Since the 1980s, the US government's Chicago-area Fermilab has been at the forefront of high-energy physics. That's in large part thanks to the Tevatron, the machine that first reached the energies needed to discover the last quark in the Standard Model. But the Tevatron has come to the end of its run; at 2pm on Friday, it will be shut down for the last time (an event that will be webcast).

The move will shift physicists' focus across the Atlantic, to the Large Hadron Collider (LHC) at CERN. The LHC is likely to enjoy a long run at the top of particle physics, but in time, it too will be superseded. What might come next? If Fermilab scientists have their way, particle physics could migrate from hadrons to muons. But getting there will take time, research, and the serious application of time-dilating relativity.

A series of bad options



The cancellation of the American Superconducting Supercollider (SSC) in the 1990s gave particle physics a hangover. It took years for the next big accelerator (the LHC) to be built, and even when it operates at its designed power, it won't reach the energies once planned for the SSC. The LHC is also a fundamentally different project, constructed in tunnels built for an earlier collider and requiring financial input from just about every country with a significant physics program. These harsh realities leave just about everyone who thinks about it wondering whether anything more powerful than the LHC will ever get built. It has also forced them to ponder exotic ways to get particles up to high energies using approaches that are fundamentally different from anything we've tried before.

During Ars' visit to Fermilab earlier this year, however, we found out that at least some researchers have pondered using the traditional approach to accelerating particles—but relying on exotic particles to produce energies that rival those of the LHC in a much smaller space. The muon is a heavier, less stable version of the electron, and it's produced in many of the collisions that occur in particle colliders like the LHC and Tevatron. With a half life of just a few microseconds, it wouldn't seem to be an obvious candidate for accelerating. But, thanks to some of the consequences of relativity, it might just fit the bill.

Particle colliders all work on the same general principle. When two particles smash together at nearly the speed of light, some of the energy they carry gets converted into mass, as per Einstein's E = mc2. But, as Fermilab director Pier Oddone noted, accelerators like the LHC and the Tevatron aren't as efficient as they otherwise might be. That's because they both collide hadrons (as the LHC's name implies), which are composites of three quarks. Thus, even when a pair of protons collide head on, only part of their total energy gets imparted to the quarks that actually do the colliding.

"The energy of hadron collisions is split among its components," Oddone told Ars. "Lepton [a fundamental particle] collisions give us all the energy—they're about 10 times better."

At the same energy, collisions between fundamental particles—those that aren't comprised of smaller constituents—pump much more of their energy into the creation of exotic matter. As Oddone put it, a 1TeV collider that smashed leptons would be about as powerful as the LHC, which operates at much higher energies.

Unfortunately, few fundamental particles make for good colliding. Quarks and gluons are unstable unless they're bound together as parts of other particles; neutrinos are too light and don't carry a charge; everything else is unstable except the electron (which is a kind of lepton). Electron colliders have been built at places like the Stanford Linear Accelerator and at CERN, where the Large Electron-Positron (LEP) Collider used to occupy the tunnels that now house the LHC. But colliders that smash electrons and/or positrons are extremely difficult to build.

The problem arises from the electron's low mass. As any charged particle is made to travel along a curved path, it loses some of its energy as radiation (called synchrotron radiation; this has turned out to be a useful source of high energy photons, which is why many older accelerators have been put back into service for imaging purposes). For a particle as light as the electron, this loss is enough to remove much of the energy put into accelerating it in the first place. Thus, without continual boosts, the particles swinging around the circular track of the LEP would quickly slow to speeds useless to physicists.

The solution? Simply run the particles along a straight path. This comes with its own problems, though—the higher the energy, the longer the necessary path and the more expensive the buildout.

So building a high energy lepton collider is a choice between two unappealing options: building a ring that would require a lot of energy to run, or building a linear collider that requires a lot of space and money. The alternative, another proton collider that's bigger than the LHC, doesn't look like it's in the cards any time soon.

Why not muons?

That list of unappealing options has many people at Fermilab starting to look seriously at muon colliders. The muon has a lot of things going for it. It's also a charged lepton, like the electron, and so it behaves a bit like an electron's heavier brother—a much heavier brother. With a mass of just over 100MeV (Mega-electron Volts), the muon is about 200 times heavier than the electron—in fact, it's over 10 percent of the mass of a proton. It should therefore behave more like a proton when it comes to acceleration and maintaining speed at high energies, while still having the advantage of being a fundamental particle.

There's just one small problem: muons aren't stable. They're relatively long-lived when it comes to exotic fundamental particles, but they typically survive only a couple of microseconds before decaying into a spray of other particles, including an electron. Microseconds don't seem like a lot to work with here... but there may be a way to make muons survive long enough to be useful.

Muons are typically produced as a result of high-energy collisions that also impart a reasonable amount of energy to the particles. Thus, the muons will typically start off moving pretty quickly. And, once a bit of further acceleration happens, some interesting effects of relativity kick in.

As particles approach the speed of light, two things happen. Since additional energy won't cause them to speed up much further, most of the energy gets converted into mass (again, to balance the books using Einstein's famous E = mc2). As a result, the muons will start off heavy and get even heavier as they're accelerated. This makes them easier to corral into useful bunches. Since the muons' negative charges push against their mass, the impact of charge repulsion drops quickly as the mass goes up. Muons that start off moving quickly and are rapidly accelerated will be easier to package into the dense bunches that lead to a high rate of collisions.

Even more significantly, strange things start happening to time as the muons approach the speed of light. In the muons' frame of reference, time slows down relative to the frame of the accelerator hardware and the humans operating it. As a result of the speed, their short two microsecond lifetime begins to stretch out.

At 10GeV of energy, Oddone said, the muons survive nearly 100 times longer than one of their stationary peers would. He thinks it might be possible to get them as high as five TeV, which would provide a very large lifetime boost. The end result is that the hardware would have a lot more time to get the muons up to speed and smash them together before they'd typically decay on their own.