Full episode with Harry Cliff (Apr 2020): https://www.youtube.com/watch?v=8A-5gIW0-eI
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Harry Cliff is a particle physicist at the University of Cambridge working on the Large Hadron Collider beauty experiment that specializes in searching for hints of new particles and forces by studying a type of particle called the "beauty quark", or "b quark". In this way, he is part of the group of physicists who are searching answers to some of the biggest questions in modern physics. He is also an exceptional communicator of science with some of the clearest and most captivating explanations of basic concepts in particle physics I've ever heard.

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I work on a detector called LHCb, which is 1 of these 4 big detectors that are spaced around the ring.

We do slightly different stuff to the big guys.

There's 2 big experiments called ATLAS and CMS, 3,000 physicists and scientists and computer scientists on them each.

They are the ones that discovered the Higgs and they look for supersymmetry and dark matter and so on.

What we look at are standard model particles called b-quarks, which depending on your preferences, either bottom or beauty, we tend to say beauty because it sounds sexier.

But these particles are interesting because we can make lots of them.

We make billions or hundreds of billions of these things.

You can therefore measure their properties very precisely.

So you can make these really lovely precision measurements.

And what we are doing really is a sort of complimentary thing to the other big experiments, which is, they, if you think of the sort of analogy they often use is, if you imagine you're looking in, you're in the jungle and you're looking for an elephant, say, and you are a hunter.

Let's say the elephant's very rare, you don't know where in

Either you can go wandering around the jungle and try and find the elephant.

The problem is if there's only 1 elephant and the jungle's big, the chances of running into it were very small.

Or you could look on the ground and see if you see footprints left by the elephant.

If the elephant's moving around, you've got a chance, your better chance maybe of seeing the elephant's footprints.

If you see the footprints, you go, okay, there's an elephant.

I maybe don't know what kind of elephant it is, but I got a sense there's something out there.

We're looking for the footprints, the impressions that quantum fields that we haven't managed to directly create the particle of, the effects these quantum fields have on the ordinary standard model fields that we already know about.

So these B particles, the way they behave can be influenced by the presence of say superfields or dark matter fields or whatever you like.

And the way they decay and behave can be altered slightly from what our theory tells us they ought to behave.

Lexie D'Ericati And it's easier to collect huge amounts of data on bee quarks.

And the only place really at the LHC, or really in high energy physics at the moment where there's fairly compelling evidence that there might be something beyond the standard model is in these beauty quarks decays.

Just to clarify, is the difference between the 4 experiments, for example, that you mentioned, is it the kind of particles that are being collided?

Is it the energies which they're collided?

What's the fundamental difference between the different experiments?

What's different is the design of the detectors.

So Atlas and CMS, they're called what are called general purpose detectors.

And they are basically barrel shaped machines.

And the barrel captures all the particles that go flying out in every direction.

So in a sphere effectively, they come flying out and it can record all of those particles.

And- What's the, sorry to be interrupting, but what's the mechanism of the recording?

Oh, so these detectors, if you've seen pictures of them, they're huge, like Atlas is 25 meters high and 45 meters long, they're vast machines, Instruments, I guess you should call them, really.

They have layers, concentric layers of detectors, different sorts of detectors.

Close into the beam pipe, you have what are called, usually made of silicon, they're tracking detectors.

They're little strips of silicon or pixels of silicon, and when a particle goes through the silicon it gives a little electrical signal and you get these dots, electrical dots, through your detector which allows you to reconstruct the trajectory of the particle.

That's the middle, and then the outsides of these detectors you have things called calorimeters which measure the energies of the particles and then right at the edge you have things called muon chambers which basically these muon particles which are the heavy version of the electron, they are like high velocity bullets and they can get right to the edge of the detectors.

If you see something at the edge, that's a muon.

That's all being fed out to, you know, computers.

So we, because we're looking for these bquarks, bquarks tend to be produced along the beam line.

So in a collision, the bequarks tend to fly sort of close to the beam pipe.

So we built a detector that's sort of pyramid cone shaped, basically, that

have your collision, stuff goes everywhere.

We ignore all the stuff over here and going off sideways.

We're just looking in this little region close to the beam pipe where most of these be quarks are made.

aspect of the sensors involved in the collection of the b quark trajectories?

the differences is that 1 of the ways you know you've seen a b

quark is that b quarks are actually quite long lived by particle standards.

So they live for 1.5 trillionths of a second,

which is if you're a fundamental particle, it's a very long time.

Because the Higgs boson I think lives for about a trillionth of a trillionth of a second, or maybe even less than that.

And they will actually fly a little distance before they decay.

