Milestone particle physics study could change science as we know it

You may never have heard of muons before, but these particles are crucial to our understanding of the universe.

In a milestone study published Wednesday in the journal Nature, researchers announce that they have overcome one of the biggest challenges to studying these fundamental particles. The study heralds a new era of research into the properties and structure of matter — and that includes us humans.

“This lets us explore a completely different regime of fundamental physics which would have been almost impossible to reach previously,” lead researcher Chris Rogers tells Inverse.

Muons are fundamental particles — particles not made of any other particles — and are very similar to electrons, but 207 times heavier. That means they carry much more energy than electrons. They are crucial to our understanding of our physical world, because they provide information about the properties and structure of matter.

But studying muons is difficult. Fundamental particles like muons (think the neutrino) are only visible at high energies and for microseconds. Physicists have to use particle colliders and accelerators to glimpse their properties. These instruments slam particle beams together at a very high speed and capture data from the resulting explosions, allowing researchers to peek into the subatomic world. This is exactly what physicists have been doing at the Large Hadron Collider, which infamously smashed proton beams together in order to discover — and study — the theorized Higgs boson, another elusive fundamental particle.

“We'd known about muons for many years, but we never managed to put them into a particle accelerator before.”

Now, after decades of research, Rogers and a team of hundreds of international scientists have outlined how to get closer to creating a first-of-its-kind particle collider that can blast beams of muons at much higher energies than any existing colliders. The invention has the potential to reveal exotic particles that exist only in theory, or even entirely new particles scientists had never thought of before.

“When you smash beams of particles together, what happens is you make all sorts of exotic new particles, like force carriers or like novel forms of matter,” Rogers says.

“And by looking at all the different sorts of particles which can be created, we can try and understand things about how matter gets stuck together.”

How matter sticks together is at the basis of our understanding of how everything in the universe is made — because everything is made of matter.

“This result is a fundamental landscape change. It doesn’t happen very often — I’d say every 10 to 15 years,” Fermi Lab physicist Vladimir Shiltsev, who was not involved in the new study, tells Inverse. “It’s like a lego piece, to build our future plans. A piece of lego that you can multiply and play with and use to build new things. We can move forward in previously unexplored ways.”

Despite their obvious importance, colliders like the famous Large Hadron Collider, are massive instruments that cost huge amounts of money and are very hard to build and manage. So far, there is no particle collider designed for muons, which means there is no way of studying and understanding them — even though muons were among the first particles to be ever discovered, in 1936.

“We'd known about muons for many years, but we never managed to put them into a particle accelerator before,” Rogers says.

“We have demonstrated that it's possible to build a completely new sort of particle accelerator. And accelerators themselves have been used as engines of discovery for many decades, so the applications for muon accelerators… we haven't even started imagining yet.”

How to make a muon collider

The researchers cracked the problem of “ionization cooling of muons” — basically cooling the smashing beams of particles so they’re easier for current instruments to capture and analyze, rather than having the muons flying all over the place uncontrollably.

Here's how Rogers puts it:

We had to develop a technique which can take this messy beam and turn it into a really nice laser-like beam: cooling the beam. So if you imagine the beam is like a hot gas, it's flying along really fast. What we want to do is reduce the temperature of that gas. The temperature of the muons at production is roughly 10 billion degrees Celsius. We're trying to go from this diffused gas to something which is more like a laser beam.

Cooling techniques have been discovered before, but they usually take minutes or hours to do. What we developed is a radically different sort of cooling, which can take effect on timescales of billions of a second.

The muon collider itself would be similar in scale to either the Tevatron, which was a machine which was built at Fermilab near Chicago, or the Large Hadron Collider. You're looking at a machine which is several kilometers in length, with these extremely strong magnets, and the cost scale is similar to those machines.

Science as we don't know it

“Muons themselves are interesting beasts and there are applications which we will really only just starting to understand if we start building beyond accelerators,” Rogers says.

Muons' properties hint at these future applications. For example, muons are much more penetrating than X-rays, which means they can be used to look inside things that are too thick for x-rays to image. This includes pyramids and volcanoes, for example.

“Muons may provide a completely new way to reach further into the energy frontier of particle accelerators that simply cannot be achieved at the present time, and we might not have a way to get there otherwise," physicist Robert Ryne of the Lawrence Berkeley National Laboratory, tells Inverse. Ryne was not involved in the study.

But while the study represents a milestone along the road to developing the first-ever muon colliders, don't hold your breath too soon. There are many more years of research ahead for Rogers’ team to turn this into a reality, he says. But when we do get there, what we find may be truly unimaginable.

"On the path to developing accelerators for particle physics, with every passing decade, that research has led to things that have had big impacts on people’s lives," Ryne says.

Abstract: The use of accelerated beams of electrons, protons or ions has furthered the development of nearly every scientific discipline. However, high-energy muon beams of equivalent quality have not yet been delivered. Muon beams can be created through the decay of pions produced by the interaction of a proton beam with a target. Such ‘tertiary’ beams have much lower brightness than those created by accelerating electrons, protons or ions. High-brightness muon beams comparable to those produced by state-of-the-art electron, proton and ion accelerators could facilitate the study of lepton–antilepton collisions at extremely high energies and provide well characterized neutrino beams. Such muon beams could be realized using ionization cooling, which has been proposed to increase muon-beam Brightness. Here we report the realization of ionization cooling, which was confirmed by the observation of an increased number of low-amplitude muons after passage of the muon beam through an absorber, as well as an increase in the corresponding phase-space density. The simulated performance of the ionization cooling system is consistent with the measured data, validating designs of the ionization cooling channel in which the cooling process is repeated to produce a substantial cooling effect. The results presented here are an important step towards achieving the muon-beam quality required to search for phenomena at energy scales beyond the reach of the Large Hadron Collider at a facility of equivalent or reduced footprint.