:59Neutrinos aren't the only mysterious components of the universe whose secrets scientists hope to unravel. And for the vast history of our universe, from a few minutes after the Big Bang till 10 billion years after the Big Bang, dark matter dominated our universe. Next, have scientists finally had a run-in with the elusive particle? This episode of Big Picture Science is on a collision course. Quick question.

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21:35While neutrinos are extraordinarily abundant in the universe, each one is nearly weightless. Meanwhile, the primary component of matter in the universe, dark matter, both has mass and exerts a powerful gravitational pull on everything around it. Yet, dark matter is invisible. It can't be seen by telescopes. We know about it through its gravitational effects.

22:06For example, dark matter provides mass and momentum to galaxies, causing them to rotate faster than their visible matter can account for. It's phenomenally important, even though we don't know what it is. In our current understanding of the universe, there's five times more dark matter in our universe than regular matter. And for the vast history of our universe, from a few minutes after the Big Bang till 10 billion years after the Big Bang, dark matter dominated our universe and shaped what our universe became.

22:42And there are many theorists who think even the first stars were seeded by dark matter. So dark matter is just kind of inherently important in our universe. And so it is just a huge mystery for those of us working in physics, particle physics, particle astrophysics, and astrophysics to go after. Weird is the adjective that often accompanies descriptions of dark matter, even by the physicists who study it. And still, they persist. Because if we could just figure out what it was, we'd know what kind of matter was holding the universe together.

23:16I'm Kimberly Palladino, a professor of physics at the University of Oxford, where I work on searches for dark matter by building big detectors deep underground. Nearly a mile underground in South Dakota is the ultra-sensitive detector for LUX Zeppelin, an international project hunting for dark matter. It's located at SURF, which, you'll recall, is the research facility that is home to the receiving end of the DUNE neutrino experiment. Both experiments are nestled deep in a former gold mine to shield them from cosmic disturbances.

Detecting xenon and WIMPs

23:49You go down in very, very large elevators, and you walk through a mine that, you know, they've got lights set up, you're dressed as a miner, you have to follow all of the safety protocols as a miner, and then you suddenly arrive at a physics laboratory. It's there that scientists look for particle interactions in vats not of liquid argon, but of liquid xenon, a heavier atom. In 2023, the detector got a hit, a flash that could have been caused by the interaction of xenon with a WIMP, or Weakly Interacting Massive Particle.

24:26Now, a WIMP is one explanation for what dark matter may be, but after that tantalizing ping was a long wait, while experts combed through the data to confirm their suspicions. It felt like being a teenager having a crush, where you don't want to get your hopes up too much that the other person likes you, but you also read every little secret sign coming from them that they could like you back. That's what it's like as a scientist seeing perhaps a hint of the signal you're looking for.

24:58After three years of checking the data and ruling out the alternatives, the team announced their suspected findings. Dr. Palladino told Roland that the unusual event they witnessed, that flash produced by a collision with a xenon atom, could be a dark matter particle revealing itself. So we are figuring that dark matter is a new particle that is not too dissimilar from the fundamental particles that we already know about. And we expect that sometimes it will just bump into regular matter, and we expect that to be more likely for it to bump into bigger, heavier things.

25:37So we think it's going to hit the nucleus of the atoms making up our target material instead of the electrons. And initially we think that this bumping into it really acts very similar to billiard balls scattering, and a lot of the time it would have very glancing blows off of the target balls, and at least sometimes impart more energy. But we're looking for some brand new physics that we've never detected before, because that's, you know, fundamental to the idea of searching for dark matter. And so theoretical physicists can write down all of the different ways that particles can interact, and it doesn't necessarily have to match that simplest case.

26:18But in every case, it sounds like we can think of the dark matter particles. These would be like cue balls raining down on us from outer space, and your job is then to give them as many sort of red balls to hit, and then just wait to see if any of them moves. Exactly. And so we have to make sure that there's no other normal particles coming to at us either, that there's not kind of standard run-of-the-mill radioactivity around. And we put in a lot of effort to remove all of those standard interactions so that we're prepared to see even just one dark matter interact.