So they will fly a few centimeters maybe, if you're lucky, then they'll decay into other stuff.

So what we need to do in the middle of the detector, you want to be able to see, you have your place where the protons crash into each other, and that produces loads of particles that come flying out.

So you have loads of lines, loads of tracks that point back to that proton collision.

Then you're looking for a couple of other tracks, maybe 2 or 3, that point back to a different place that's maybe a few centimetres away from the proton collision.

That's the sign that a little B particle has flown a few centimetres and decayed somewhere else.

So we need to be able to very accurately resolve the proton collision from the B particle decay.

So the middle of our detector is very sensitive and it gets very close to the collision.

So you have this really beautiful, delicate silicon detector that sits, I think it's 7mm from the beam.

The LHC beam has as much energy as a jumbo jet at takeoff, so it's enough to melt a ton of copper.

So you have this furiously powerful thing sitting next

So those aspects of our detector that are specialized to measure these particular B quarks that we're interested in.

And is there, I mean, I remember seeing somewhere that there's some mention of matter and antimatter connected to the B, these beautiful quarks.

Yeah, so there is a connection, which is that when you produce these B particles, these particles, because You don't see the b quark, you see the thing the b quark is inside.

So they're bound up inside what we call beauty particles, where the b quark is joined together with another quark or maybe 2 other quarks, depending on what it is.

There are a particular set of these b particles that exhibit this property called oscillation.

So if you make, for the sake of argument, a matter version of 1 of these B particles, as it travels, because of the magic of quantum mechanics, it oscillates backwards and forwards between its matter and antimatter versions.

So it does this weird flipping about backwards and forwards.

What we can use this for is a laboratory for testing the symmetry between matter and antimatter.

If the symmetry between antimatter and antimatter is precise, it's exact, then we should see these B particles decaying as often as matter as they do as antimatter, because this oscillation should be even.

It should spend as much time in each state.

But what we actually see is that 1 of the states it spends more time in.

It's more likely to decay in 1 state than the other.

So this gives us a way of testing this fundamental symmetry between matter and antimatter.


Speaker 0: I work on a detector called LHCb, which is 1 of these 4 big detectors that are spaced around the ring. We do slightly different stuff to the big guys. There's 2 big experiments called ATLAS and CMS, 3,000 physicists and scientists and computer scientists on them each. They are the ones that discovered the Higgs and they look for supersymmetry and dark matter and so on. What we look at are standard model particles called b-quarks, which depending on your preferences, either bottom or beauty, we tend to say beauty because it sounds sexier.

But these particles are interesting because we can make lots of them. We make billions or hundreds of billions of these things. You can therefore measure their properties very precisely. So you can make these really lovely precision measurements. And what we are doing really is a sort of complimentary thing to the other big experiments, which is, they, if you think of the sort of analogy they often use is, if you imagine you're looking in, you're in the jungle and you're looking for an elephant, say, and you are a hunter.

Speaker 1: Let's say the elephant's very rare, you don't know where in

Speaker 0: the jungle, the jungle's big. There's 2 ways you go about this. Either you can go wandering around the jungle and try and find the elephant. The problem is if there's only 1 elephant and the jungle's big, the chances of running into it were very small. Or you could look on the ground and see if you see footprints left by the elephant.

If the elephant's moving around, you've got a chance, your better chance maybe of seeing the elephant's footprints. If you see the footprints, you go, okay, there's an elephant. I maybe don't know what kind of elephant it is, but I got a sense there's something out there. So that's sort of what we do. We are the footprint people.

We're looking for the footprints, the impressions that quantum fields that we haven't managed to directly create the particle of, the effects these quantum fields have on the ordinary standard model fields that we already know about. So these B particles, the way they behave can be influenced by the presence of say superfields or dark matter fields or whatever you like. And the way they decay and behave can be altered slightly from what our theory tells us they ought to behave.

Speaker 2: Lexie D'Ericati And it's easier to collect huge amounts of data on bee quarks. Al-Khalili

Speaker 1: We get billions and billions of

Speaker 0: these things. You can make very precise measurements. And the only place really at the LHC, or really in high energy physics at the moment where there's fairly compelling evidence that there might be something beyond the standard model is in these beauty quarks decays.

Speaker 2: Just to clarify, is the difference between the 4 experiments, for example, that you mentioned, is it the kind of particles that are being collided? Is it the energies which they're collided? What's the fundamental difference between the different experiments?

Speaker 1: The collisions are the same. What's different is the design of the detectors.

Speaker 0: So Atlas and CMS, they're called what are called general purpose detectors. And they are basically barrel shaped machines. And the collisions happen in

Speaker 1: the middle of the barrel. And the barrel captures all the particles that go flying out in every direction.

