Thank you very much for the opportunity to come and speak to you, and thanks very much for coming along.
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.
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.
And I can keep breaking that into two and into two and into two.
And one of the questions that even the ancient Greeks asked was, could you could you do that forever?
Or is there some point at which you got to a piece of material that was indivisible?
You couldn't break it into two anymore. And it turns out the answer to that question was yes.
If you go long enough, you end up with bits of stuff that cannot be divided anymore.
And that's what we call fundamental particles.
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.
So how small is small? Let's try and put this in perspective a bit.
So this huge thing here is a red blood cell. So in real life, that's a millionth of a of a metre across.
And then you have an atom over there, which is 10,000 times smaller than that.
And the atom isn't even the smallest thing that you can think of.
So in the early 20th century, physicists discovered that you could split the atom into even smaller bits.
Which anyone who's watched Chernobyl recently will know all too well.
But if you go further, this this is this is our atom from the previous life.
And that thing is 100 million times larger than an electron.
And this electron in the top left, that is one of the smallest things in the universe.
You cannot, as far as we can tell,
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.
They are little lumps of stuff.
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.
So just to put this in context, at most the electron is that big.
That's all not read out all the zeros, that's ten to the -18 metres.
But it's actually probably even smaller than that. It might be so small, in fact, that it doesn't actually have a size.
So in our theories, we don't put a size onto the electron.
It is an infinitely small point in 3D space.
So one thing you might ask well is I mean, if something so tiny, how can you even attempt to see something like that?
And the answer, the genuine answer is you can never see one directly,
but you can build machines and use techniques that help you see where these things go.
And that's the that's the topic of the talk, and that's where we get into thinking in 3D.
So a nice analogy that I came across quite recently is you can see tracks left in snow.
So the animal that left these tracks isn't there anymore. Right. It's it's been there and long since gone.
But the tracks it leaves behind tells you something about what that animal is, maybe how big it is and the direction it took.
And this is a really close analogy for how we actually look for fundamental particles in physics.
So what we do is look for the traces that they leave behind inside our detectors.
And what's quite cute is that different particles have their own distinctive traces.
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.
They all look different to to our detectors. And these are not particle traces, in case you're wondering, these are different animal tracks.
So we have a racoon on the top left. I only chose like cute animals and then a squirrel.
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.
Now. Where is all this happening and why is it important?
Well, as we heard in the introduction, I work at CERN, which is based on the border between France and Switzerland.
Very nice scenery. And then there's this huge tunnel, which is 27 kilometres long as the size of the Circle Line in London.
It's not actually aboveground. This is 100 metres underground and they didn't paint the whole countryside yellow.
This is just a just for your reference. So inside this tunnel is what we call a particle accelerator.
And in this accelerator, we send a beam of particles in one direction and they go at basically the speed of light.
So these things are moving incredibly quickly with incredibly high energies.
And then we send another beam in the opposite direction. And at a few different points on the ring, you can see them.
You can see them No. Two down here. These are different experiments.
And in those experiments, we bash them head on.
Right. So we have two beams travelling in opposite directions with incredibly high speeds, really, really large energies.
And we bring them together. And these beams are less than a width of a hair.
So you have a beam that's 27 kilometres long as thin as a hair.
And you've got another one. So you're colliding two incredibly energetic hair, thin beams at these points.
And the point of all that is to cram an incredible amount of energy and heat into a really, really small area of space.
And that's interesting because that's exactly what the universe looked like in the first moments after the Big Bang.
So in a very real sense, this Large Hadron Collider is a bit of a time machine.
We can recreate the conditions of the very early universe in these collisions of particles.
So you bash these you bash these beams together, and out comes hundreds of very interesting,
exotic, heavy particles that don't exist anymore in the normal universe that we live in.
They only existed in the very early universe, and that's what we're interested in studying at this machine.
So at each of these points where we where we collide, the particles, we want to know what's coming out of those collisions.
Right? That's what we're really interested in. So you have to build a machine to take pictures of it.
And that's what this thing is. This is one example.
There are several of these huge machines at CERN. This isn't the one I work on, but it is the largest.
To put this in context. The two beams pass sort of into the board.
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.
This thing is 25 metres tall and 45 metres long.