26:53And so that's why you're underground. That screens out the cosmic rays, which would otherwise, as it were, fog your view. And then you've removed all possible radioactivity that might confuse you. And yet you still just have to wait, and you still get all that sort of muck from time to time. Yeah, we still have to wait. And all we can do, we can never bring these numbers perfectly down to zero. But what I'm hearing here is, I mean, this really brings to me the complexity of this kind of experiment. First of all, you can't see the dark matter particle because it's invisible.

27:24So you're looking for the influence of the dark matter particle on an atom in your detector, but you don't even see the atom directly. You see these flashes of light because there's a disturbance that's caused that then sort of moves electric charge around. And then you sort of have to interpret that. This is pretty complicated stuff. It is. We have to calibrate our detector because not only do we see those flashes of light, but we have slightly different likelihoods of collecting light and charge from different areas of our detector or potentially at different times.

27:54And so we have to put all of those corrections into our data first, and we have to constantly monitor the state of our detector. I mean, it's a sensible question to ask. Basically, this is a bath of xenon, as I understand it, this xenon gas. A very cold bath, yes. Right. And it's sort of surrounded by light sensors, is it? Light detectors or cameras or what? Yeah. Actually, it's in a giant cylindrical detector. It's very human size. It's one and a half meters tall for the central region and one and a half meters wide.

28:27And the walls are made of just, you know, white PTFE, commonly called Teflon, that for this type of, it's UV light that we're looking for from these interactions. That's, it's almost 100% reflective. And then we have the photo sensors on the top and the bottom. So what was different, I think it was in June 2023, that sort of, at some point, leapt out and said, hang on, have we actually got something at last? Yeah. So the event we saw was at a higher energy and a nuclear recoil.

29:00So for this event in June 2023, we see two flashes of light, which is what make up our events. And the ratio of the first amount of light that comes from scintillation, when the atom is first hit, to the secondary signal, S2 light, that is created because when an atom gets hit, it can ionize some electrons. They get separated and they make new flashes of light. And so between the ratios of light in those two signals, you can tell generally if you hit an electron or if you hit a nucleus and you get a lot more of that early scintillation light.

29:36And so this event, it's really standing out alone as a potential signal event. What was the mood when you started to realize that this might be something, you know, what you've been waiting for years to see? It's, it's, the mood is a little bit of everything. You're excited, you're confused because it, again, it wasn't in the most standard place it could have been. There's a deep, deep fear that you've made a mistake and you've made, I think at this point, we're only hopeful that if we've made a mistake, it's at least not a mistake that anyone has, has fully called us out on said, oh, you know, you numbskulls, you forgot the X, Y, Z.

30:16But it's, it's always a fear that you're not as good a scientist as, as you want to be. So, so then we start triple and quadruple checking the things we had already double checked.

30:28And eventually you choose to publish. This might be the first sighting of a bit of dark matter, which has puzzled astronomers and physicists for so long. What about the, the mass of this particle? I mean, there's, I guess if you see a recoil, you can get a recoil from something light traveling fast and hitting it or something heavy sort of coasting along. Can you interpret the data to tell you how heavy this dark matter particle might have been? We can get a little bit of a sense because we know that, that momentum still has to be conserved.

30:58And so that tells us kind of at a minimum mass, the particle is probably, we'd say 200 GeV over C squared. This is a particle physics unit, but it means it's about 200 times the mass of a proton. But it easily could go up to more than a thousand times the mass of a proton. So that would be quite heavy. I mean, that's actually, I suppose, a little bit heavier than the nucleus of a xenon atom. That's where you're getting that from, is it? Yes. And for it to be able to give this much energy going, we also are assuming that because we know how fast dark matter would be traveling in our own galaxy before it would escape, that limits how much energy these different particles could have.

31:40And that tells us again, how much energy it could impart. So somewhere between 200 and a thousand times the mass of a proton, probably not going desperately fast. I mean, it's sort of, it does feel like the information is quite limited on just a single detection like this. Oh, it's incredibly limited. Again, with one event, you know, we have very little to go on and yet it becomes all the more of a mystery, a clue. It does feel like, you know, you're in the middle episode of a mystery series.

