Muons: scientists achieve focused beam of 'heavy electrons'

18 February 2020

Interview with 

Chris Rogers, Rutherford Appleton Laboratory

MICE_Experiment

Scientists working on some apparatus for the muon ionisation cooling experiment.

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Scientists have unlocked the next stage in the cutting edge of experimental physics. It’s an upgrade to facilities like the Large Hadron Collider, where beams of tiny particles race round at nearly the speed of light, and then smash into each other to reveal what they are individually made of. The results shed light on the fundamental nature of the universe. Experimenters’ ideal next step is to up the ante using ‘muons’ - essentially a heavy electron - that can collide at even higher energies. But these have been hard to make into a focused beam. Now an international collaboration has managed to create that beam - in what they call the muon ionisation cooling experiment, or MICE. Phil Sansom got some “concise MICE advice” from research leader Chris Rogers...

Chris - We've demonstrated a technique whereby we can take a beam of particles called muons and we can squeeze them right down, and accelerate them up to really high energies. Because of the unique properties of muons we can actually explore physics which is even beyond the energy scale which is available in the Large Hadron Collider.

Phil - Wow. This is real futuristic.

Chris - Yeah, right. That's it. And no one's developed a technique like this before which can really be used to handle muon beams.

Phil - What exactly is a muon, to start with?

Chris - So a muon is like a really heavy electron. You actually have muons going through you pretty much every second of every day, which come from cosmic rays.

Phil - I have a muon in me right now?

Chris - That's pretty much right.

Phil - What do they look like?

Chris - Just like electrons except for a couple of special properties. One is that they're much heavier than electrons, almost 200 times heavier than electrons; and the other one is that they decay radioactively, so they only live for two millionths of a second.

Phil - That's bizarre. How do you even deal with them?

Chris - We have a special trick up our sleeve. If you accelerate particles to really high speeds, as the particles get closer and closer to the speed of light they live for longer and longer. It's Einstein's time dilation phenomenon.

Phil - How do you make the muons in your lab?

Chris - We take a beam of protons, accelerate those protons, and then bash them into a target. All sorts of other particles come out, and some of those particles are muons.

Phil - And how have you been trying to deal with them in this particular experiment? Because this is the first time you've managed to get them into a beam, correct?

Chris - We've had muons in a beam before, but we've never really managed to prepare a beam so that it would be suitable to accelerate them, much more like a laser beam if you like. We pass that muon beam through an absorbing medium, and as the muons go through the absorber they lose energy. All of that hot gas slows down as it goes through the absorber. So then we need to accelerate that beam back up using a conventional particle accelerator technique.

Phil - What's this material you're filtering them through? Is it something special and strange?

Chris - We use either liquid hydrogen, cooled down to a few tens of Kelvin; or we use lithium metal with hydrogen embedded into the metal.

Phil - Those are strange and weird.

Chris - They're pretty cool bits of kit which we use to do it.

Phil - Why those?

Chris - When the muons go through the absorber they knock the electrons off the atoms, and when we knock the electrons off the muons lose energy. That's what's called ionisation and that's why the technique's called ionisation cooling. Now there's another thing which happens: they bash into the centre of the nucleus and they scatter off, flying off in all sorts of different directions. Now we don't want that, so we have to pick special materials where the nucleus of the atom is as small as it possibly can be. Hydrogen has the smallest nucleus of any material, and lithium has a pretty small nucleus as well.

Phil - Is that why it's taken so long to figure this out? Because you're trying to get materials with small enough nucle... Nuclei? Nucleuses?

Chris - Nuclei.

Phil - Nuclei!

Chris - It's not just the material which we have to consider; we have to combine that with a particle accelerator lattice. And combining those two different things into one experiment was really tricky.

Phil - What did it feel like when you finally managed it for the first time?

Chris - It was pretty cool. In fact, we only cooled the beam by about 10% of the full cooling channel which you would need in a real muon collider facility. But that was pretty cool.

Phil - And what does this mean for physics? Is there really exciting science coming up that could potentially use muon beams?

Chris - The aim of our experiment then is to take this technique and then put it into a thing called a muon collider, where we collide beams of muons together. Muon colliders are really exciting because they let us reach much higher energies than are available using even existing facilities like the Large Hadron Collider at CERN.

Phil - It's like a Large Hadron Collider upgrade?

Chris - It would be a Large Muon Collider.

Phil - Large Muon Collider. That's cool.

Chris - Should be!

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