It's actually the largest technological construction ever built.
So this is the largest science experiment ever conducted.
And it's a big concern shared of what you just told me, that we're looking for the smallest things in the universe,
and yet you've built the biggest machine in the world in order to do that. It's not just to show off that.
The reason is that they're incredibly energetic, these particles that come on.
So you need to put a lot of stuff in the way to slow them down and actually measure them.
Otherwise they'll just fly off.
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.
So we fill this huge space with basically a 3D digital camera.
So you actually you have the particles colliding and they they come in in all directions in three dimensions.
And the job of this detector is to trace these particles as they move through the machine.
And I have a little video. Hopefully it will work.
So this is this is typically what we call it. This is an event display.
So this is a computer graphic reconstruction of what these particles look like when they're produced.
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.
And that's that's the point at which these collisions are occurring and all of those orange lines and bars and things.
Those are the traces left behind by different particles.
And the bars, the green and the and the blue bars are measures of how much energy they have.
So this picture is is fully three dimensional.
So that that. The physics of this collision is captured in 3D, which sort of gave me the idea for this talk.
It really fits rather nicely with with the ideas of the exhibition.
So this is all well and good, but what can we actually do with pictures like this?
Is it is it just curiosity or can we actually do some physics with it?
So I'll give you an example. The one particle that wasn't known before the LHC was built was the Higgs boson.
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.
And this is a picture of of a of a real life Higgs boson produced in a CERN collision.
And the Higgs doesn't hang around very long. It actually lasts for less than ten to the -20 2 seconds.
So basically, you make it and then it's gone. But what's left behind are the things that it actually turns into.
And in this case, you can see these see these two big green bar things.
Those are two particles of light, two photons. And they're not just random particles of light.
These are the two photons that the Higgs boson turned into.
So if you have a camera that's capable of tracing those lines.
Then you can add up the energies of those two photons and figure out how heavy the thing was that they came from.
And in doing that, that's how you discover a particle. You look at what they turn into.
You add them together and ask, How heavy was that thing?
So the Higgs the Higgs can do not just that. It has a few different party tricks.
Here's another one. So instead of two photons, you can produce two electrons.
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.
We've produced the Higgs and then it decays into tracks that our 3D camera is capable of capturing.
So this is two millions and two electrons. Any gas is what this one is. This is another Higgs boson decay.
GOODMAN Yeah, for millions based on the fact that there's two right things here and four things here.
So, yeah, I have a Ph.D. in this issue. It's great.
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.
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.
As they come on, we can add them together. And on the right hand side, we have mass.
So mass and energy are basically equivalent things in physics.
And they have the C squared, which you can think of as just a currency exchange rate.
So we've got these particles with four energies.
If we add those energies up, then we know what the mass of the thing that produced them was.
And you can think of the mass as kind of a passport photograph. Different particles have different, very distinctive masses.
So if you know what the masses that you've discovered a new particle and this is exactly what happened at CERN.
So over time, this graph has more and more and more data in it.
This is as the collisions run in. And do you do you notice a bump around 125?
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.
Any science experiment usually has other stuff that looks like what you're interested in,
but actually isn't that bump above the smoothly varying background that is the Higgs boson.
And they're all sitting at one very particular point in the graph because that's the mass of the Higgs boson.
So remember I said, if you can figure out what the mass is, then you've discovered a new particle.
And all of these data points just correspond to two different pictures, just like this one.
And when you add them all up and run your experiment for long enough, you can you can make discoveries like this one.
So at CERN, this was this was actually one of the main reasons why the Large Hadron Collider was built.
The Higgs was the last piece of the Standard Model jigsaw, and it took almost sort of 100 years to fill it all in.
But now that we've discovered it, we have good reason to suspect that it isn't the final story.
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,
but also discover new things, maybe dark matter or other interesting phenomena that we that we have no idea about just yet.
So that's that's the whole idea. And I'll leave you with a few take home messages.
So fundamental particles are the smallest things in existence.
And as far as we know, you can't make them any smaller. They might even have no size.
And we use these huge three dimensional digital cameras to trace their paths when we produce them.
And it's really these traces that help us to understand the building blocks of the universe and even discover brand new particles.
So thank you very much.