32:14And is this the clue that's going to break it open or is it a red herring? I mean, I guess you know how long you've been running LZ, I guess. And you've got, you know, one detection in, I don't know what it was, a year or so of watching. So you might hope that, let's say, once a year you might see something like this. But also it'd be quite nice to see a collision which is slightly different and that would then start to build up a picture. But it would be at best a very slow process.

32:46And if you were just lucky on this time, that's not going to help. What's the thinking in the group? The thinking is definitely we have to analyze more data. The data set we analyzed stopped in 2024. So we have essentially two more years of data since then, as well as the fact that our detector is still running and will run for longer. So we are working to define what our final search strategy for the next data set will be to do it as efficiently but as rigorously as possible.

33:19Because now that whole process of bias mitigation is much, much harder because we already know we saw one event in a previous data set. Gosh. So even seeing one thing you're saying can actually contaminate the mathematics of how you interpret future events. Less than the mathematics, but more the human behavior that does the analysis behind it. Kimberly Palladino, it's been an absolute pleasure talking to you about this. Thank you so much. Kimberly Palladino is an astrophysicist, a professor of physics at the University of Oxford, and the project scientist for Lux Zeppelin, an international collaboration searching for dark matter.

34:00Roland, as she said there at the end, because she and the other scientists are human doing the science, they have to temper their natural excitement at the possibility that they've isolated dark matter while they comb through the rest of the data. I mean, they have to continue thinking rationally. As you say, the anticipation, they've been looking for this dark matter for 30 years with bigger and bigger and bigger experiments like Lux Zeppelin.

34:33And I have to say, I think a lot of us had given up on the idea this would ever work. So the fact they got this one ping in the machine, no matter the fact it doesn't tell them an awful lot, of course they were excited. And of course, they really did not want to look fools, which is why it took them so long to actually reveal the result. They just wanted to make sure there wasn't something going on in that machine that they mistook. Something electrical short, shall we say, that fooled them. They had to be so careful.

35:04Roland, why do these particle detectors have to be so big? I mean, there's this discrepancy between what they're looking for, which is so small, and the size of the detectors themselves, which tend to be massive. Well, I guess it comes down to two things. Often these things are incredibly energetic, in which case you need a big volume of stuff in which to sort of pick out the details of what's going on. You know, you have a particle shooting through some detector. You want to make sure you actually get all the information.

35:36So that's one reason why they're massive. And again, in the case of Lux Zeppelin or the experiment that we're talking about with the neutrinos, these are very elusive particles. So they're probably streaming through us all the time. And the only way that you're going to pick up one or two of them is if you have an absolutely massive detector. So they're hoping to build another detector that's going to be about 10 times as big as Lux Zeppelin, because that way they can increase the chance of actually detecting any of these particles.

36:08And I guess compared with the size of the original laboratory, which is the universe itself, these detectors and these laboratories are pretty small. I think you should be on their PR team, Molly. Absolutely. You know, they get a big bang for their buck, even if you have to wait 30 years for that pop. Coming up, the Large Hadron Collider may be temporarily shut down, but that doesn't mean the work there has stopped.

36:44We have, from the previous run that just finished, this huge amount of data that we've recorded. And we've only really started to scratch the surface of that data sample. So there's going to be a lot of work in the next few years, really getting all the physics we can out of that amazing data resource. Next, another search for yet more exotic building blocks of the universe. This episode of Big Picture Science is on a collision course.

37:14We'll be right back.

The Large Hadron Collider upgrade

37:44We mentioned that the celebrated Large Hadron Collider, or LHC at CERN, that straddles Switzerland and France and where the Higgs boson was discovered, has gone quiet as it submits to upgrades. But that doesn't mean that science has stopped at the world's largest laboratory for particle physics.

38:15Hello, I'm Harry Cliff. I'm a particle physicist at the University of Cambridge, and I work on the LHC-B experiment at the Large Hadron Collider at CERN. The LHC-B experiment is one of four massive detectors housed around the LHC, which have been watching high-energy collisions between protons for over a decade now. As scientists try to work out the secrets of the universe, the Higgs boson was their big catch. Scientists at CERN have not made headlines since, though they have been tidying up many of the details in our understanding of the forces and particles that make up the cosmos.