Speaker 0: So in a sphere effectively, they come flying out and it can record all of those particles. And- What's the, sorry to be interrupting, but what's the mechanism of the recording?

Speaker 1: Oh, so these detectors, if you've seen pictures of them, they're huge, like Atlas is 25 meters high and 45 meters long, they're vast machines, Instruments, I guess you should call them, really. They're kind of like onions.

Speaker 0: They have layers, concentric layers of detectors, different sorts of detectors. Close into the beam pipe, you have what are called, usually made of silicon, they're tracking detectors. They're little strips of silicon or pixels of silicon, and when a particle goes through the silicon it gives a little electrical signal and you get these dots, electrical dots, through your detector which allows you to reconstruct the trajectory of the particle. That's the middle, and then the outsides of these detectors you have things called calorimeters which measure the energies of the particles and then right at the edge you have things called muon chambers which basically these muon particles which are the heavy version of the electron, they are like high velocity bullets and they can get right to the edge of the detectors. If you see something at the edge, that's a muon.

So that's broadly how they work.

Speaker 2: And all of that is being recorded.

Speaker 0: That's all being fed out to, you know, computers.

Speaker 2: That data must be awesome.

Speaker 1: OK. So LHCb is different. So we, because we're looking for these bquarks, bquarks tend to be produced along the beam line.

Speaker 0: So in a collision, the bequarks tend to fly sort of close to the beam pipe. So we built a detector that's sort of pyramid cone shaped, basically, that

Speaker 1: just looks in 1 direction. So we ignore,

Speaker 2: if you

Speaker 0: have your collision, stuff goes everywhere. We ignore all the stuff over here and going off sideways. We're just looking in this little region close to the beam pipe where most of these be quarks are made. So is there a different

Speaker 2: aspect of the sensors involved in the collection of the b quark trajectories?

Speaker 0: There are some differences. So 1 of

Speaker 1: the differences is that 1 of the ways you know you've seen a b

Speaker 0: quark is that b quarks are actually quite long lived by particle standards. So they live for 1.5 trillionths of a second,

Speaker 1: which is if you're a fundamental particle, it's a very long time.

Speaker 0: Because the Higgs boson I think lives for about a trillionth of a trillionth of a second, or maybe even less than that. So these are quite long-lived things. And they will actually fly a little distance before they decay. So they will fly a few centimeters maybe, if you're lucky, then they'll decay into other stuff. So what we need to do in the middle of the detector, you want to be able to see, you have your place where the protons crash into each other, and that produces loads of particles that come flying out.

So you have loads of lines, loads of tracks that point back to that proton collision. Then you're looking for a couple of other tracks, maybe 2 or 3, that point back to a different place that's maybe a few centimetres away from the proton collision. That's the sign that a little B particle has flown a few centimetres and decayed somewhere else. So we need to be able to very accurately resolve the proton collision from the B particle decay. So the middle of our detector is very sensitive and it gets very close to the collision.

So you have this really beautiful, delicate silicon detector that sits, I think it's 7mm from the beam. The LHC beam has as much energy as a jumbo jet at takeoff, so it's enough to melt a ton of copper.

Speaker 1: So you have this furiously powerful thing sitting next

Speaker 0: to this tiny, delicate silicon sensor. So those aspects of our detector that are specialized to measure these particular B quarks that we're interested in.

Speaker 2: Yeah. And is there, I mean, I remember seeing somewhere that there's some mention of matter and antimatter connected to the B, these beautiful quarks. Is that, what's the connection? Yeah, What's the connection

Speaker 0: there? Yeah, so there is a connection, which is that when you produce these B particles, these particles, because You don't see the b quark, you see the thing the b quark is inside. So they're bound up inside what we call beauty particles, where the b quark is joined together with another quark or maybe 2 other quarks, depending on what it is. There are a particular set of these b particles that exhibit this property called oscillation. So if you make, for the sake of argument, a matter version of 1 of these B particles, as it travels, because of the magic of quantum mechanics, it oscillates backwards and forwards between its matter and antimatter versions.

So it does this weird flipping about backwards and forwards. What we can use this for is a laboratory for testing the symmetry between matter and antimatter. If the symmetry between antimatter and antimatter is precise, it's exact, then we should see these B particles decaying as often as matter as they do as antimatter, because this oscillation should be even. It should spend as much time in each state. But what we actually see is that 1 of the states it spends more time in.

It's more likely to decay in 1 state than the other. So this gives us a way of testing this fundamental symmetry between matter and antimatter.

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Beauty Quarks (Harry Cliff) | AI Podcast Clips