Facts So Romantic

The Quirky Muon Just Might Spur a Physics Breakthrough—Again

Folks in the Midwest may have been surprised to see a massive electromagnet being towed up the Mississippi River and driven through the flatlands of Illinois in July. The electromagnet was on its way from its original home at New York’s Brookhaven National Laboratory to Fermilab, near Chicago. In 2016 it’s scheduled to be the centerpiece of the Muon g-2 experiment (pronounced “gee minus two”), designed to look for tantalizing hints of physics beyond what’s known as the Standard Model of particle physics.

The Muon g-2 experiment will be looking to catch a muon, a very short-lived particle, in the act of converting into its lighter cousin, the electron—a kind of “flavor violation” that would indicate that a new particle, or force, or something else entirely must be responsible. To see this change, researchers will make precise measurements of a wobble that occurs when a muon is placed in a magnetic field, thanks to all those virtual particles popping in and out of existence in the quantum vacuum. If the value of the wobble disagrees with the exacting prediction of the Standard Model, then we’ll know that there is some “new physics” involved, as scientists say.

Unlikely? You betcha. But it’s not much more unlikely than the discovery of the muon itself back in 1936, when scientists thought their model of particle physics was pretty much complete. That was when two Caltech physicists, Carl Anderson and Seth Neddermeyer, were studying cosmic rays and noticed that some particles didn’t curve as expected when they passed through a magnetic field. A year later, cloud chamber experiments confirmed this was, indeed, a new particle.

It was such a surprising development that I.I. Rabi famously declared, “Who ordered that?” And it led to a much more complicated version of the Standard Model, with not one, not two, but three generations of matter particles (quarks and leptons), not to mention four varieties of force-carrying particles (gauge bosons)—a veritable particle zoo. We just don’t encounter those second and third generations very often outside of particle accelerators, because they are so heavy they decay into their first-generation cousins almost immediately.


That doesn’t mean the humble muon can’t have any practical applications. Los Alamos scientists are developing muon-based radiography techniques to track the illegal transport of nuclear materials in trucks or cargo containers at key border crossings. In Japan, muon detectors could help detect when active volcanoes might be preparing to erupt.

By far the most colorful application is the use of muon detectors to map Mayan pyramids—a kind of X-ray-like imaging technique, only using muons instead of high-energy photons in the X-ray regime. The denser the object being imaged, the more muons that are blocked, casting a shadow; if there are gaps in the internal structure—say, a hidden burial chamber—this will show up as clear areas on the resulting image. It’s better than ground-penetrating radar for imaging things more than 100 feet below the surface.

Back in the 1960s, physicist Luis Alvarez famously collaborated with Egyptian archaeologists to gain access to a hidden chamber in Khafre’s pyramid at Giza. He built a special spark chamber as a muon detector, wrapped in iron sheeting, and used it to image the entire structure. When he analyzed the data, there was a clear area in the pyramid that stopped fewer muons than expected—in other words, there was a void in the dense structure there, hinting at hidden rooms that had not yet been discovered. Alas, they didn’t find any such rooms, but Alvarez’s work did establish proof of principle for the technique, known as muon tomography.

Others are keen to follow in Alvarez’s footsteps, like Roy Schwitters at the University of Texas, Austin, who is repurposing old muon detectors to map unexplored “jungle-covered mounds” that are likely to be hidden Mayan ruins in Belize. The idea is to bury several muon detectors around the base of a pyramid in La Milpa and use them to measure the trajectories of the muons that pass through it to map out whatever is inside. It will be like taking a CT scan of the interior: Bright spots would indicate voids, potential vaults or chambers.

Now we’re back at another point in science history where we think our model for particle physics is complete—as far as it goes. But physicists are still looking for new physics, whether that be supersymmetry, quantum gravity, or something else entirely, and Fermilab’s Muon g-2 experiment is just one of many designed to probe the frontier. Who knows? The muon may once again dramatically change what we we know about the universe.


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