38:52The shutdown allows the whole machine to be upgraded so they can make even bigger discoveries. It's at great expense, but Dr. Cliff told Roland that that's how they make sure that the LHC continues to be a smashing success. The LHC, it's the biggest scientific instrument ever built by human beings. By some measure, it's the biggest machine that's ever been built by human beings. It's a gigantic circular particle collider. It's 27 kilometers in circumference, and it's buried about 100 meters on average underneath the Swiss-French countryside just outside Geneva, which is the location of CERN, the European Organization for Nuclear Research.

39:31So, I mean, what the LHC is, is, you know, it's a gigantic accelerator. What it does is it takes protons, which are the nuclei of hydrogen atoms. It whizzes them around this 27-kilometer ring, and they get faster and faster every time they orbit the ring. And then eventually, when they're going at tremendously high speeds and have huge amounts of energy, so this is when they're going at 99.9999991% the speed of light, they're brought into collision. So they collide into each other inside these gigantic detectors, which are these kind of cathedral-sized, three-dimensional, highly precise digital cameras that record what happens in these collisions.

40:09And the reason we're doing this is what the LHC really is, is a machine for making exotic types of fundamental particle that don't normally exist in nature. So the reason you accelerate these protons is when they collide, their kinetic energy is converted into new forms of matter. So that could be the Higgs boson, as it was in 2012, or it could be dark matter particles or other strange things that we might hope to discover. So really what we're doing with the LHC is kind of exploring this very high-energy regime and seeing what new fundamental particles we can create out of the kinetic energy of these collisions.

40:45And when one of these things is bored in the middle of one of these detectors, it sort of instantaneously falls to pieces and throws out debris in every direction. So you never have the thing itself. You're looking at the debris that flies out and trying to work backwards what was there for an instant. Yeah, that's exactly right. So if you take the Higgs as an example, I mean, a Higgs boson lives for a fantastically short period of time. I think it's something like 10 to the minus 25 seconds. That's a millionth of a millionth of a millionth of a millionth or something like that. Yeah, something of that order. So it's, you know, it's so short that it's no time for the particle to hit any sensitive bit of your detector.

41:22What happens is the Higgs, it doesn't exactly fall apart. It transforms. It actually converts into other particles. And one thing it can do, for example, is turn into two photons. So two very high-energy gamma rays that will fly out more or less back-to-back in your detector. And you'll see two big energy deposits on opposite sides of the detector. And then analysts will then trawl through the data and search for the very small fraction of collisions where something like this has happened. And then reconstruct the Higgs boson by adding together the energies of these two photons.

41:53So that's the kind of way that these things are discovered in general. So, yeah, you very rarely see, almost never see the thing that you're actually trying to find. You see the after effects of it and then reconstruct its presence. So you've been doing this now on and off at CERN for, I guess it's since 2010 or 2011, it started working properly. And it's had sort of tweaks to its engineering every now and again. But they have just closed it down for several years for a massive revamp. Is that right? Yeah, that's right. So we've just entered this period known as Long Shutdown 3.

42:27So we've had Long Shutdown 1 and 2 already. And what Long Shutdown 3 is really going to be is a major upgrade of the accelerator and some of the detectors. So there's this this fact that you have when you've been running an experiment for several years, as we have with the LHC, every new year of data that you take is a smaller and smaller fraction of what you've already got. So it has limited impact if you just keep running the thing. So the way you make progress, the way you get more sensitive, the way you can find rarer and rarer processes is to dramatically increase the rate at which you record data.

42:59So the idea of this upgrade of the LHC, which is called the high luminosity upgrade, what that basically means is we're going to upgrade the luminosity or the brightness of the beam. So how intense the collisions are will mean that you get a sort of factor of 10, more or less, increase in the rate of collisions. So that means that a year of data in this new era will be worth 10 years worth of data in the old era. So that means we can then dramatically increase our sensitivity to all kinds of different processes and particles and make very, very precise measurements. So that's the kind of, but to do that requires a pretty major overhaul of the accelerator and the detectors, because you have to have a way of, basically the way you achieve this is by squeezing the proton beams to even narrower.

43:40You imagine these proton beams are like very, very thin, far thinner than a human hair. The thinner you make them, the more intense the collisions become, because it's a bit like having flocks of birds where the birds are more tightly packed when they fly through each other. Less space in between. Exactly. You're reducing the empty space and having more head-on collisions. I mean, the engineering on the machine in any case is pretty astonishing. I mean, you're talking about these protons zooming around at just about the speed of light and they're being steered by, basically they're being steered by magnets.

