Technical foundation for a muon collider laid at J-PARC

A particle collider is a machine that energises two beams of subatomic particles and smashes them head on.

The Large Hadron Collider (LHC) in Europe is the world's largest and most famous particle collider. It accelerates (with the effect of energising) two beams of protons to nearly the speed of light and has them collide. When they do, energy is released in the same way the collision of two cars releases sound, heat, and kinetic energy. The existing kinetic energy of the beams is redistributed into the mass and kinetic energy of new particles. By studying this process, physicists can learn a lot about their properties.

For example, this is how they made one of the headline discoveries of the 21st century: the Higgs boson particle in 2012. Proving the particle exists allowed physicists to confirm that their theory about how subatomic particles get mass is right. That theory is in turn related to many properties of our universe, including its size, the formation of galaxies, and the inner lives of all the universe's stars, including our sun. For first proposing that theory in 1964 (together with four others), Peter Higgs and François Englert were awarded the physics Nobel Prize in 2013.

This said, the properties of the Higgs boson, which physicists have since examined in more detail, have raised more questions about the universe. Two examples include the mysterious nature of dark matter and why neutrinos have mass even though the theory that explains all subatomic particles says they shouldn't.

While scientists have built and are operating clever experiments to test different explanations for these anomalous entities, they are also discussing the possibility of building more powerful colliders. The LHC has been able to access a collision energy of up to 13.6 TeV, or about 14,000-times the energy of a proton at rest. Scientists are currently deliberating proposals for colliders that can do better.

The machines in these proposals have taken three forms: a linear electron-positron collider, a circular electron-positron collider, and a circular proton-proton collider. Each of these machines will cost several billion dollars to build and will require many countries to fund and manage them.  So scientists have to be able to justify which collider they'd like to build and then convince governments to pay.

The most common argument has been that participating in such sophisticated experiments will also lead to spin-off benefits that will give countries the edge in other spheres, including in medical diagnostics and materials of the future. Increasingly, the question of scientific leadership has also become relevant: India is looking for some of it en route to its goal to become an economically developed country by mid-century; the US is trying to not lose it to China; China is working to take more of it from the US; and so on.

The point is that there is more at stake here than 'simple' problems in physics, although these questions are weighty in their own right.

The problem currently is that all three types of machines — a linear/circular electron-positron collider or a circular proton-proton collider — are beset by important disadvantages of their own, and different scientists have focused on them (in addition to their price tags) as they try to decide the way forward.

A circular proton-proton collider like the LHC but bigger can scale a collision energy of 100 TeV. However, it will need to deal with the fact that protons are composite particles, i.e. they're made up of smaller particles. When they collide head on, only a small fraction of energy is used to 'make' new particles; the rest is exchanged between the constituent particles.

Both electrons and positrons are elementary particles on the other hand and generate 'clean' collision data. But when an electron (or a positron) is circulated in a magnetic field through the collider while it's being accelerated, its small mass means it releases much of the energy it acquires as light. Thus circular electron accelerators consume a lot of energy to achieve their results.

A linear collider doesn't have this problem since the particles are accelerated in a straight line, but because they can't go round and round to accelerate more and more, the machine needs to be really long. In some designs they are a few tens of kilometres long: finding a suitably large patch of land is difficult, and maintaining the integrity of the beam across that distance more so. And because each group of particles collides only once and is dumped, the collider must produce, accelerate, and dispose of fresh ultra-intense particle beams at a high frequency, increasing its wall-plug power demand.

In this scenario, some scientists are also mulling a new type of collider that hasn't been built before — one for muons. Unlike protons and like electrons, muons are elementary particles and thus lead to clean collisions. A muon is also about 200-times heavier than an electron, so it loses more than a billion-times less energy as light when it's being accelerated in a circle.

Thus, as scientist Diktys Stratakis of Fermilab in the US wrote, "A muon collider ring with a circumference of 10 km could have the same potential as a 100 km proton collider ring, if proven to be feasible."

But of course it's not a silver bullet. Perhaps the single biggest issue is that muons are much less stable than protons or electrons. Each muon has a lifetime of about 2.2 microseconds at rest. So producing a sufficiently dense bunch of muons is difficult. The collider must also be able to create large, powerful magnetic fields fast enough before the muons decay. And when muons do decay, they emit electrons or positrons that the machine's various components must be shielded from. So building a muon collider entails a lot of innovation first.

A team of scientists in Japan recently reported in Physical Review Letters that they had taken a crucial step forward: they were able to cool (de-energise) a bunch of muons, then accelerate them for the first time using a device called a radiofrequency cavity. This is significant because this end-to-end feat has never been demonstrated before and as such represents the first major problem to solve when building a muon collider.

The scientists — from Canada, China, and Japan — performed their feat at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai. They achieved it in six steps.

1. A beam of 0.003 TeV protons strikes a graphite target and produces 'hot' muons.

2. A slender aluminium foil in front of the target slows them down a little.

3. The muons are further slowed by an 8-mm thick silicon dioxide aerogel disc. As a muon slows nearly to a halt, it bonds with an electron in the aerogel to create a muonium atom: a positively charged muon plus a negatively charged electron.

4. An ultraviolet laser knocks off the electrons to free very low energy muons — about as much energy they'd have at room temperature.

5. Electrostatic lenses and steering plates impart a small amount of energy to the muons and focus them, like getting people at a venue to gather in a single room.

6. The muons are subjected to electric fields alternating at 324 MHz inside a 3-m-long tube, accelerating them. (This is the radiofrequency stage.)

The feat is the first ever demonstration that started with muons jiggling around at a room-temperature level of energy (around 25 meV) and ended with muons moving in a common direction with about 100 keV of energy — an energy boost by a factor of 4 million.

While 100 keV is still seven orders of magnitude away from 2 TeV, the Japan team's feat is remarkable because it 'solves' the very first and possibly  hardest challenge presented by a muon collider: catching 'live' muons before they 'die'. The team's setup stopped fast-moving muons, cooled them to 25 meV, stripped the electrons, and injected them into a radiofrequency cavity in 2.28 microseconds, i.e. within the muons' lifetime. In a manner of speaking, if a 2-TeV muon collider is a skyscraper, the study lays the foundation.

The J-PARC team was also able to cut the transverse emittance — a measure of how the beam spreads — by 200-times horizontally and 400-times vertically relative to the raw muon beam at the beginning. This two-order-of-magnitude reduction is paramount for the lightly energised muon beam to enter the next, more powerful accelerators.

"Although the beam produced by the J-PARC team is of good quality (in terms of having low emittance), its energy and intensity are not yet high enough for the experiments that researchers eventually hope to make. Nevertheless, the demonstration of the potential to re-accelerate cold muons is an exciting step forward," Chris Rogers, a scientist with the ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in the UK, wrote in Physics.