Quarks, Axions and other Particles - Sixty Symbols

Added: 2026-05-19

[00:00:00]Hey everyone, make sure you check out the end of the video so I can tell you about this new toy I've got. Do you think there could be some unknown physics emergent or fundamental hidden down at far subatomic scales? A proton is 10us14 and plant length is 10us35.

[00:00:18]There is a lot of room down there.

[00:00:20]>> Um, I really hope so. My whole career has been based on the idea that there's something interesting between the proton and the plank scale. Definitely there's something there. I mean, you know, there's lots of stuff that we we work with where you can sort of talk about physics that can happen at those sorts of scales like barioenesis, the physics of inflation, right? Which is the process of how the un we look around, we're in this big universe, right? The universe is pretty big, right? And it's looks pretty similar in all directions.

[00:00:48]We know that from the cosmic microwave background radiation. That's quite hard to explain why regions of space and time that should have really been you know seemingly all the light from that direction all the light from that direction has got the same sort of temperature you know why and it's this homogeneity and isotropy of the universe which we have to explain and the best way to explain it is with a theory of inflation where the universe sort of had this went through this exponential expansion in its very early days. Well that all happens at these sorts of scales typically in that little window of possibility that you're talking about. So there's loads of physics in that window that you could imagine.

[00:01:20]>> I began life working on what are called kutleline theories. These are theories of extra dimensions and these have the the sizes of those extra dimensions are sort of between these scales and so they have associated particles with masses to that can be up towards the plank scale.

[00:01:36]I then moved on to think about cosmic strings. Cosmic strings are objects which have a a diameter that is way way smaller than the diameter of a of a proton orbiting, you know, flying around the universe, distorting light. We're looking for them in the gravitational waves. You could have an explanation of of why the Higs particle is so light compared to, you know, say the fundamental scale of gravity. You know, there are these questions that are unanswered and they're all could have answers with new physics in those windows.

[00:02:08]and more more recently I've been thinking about inflation and inflation again is at these huge scales with these massive fields so please show up but they've not shown up yet and that's one of the big problems that faces us so I think definitely there is new physics in that window well I should never say definitely there might not be but you know I would like to think there are otherwise why are we here thinking about these theories that's a whole other question what do you think is the most likely new particle to be discovered if any >> it's got to be a super symmetric part particle that I well I don't know if it's the most likely it's the one that people are hoping for right I mean if you if you like wimps as a form of dark matter the the favorite candidates are from super symmetry and that was one of the there was two main reasons the LHC was built right the first was to find the Higs and the second was to find the dark matter particle in particular evidence of super symmetry I still think there's a strong argument why you would like super symmetry. It has lots of other nice features and the fact that we've not seen it yet moves you into different regions of the available parameter space of super symmetry. It it takes you out of what was the favorite region, but it doesn't mean it's not there. And I'd have thought that remains to probably the most likely >> right. I think it's probably an axon. I think an axon may well be discovered.

[00:03:40]So, okay. So, an axion is is a is a a type of particle that was originally introduced to solve something called the the strong CP problem. We don't go into details of what that is, but one of the things it's used for now is is as a as a a candidate for for dark matter. And I suppose more generally I would say in terms of the next particle to be to be found will probably be a candidate for dark matter at least a portion of dark matter I think and and it seems a lot a very popular idea for it at the moment is is is an is an axion axons are prevalent string theory you can get axons by the dozen actually more than a dozen in string theory so they're very prevalent from fundamental theory easy to get them they can be dark matter candidates um But yeah, I think I feel like we are probably on the brink of dark matter. I think within my my sort of lifetime as a as a physicist, you know, before my retirement and all that, I think we'll probably find dark matter, a dark matter particle of some sort. You never know, there might be modified gravity out there that to explain the large scale acceleration of the universe and that has its own particles. And so one of the things I have been thinking about with colleagues is the idea of using um accelerator data data from the LHC to search for particles associated with the modified gravity called chameleon theories and symmetron theories but they're very difficult to find. The third possibility is the axion and that I've just that's popped into my head.

[00:05:13]This is the that in fact many people who are who worked on WIMPs as dark matter have moved into axion physics as which is an alternative dark matter candidate.

[00:05:23]They're very light particles. Wimps are quite heavy particles. Dark matter wh axons are very light particles and there that's those are the two favored dark matter candidates and so there's a possibility that that will turn out to be the one that's detected. Coming back to the first one, the super symmetry particle.

[00:05:42]>> Does that have a name if that particle or what kind of what or would it >> Well, it depends. I mean, they could be neutralos, they could be they've got lots of different variants, combinations that can give you dark matter um of depending which regions of parameter space you're in. So, they're they're usually composite particles made up of a series of of of super symmetric um candidates if you of super symmetric partners. this just to just okay I'm going on a little bit too long but but you've got your particles and your particles come in the form of the forces of nature and the particles that make up the matter the forces are called are defined by their spins is one way of defining them and so they've got integer spins they're called bzons the the matter particles the electrons the protons the quarks have got half integer spins they're called firmians what super symmetry said was that For every Bzon with an integer spin, there is a corresponding firmion with an half integer spin. It's got an identical particle, but it's just got a different spin, the same mass, and for every firm, there's an identical bzon. So, they they doubled the number of particles. And so, it was quite easy then to concoct combinations of these super symmetric particles which would provide the dark matter. They don't interact very much.