44:10You can't touch them, but they have to hit each other. You know, if they're that thin, you have to steer them pretty accurately to make sure they really meet in the middle to get any collisions at all. The engineering is astonishing. Yeah, I think someone described it once. It's like firing two knitting needles at each other from opposite sides of the Atlantic and getting them to hit midway. That's the kind of level of accuracy you require to actually achieve this. So it is stupendous. I mean, I'm not an accelerator engineer. I'm in awe of the fact they make these things work at all, really. It's kind of semi-miraculous, I think.

Precision measurements and funding

44:42And to go back, the idea is you're saying that all the easy things to find, in other words, the things that were popping up all the time, you've already discovered those. So now you have to find the things that only happen once every, you know, million or every billion or trillion collisions. And those are the ones that are going to have the clues that you need for the next level of physics. Yeah, to some extent. I mean, it's not just about finding rarer processes, but I mean, a lot of it is about precision. So, I mean, there are kind of two ways that you can discover new particles.

45:12And one of them is to directly create the new particle in a collider like the Higgs. So that was how the Higgs was found. You make the Higgs boson, you make many, many Higgs bosons, and then you spot that there's a kind of excess of particles in a certain region. But another way you can do it is to look at the indirect influence of new particles on things you already know exist. So this is kind of what we do at LHCB, where we study these bottom quarks. And bottom quarks are very interesting things because they decay, because they're very heavy.

45:44They can decay in many, many different ways to lots of other standard model particles. And the way they decay, this takes place through the fundamental forces of nature, in particular the weak force, which is one of the three quantum mechanical forces that we know of. And what can happen is let's imagine that there's another force that we haven't discovered before, where the particle of that force might be very, very massive and so massive that you can't actually create it directly at the collider. But that particle through quantum mechanics can still have an influence on the decay of your B quark and actually change, for example, how often the bottom quark decays in a particular way or change the angles that particles come flying out at.

46:24So by making these very, very precise measurements of properties of known particles and then comparing to theory, you can indirectly see evidence of these influences of things that might live at much, much higher energy that you can't actually access directly. So for you, getting more of these interactions improves the precision. So that's pure statistics, effectively, that then means that you can make very precise measurements on the particles you know about and then think about the ones that you might not know about yet. Exactly, exactly. And the sort of more precise you are, the smaller your uncertainty on your measurement, the more sensitive to what you are to small effects that could change the result from what you'd expect in the standard theory.

47:03You've now got a four-year wait, three maybe, four-year wait, and then it's going to be turned on again. I mean, I presume the anticipation is quite high for what happens next, but equally a certain degree of frustration while you have less to do? Yeah, I mean, I think, well, my colleagues on Atlas and CMS, they're going to be very, very busy with their upgrade because they're going to be basically rebuilding their detectors more or less in the next few years. So they will be kept off the streets for sure. On LHCB, I mean, so we had our upgrade, our first upgrade in the previous long shutdown.

47:39So what that means now is we have, from the previous run that just finished, this huge amount of data that we've recorded, which is, you know, factors larger than what we had at the end of the last run in 2018. And we've only really started to scratch the surface of that data sample. So there's going to be a lot of work in the next few years, really getting all the physics we can out of that amazing data resource. So we'll have plenty to do in the meantime. I mean, one thing probably worth mentioning, I mean, we are actually at LHCB in a rather uncertain situation now because our upgrades have been sort of out of phase with Atlas and CMS.

48:14And that was done basically so that not all the experiments were asking for money at the same time from the funding agencies. So that was a kind of strategic thing. You've been kind to the taxpayer. In fact, trying to, yeah, smooth out the bumps, basically. But so we have an upgrade planned for the middle of the 2030s that will allow us to really fully exploit the high luminosity LHC, increase our data rate even more by another order of magnitude, basically. And that project had been funded by the UK government and by many other international partners, most of the key partners in LHCB had committed funds.

