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Thank you very much for the opportunity to come and speak to you, and thanks very much for coming along.

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So the title of the talk is Tracking the Invisible, and it's really a quest to see things that are tinier than anything else in existence.

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So just how tiny is tiny? Well, if I if I start off with a ball of Play-Doh, I can break it into two bits easily enough.

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And I can keep breaking that into two and into two and into two.

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And one of the questions that even the ancient Greeks asked was, could you could you do that forever?

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Or is there some point at which you got to a piece of material that was indivisible?

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You couldn't break it into two anymore. And it turns out the answer to that question was yes.

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If you go long enough, you end up with bits of stuff that cannot be divided anymore.

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And that's what we call fundamental particles.

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And my job as a particle physicist is to study these tiny bits of the universe which make up everything in this room, everything in the galaxy.

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So how small is small? Let's try and put this in perspective a bit.

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So this huge thing here is a red blood cell. So in real life, that's a millionth of a of a metre across.

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And then you have an atom over there, which is 10,000 times smaller than that.

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And the atom isn't even the smallest thing that you can think of.

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So in the early 20th century, physicists discovered that you could split the atom into even smaller bits.

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Which anyone who's watched Chernobyl recently will know all too well.

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But if you go further, this this is this is our atom from the previous life.

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And that thing is 100 million times larger than an electron.

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And this electron in the top left, that is one of the smallest things in the universe.

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You cannot, as far as we can tell,

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you can take an electron and put it in a vise or take a kitchen knife to it and chop it into electrons or what we call indivisible quanta.

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They are little lumps of stuff.

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And with these indivisible particles, you can build any atom that you want, and thus anything, anything goes in terms of all of the periodic table.

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So just to put this in context, at most the electron is that big.

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That's all not read out all the zeros, that's ten to the -18 metres.

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But it's actually probably even smaller than that. It might be so small, in fact, that it doesn't actually have a size.

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So in our theories, we don't put a size onto the electron.

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It is an infinitely small point in 3D space.

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So one thing you might ask well is I mean, if something so tiny, how can you even attempt to see something like that?

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And the answer, the genuine answer is you can never see one directly,

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but you can build machines and use techniques that help you see where these things go.

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And that's the that's the topic of the talk, and that's where we get into thinking in 3D.

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So a nice analogy that I came across quite recently is you can see tracks left in snow.

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So the animal that left these tracks isn't there anymore. Right. It's it's been there and long since gone.

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But the tracks it leaves behind tells you something about what that animal is, maybe how big it is and the direction it took.

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And this is a really close analogy for how we actually look for fundamental particles in physics.

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So what we do is look for the traces that they leave behind inside our detectors.

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And what's quite cute is that different particles have their own distinctive traces.

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They don't all look the same. So we can actually use these machines to tell whether it was an electron or a heavy cousin of the electron emission.

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They all look different to to our detectors. And these are not particle traces, in case you're wondering, these are different animal tracks.

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So we have a racoon on the top left. I only chose like cute animals and then a squirrel.

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And this one is not so cute, but it's a crow. But it gets across the idea that different tracks tell us different things about different particles.

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Now. Where is all this happening and why is it important?

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Well, as we heard in the introduction, I work at CERN, which is based on the border between France and Switzerland.

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Very nice scenery. And then there's this huge tunnel, which is 27 kilometres long as the size of the Circle Line in London.

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It's not actually aboveground. This is 100 metres underground and they didn't paint the whole countryside yellow.

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This is just a just for your reference. So inside this tunnel is what we call a particle accelerator.

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And in this accelerator, we send a beam of particles in one direction and they go at basically the speed of light.

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So these things are moving incredibly quickly with incredibly high energies.

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And then we send another beam in the opposite direction. And at a few different points on the ring, you can see them.

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You can see them No. Two down here. These are different experiments.

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And in those experiments, we bash them head on.

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Right. So we have two beams travelling in opposite directions with incredibly high speeds, really, really large energies.

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And we bring them together. And these beams are less than a width of a hair.

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So you have a beam that's 27 kilometres long as thin as a hair.

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And you've got another one. So you're colliding two incredibly energetic hair, thin beams at these points.

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And the point of all that is to cram an incredible amount of energy and heat into a really, really small area of space.

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And that's interesting because that's exactly what the universe looked like in the first moments after the Big Bang.

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So in a very real sense, this Large Hadron Collider is a bit of a time machine.

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We can recreate the conditions of the very early universe in these collisions of particles.

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So you bash these you bash these beams together, and out comes hundreds of very interesting,

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exotic, heavy particles that don't exist anymore in the normal universe that we live in.

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They only existed in the very early universe, and that's what we're interested in studying at this machine.

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So at each of these points where we where we collide, the particles, we want to know what's coming out of those collisions.

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Right? That's what we're really interested in. So you have to build a machine to take pictures of it.

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And that's what this thing is. This is one example.

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There are several of these huge machines at CERN. This isn't the one I work on, but it is the largest.

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To put this in context. The two beams pass sort of into the board.

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So one goes into the board and another one comes out of the board. And in the middle of this machine, you bash them together.

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This thing is 25 metres tall and 45 metres long.

