Particle physicists are always looking to up the energy in their colliders, to unlock more basic physics and perhaps unveil something more exciting than the Higgs boson: new forces, new forms of matter that could resolve the dark matter mystery or even some asymmetry in quantum physics that could explain why the matter in the known universe didn’t come with a counterpart matched set of antimatter that would have sent everything up in a puff of gamma rays moments after the Big Bang (the way that current theories seem to predict).
Muon colliders could hold that key. It’s early days, but Chris Rogers, who leads an international team that reported in the journal Nature a radical new way to cool muons, believes muon colliders have potential.
The Muon Ionization Cooling Experiment (MICE) collaboration developed the technique at the ISIS proton accelerator just outside Oxford in the UK.
“Ionisation cooling was invented in theory about 40 years ago, but no one’s done it before – it’s very challenging,” Rogers says.
Muons are similar to the electrons that orbit around every atom in our bodies, but 200 or so times heavier. Unlike their lighter cousins, muons are not stable, on average living a mere 2.2 microseconds before decaying into some smaller particle in a flash of energy.
This is the hurdle the MICE team had to overcome: any muon collider would require some muons first to be created, then herded into orderly columns that can be directed towards each other, all within a few millionths of a second.
The MICE team’s accomplishment is to begin the herding process. Muons are created by firing protons into a metal target, producing a disorderly cloud of high energy muons that need to be slowed down before being uniformly accelerated into the required narrow beam, by high voltages and strong magnets.
“Scientists have done beam cooling before, but the techniques that were in existence before now take minutes or hours to take effect,” says Rogers. “So we had to demonstrate a radically different form of cooling that would take effect within a few microseconds.”
The team’s new approach was to funnel the muons into a liquid, where they were slowed down by collisions with the electrons orbiting the liquid’s atoms, like a bowling ball careering through a collection of pool balls.
With the muons’ hefty momentum, the electrons are knocked clear of their atoms, ionising the liquid molecules (liquid hydrogen or lithium hydride) giving the technique the name ionisation cooling.
To funnel as many of the newly-created muons into the liquid as possible, the MICE team used the biggest magnets they could – superconducting electromagnets cooled to close to absolute zero that exert forces of tens of tonnes on one another.
“You have this very delicate conductor material floating on a bobbin in a vacuum and you’re trying to support these massive forces and hold the whole thing down with pretensioned insulating straps,” Rogers says. “It’s quite fiddly, the engineering challenges were quite significant.”
The MICE team persevered for the best part of a decade, and succeeded in cooling the muons by around 10%, via the almost instantaneous process of ionisation.
Although 10% seems insignificant for muons created with energies that propel them at a significant fraction of the speed of light, the team believes that a chain of ionisation coolers could get the muons a fair way towards the 10-thousand-fold energy reduction required.
If muons can be wrangled into a beam, the rewards would be rich. As simple particles, their collisions are much more direct than those of protons, which have a complex internal structure comprising quarks and the gluons that hold them together.
“When you collide protons, it’s like throwing together two bags of snooker balls. Each of the quarks holds only a fraction of the total energy of the beam,” Rogers says.
While CERN considers a successor to the LHC that would need to be a 100-kilometre ring, Rogers points out that the same energy regime could be reached with a muon collider of a similar scale to the LHC’s 14 tera electron-volts (the energy of a trillion car batteries connected together).