48:46And then just at the end of last year, just before Christmas, we were told without any warning that the UK had withdrawn funding for that project. And there's been this kind of long running debate and kind of struggle to try to restore funding to this project since then. But we've been unsuccessful. So actually, we're in a very difficult position at the experiment that we don't actually know whether we're going to be able to keep running when the machine restarts in the 2030s. And, you know, that would be a huge missed opportunity because this experiment is one of the best ways we're going to have of knowing whether a future collider might discover something.

49:20Well, you've got you've I mean, you've brought me to something which I did want to ask about, which is, as it were, the public, but maybe more importantly, the political appetite for these kinds of experiments, because they are expensive. They employ a lot of people. And I suppose you need to persuade us that it is still producing in the kind of information we're paying you for. Yeah, absolutely. I think, you know, there are several arguments that you make for investment in fundamental research. I think the most important one of all is, you know, we do this stuff to understand the world we live in and to understand, you know, our origins.

49:57I mean, what we're doing at the LHC in general is, you know, answering really basic questions like, you know, what is the world made of? And also, you know, how do we get here? We're not only studying the makeup of matter, but we're also understanding processes that took place in the very early universe that gave rise to the world that we see around us today. You know, it's the same reason people do astronomy. I think it's actually interesting that in particle physics, I think because you build these big machines and you're in a laboratory, there's some expectation that you're somehow producing some useful technology.

50:27Whereas I don't think astronomers are kind of held to the same standard that it's kind of accepted that studying the night sky is a thing that of intrinsic value in its own right. And maybe that's because we don't have such pretty pictures to show people, but the kind of the philosophy is the same, I think. But I mean, that is the kind of core argument. I mean, you wouldn't build these machines for any other reason than actually to learn about the world. But, you know, there's, of course, many, many other supplementary benefits that you get from this sort of research. You know, classically, you get, you know, spin-off technologies like superconducting magnets for medical applications like MRI scanners, proton therapy machines.

51:02The classic example of the World Wide Web being invented at CERN by Tim Berners-Lee in the 90s as a way of sharing information across networks amongst scientists. And, you know, you can point to many, many other innovations that have come out of this kind of research of that nature. So there's, you know, there's many things in terms of practical benefits this research brings. But I think ultimately it's the question you need to ask yourselves is, you know, do we want to carry on understanding the universe and understanding these sorts of basic questions? And if we do, then, you know, along with big telescope projects, colliders are the other end of that toolbox, I suppose.

51:34And if we don't continue to do this kind of research, we will not be able to answer questions like, you know, the nature of dark matter, what happened in the very early universe? Because colliders are the only way that we have really of accessing that kind of information about the really, really small high energy scale. Harry Cliff, thank you for talking to Big Picture Science. Pleasure. Thanks. Harry Cliff is a particle physicist at the University of Cambridge and a researcher with the LHC-B experiment at the Large Hadron Collider at CERN.

The value of fundamental science

52:03And, Roland, that brings us to the big picture perspective in this show, the big picture about these small particles. I guess I want to come up with a defense, particularly as we, you know, Harry Cliff, we got on to the price of doing all this. I think it's incredibly important to understand all these intricacies of the universe, because we don't understand our place in it without understanding, you know, how everything works, the clockwork, as it were, of all the forces and things that make up the universe.

52:39You know, sometimes I think how diminished we'd be if we looked up at the sky at night and thought that this was some kind of black umbrella with little light bulbs shining at us. You know, how wonderful it is to know that we are part of this enormous cosmos that stretches out for billions of light years, filled with balls of burning, fusing plasmas and black holes exploding and so on. How could we understand, how could we appreciate our place in nature if we didn't know that we were all related because we all evolved in the same way?

53:15So, for me, it might be expensive, but it is absolutely human to try and understand what the universe is made of. So, that's my defence of why people like Harry Cliff, like Kate Shaw, like Kimberly Palladino should keep doing this important work. This show would not be possible without that enthusiasm and the smashing contributions of senior producers Seth Shostak and Gary Niederhoff,

53:54producers Brian Edwards and Shannon Geary, and consultants Seacon Akpan and Roland Pease. I am the executive producer of Big Picture Science, Molly Bentley. We'd like to give a big thanks to our listeners and to our Patreon supporters. The original music in the show is by Dewey DeLay and June Miyake.

Credits and closing thoughts

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