[00:07:02]And and so that's been the surprise that we've not seen it, but it has lots of other nice features. I mean, um, one thing it tends to do is it it it allows you it provides you with a way of actually reconciling matter in with spacetime. It some people think of super symmetric spacetime. It's called super gravity. When you include gravity as a way of understanding spacetime that includes time, space, and the particles and super gravity allows you to do it.

[00:07:31]That's super symmetry with gravity in it. Also allows you to remember I talked about the hierarchy problem. Why why do particles exist at this scale the Higs particle and the top quark when the plank mass is up here and it doesn't why doesn't it shoot up? Super symmetry prevents it from shooting up. It provides a mechanism to give you this hierarchy. So there's lots of theoretical reasons for liking super symmetry. It's just not shown up.

[00:07:57]>> What does an axion look like? Like I I know what an electron is. I feel like I know what an electron is. I feel like I know what a proton is. I kind of understand neutrinos and quarks. And what's an axion? Is it got charge? Is it big? Is it small? Or >> uh I mean so it's it's its mass is quite it's quite low generally. But it depends. All these things depend. It depends what kind of axon it is. So perhaps the ones that people associate with dark matter, they're quite they're quite low low mass guys. Um >> do they have charge?

[00:08:25]>> Uh do they have charge? Uh, no. I do. They I don't No, I don't think so. No, they're they're a bit they're called pseudocalers. So, they're they're um so a I'm not aware of any axon with charge, but don't quote me on that. Um so they're pseudoscalers, which means that they're a bit like the Higs particle. The Higs particles are what's called scalar particle, but not quite.

[00:08:50]They do something different under parity flips where you reverse all the directions of space. So a Higs doesn't change its its spots under that when you do that whereas a an axion will. I mean they're no different really in that in much they're just a fluctuate a part every particle is is a fluctuation of a field. That's what it is. That's what a particle is. You have a field whether that's the electromagnetic field or the Higs field or the electron field or the axion field doesn't matter. The fluctuations of that field those are what the particles are. And so it's just like all the other guys. It's it's but it's it's it's a basonic particle. So it's it's more like I said it's a bit more like the Higs than say the electron.

[00:09:29]>> Tony, if the axion is associated with dark matter, I assume there's a lot of them around.

[00:09:36]>> What what what makes them hard to find?

[00:09:38]>> What makes them hard to find? Well, it's the the strength with which they interact with with ordinary matter is is weak. Um that's that's the issue there that that that the you know in the axon uh there's a they have a certain symmetry which kind of controls the kind of couplings that you can have to ordinary matter. Um yeah so it's just a weakness of the interaction I think with with with with stuff that we can you know we can play with. Uh we need to be able to draw the stuff into the stuff we can play with to be able to see it right. You don't want to kick kick this and you want to kick kick that and smash them together and produce an axion.

[00:10:13]Okay. Or >> shine light on them.

[00:10:15]>> Or shine light on them or have a particle accelerator and smash stuff together or or have a massive vat in the uh you know that somewhere underneath the Arctic or wherever it is or Antarctica. I can't remember which way it is. Ice cube experiment where where you know you're hoping that one of these guys is going to catch um an axion passing through. But the but the problem is they interact very very weakly. Um and so the effect is small when they do nudge these things.

[00:10:40]>> What's your favorite quark? It's got to be the top. And I think it's the top because we know so little about it. Um, and it's still playing a major role in our theories in that, for example, we still don't really know whether our universe is in a stable regime or whether it could kind of spontaneously decay. And um whether >> you're worrying me >> and whether or not it is depends upon a high precision measurement of the top and we're kind of we're in the bit which looks like it's stable at the moment but there's the top quark has an uncertainty and as old experiments measurements of mass and knowing that top quark mass is actually really important for us to be able to sort of confirm where we're going to be and it plays roles in I mean head if it's not in the right regime, it's not in the right parameter and it's not stable.

[00:11:41]>> Yeah.

[00:11:42]>> What does that mean is going to happen and when?

[00:11:44]>> Yeah. So, you're okay.

[00:11:46]>> Um it if you're in a if if we're in the unstable part, right? It's just unstable, which means it's got a lifetime way, way longer than the age of the universe. But it could it could go >> just like that.

[00:12:01]>> Yeah.

[00:12:02]>> Any moment it could just all stop. Well, if the universe when it does a transition, yeah, that that would be it.

[00:12:08]But it won't it's not going to do it because we're it >> Well, you say that now. Of course you'd say that.

[00:12:14]>> Oh, I like the sound of that. It's like It's like forget an asteroid. If the top quack goes, we're all gone.

[00:12:20]>> Yeah. Yeah.

[00:12:21]>> All right.

[00:12:22]>> My favorite quark. Um, charm.

[00:12:28]Yeah, I like charm. Firstly, it's charming, right? That's that's pretty good. Secondly, one of the things I like about it is that it was um it how it was predicted. It was found it was predict it existence was predict predicted based on uh something called naturalness. This is this idea it's kind of an ideic idea about physics is that the physics shouldn't contain large numbers and and stuff like this. And actually um Galiad and Lee they um preserving this principle of naturalism and looking at um the behavior of chaons they were able to sort of predict that there should be a new part new type of quark that should exist with a certain mass if this principle of naturalness this aesthetic principle about about nature should be preserved and indeed it was. It was found. So it's kind of like a little tick in the box for nature being beautiful. Nice. What do you think Ez was?

[00:13:24]>> The bottom.

[00:13:25]>> No, he went top.

[00:13:26]>> He went to the top, right? Okay.

[00:13:28]>> Yeah.

[00:13:30]>> Going heavy. Is he?

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