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It's actually the largest technological construction ever built.

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So this is the largest science experiment ever conducted.

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And it's a big concern shared of what you just told me, that we're looking for the smallest things in the universe,

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and yet you've built the biggest machine in the world in order to do that. It's not just to show off that.

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The reason is that they're incredibly energetic, these particles that come on.

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So you need to put a lot of stuff in the way to slow them down and actually measure them.

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Otherwise they'll just fly off.

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So you have to make lots and lots and lots of layers of electronics in order to make measurements of them as they travel through.

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So we fill this huge space with basically a 3D digital camera.

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So you actually you have the particles colliding and they they come in in all directions in three dimensions.

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And the job of this detector is to trace these particles as they move through the machine.

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And I have a little video. Hopefully it will work.

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So this is this is typically what we call it. This is an event display.

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So this is a computer graphic reconstruction of what these particles look like when they're produced.

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So when it cycles back right now, I'll just show you. So you can see they're sort of a centre of this picture.

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And that's that's the point at which these collisions are occurring and all of those orange lines and bars and things.

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Those are the traces left behind by different particles.

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And the bars, the green and the and the blue bars are measures of how much energy they have.

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So this picture is is fully three dimensional.

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So that that. The physics of this collision is captured in 3D, which sort of gave me the idea for this talk.

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It really fits rather nicely with with the ideas of the exhibition.

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So this is all well and good, but what can we actually do with pictures like this?

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Is it is it just curiosity or can we actually do some physics with it?

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So I'll give you an example. The one particle that wasn't known before the LHC was built was the Higgs boson.

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It was predicted, I think, in 1964, and it took basically 50 years between it being predicted to exist and actually being discovered at this machine.

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And this is a picture of of a of a real life Higgs boson produced in a CERN collision.

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And the Higgs doesn't hang around very long. It actually lasts for less than ten to the -20 2 seconds.

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So basically, you make it and then it's gone. But what's left behind are the things that it actually turns into.

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And in this case, you can see these see these two big green bar things.

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Those are two particles of light, two photons. And they're not just random particles of light.

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These are the two photons that the Higgs boson turned into.

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So if you have a camera that's capable of tracing those lines.

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Then you can add up the energies of those two photons and figure out how heavy the thing was that they came from.

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And in doing that, that's how you discover a particle. You look at what they turn into.

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You add them together and ask, How heavy was that thing?

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So the Higgs the Higgs can do not just that. It has a few different party tricks.

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Here's another one. So instead of two photons, you can produce two electrons.

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Those are the the green things and two neons. So this is a different looking picture, but it's it's the same type of fundamental process.

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We've produced the Higgs and then it decays into tracks that our 3D camera is capable of capturing.

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So this is two millions and two electrons. Any gas is what this one is. This is another Higgs boson decay.

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GOODMAN Yeah, for millions based on the fact that there's two right things here and four things here.

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So, yeah, I have a Ph.D. in this issue. It's great.

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You should sign up. So, yeah, the Higgs. The Higgs can do lots of different things, but ultimately it all comes down to this equation.

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So any physics talk, you have to have to throw at least one equation. And so what this tells us is the energy of these particles.

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As they come on, we can add them together. And on the right hand side, we have mass.

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So mass and energy are basically equivalent things in physics.

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And they have the C squared, which you can think of as just a currency exchange rate.

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So we've got these particles with four energies.

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If we add those energies up, then we know what the mass of the thing that produced them was.

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And you can think of the mass as kind of a passport photograph. Different particles have different, very distinctive masses.

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So if you know what the masses that you've discovered a new particle and this is exactly what happened at CERN.

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So over time, this graph has more and more and more data in it.

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This is as the collisions run in. And do you do you notice a bump around 125?

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So here we go. I think the video will repeat so. So don't worry. That bump, they're sitting on top of what we call backgrounds.

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Any science experiment usually has other stuff that looks like what you're interested in,

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but actually isn't that bump above the smoothly varying background that is the Higgs boson.

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And they're all sitting at one very particular point in the graph because that's the mass of the Higgs boson.

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So remember I said, if you can figure out what the mass is, then you've discovered a new particle.

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And all of these data points just correspond to two different pictures, just like this one.

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And when you add them all up and run your experiment for long enough, you can you can make discoveries like this one.

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So at CERN, this was this was actually one of the main reasons why the Large Hadron Collider was built.

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The Higgs was the last piece of the Standard Model jigsaw, and it took almost sort of 100 years to fill it all in.

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But now that we've discovered it, we have good reason to suspect that it isn't the final story.

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There's a lot of problems with the standard model. So the hope is that this Hadron Collider can actually not just discover what we expected was there,

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but also discover new things, maybe dark matter or other interesting phenomena that we that we have no idea about just yet.

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So that's that's the whole idea. And I'll leave you with a few take home messages.

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So fundamental particles are the smallest things in existence.

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And as far as we know, you can't make them any smaller. They might even have no size.

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And we use these huge three dimensional digital cameras to trace their paths when we produce them.

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And it's really these traces that help us to understand the building blocks of the universe and even discover brand new particles.

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So thank you very much.