At Fermi National Accelerator Laboratory, protons were always the primary particles, coursing through the circular tracks of the Tevatron, which until 2009 was the highest-energy collider in the world. But there’s a new particle making the rounds at the Batavia, Illinois, campus: the muon. This heavy but short-lived cousin of the electron is interesting both for its usefulness in testing the standard model and its potential of being used someday in a powerful collider.
“You’ve got a mind shift at the lab,” says David Hertzog, physicist at the University of Washington in Seattle and co-spokesman for a Fermilab muon experiment called Muon g-2. On 19 September, the lab announced that the US Energy Department (DOE) had granted the experiment “mission need” approval, a first step towards eventual funding. Last month, a second muon experiment, called Mu2e, received a second stage blessing from the DOE. Both experiments will share upgrades to accelerators and beams under the banner of a new ‘Muon Campus’. “In the long run, there is an interest in this synergy,” says Hertzog. “More people doing muon physics, more doing muon beams. It could maybe lead to a collider.”
The US$40-million Muon g-2 experiment will follow up on perplexing 2004 results from Brookhaven National Laboratory in Upton, New York, which found an anomaly in the spin rate of a muon within a magnetic field. At the time, the results were intriguing but not conclusively different from theoretical predictions: in the jargon, the results were statistically significant to 2.7 sigma. But since then, the anomaly has still not been explained, and progress on the theoretical front means that the 2004 results are statistically significant to 3.6 sigma, says Hertzog.
If they start taking data in 2016, as planned, the experiment within a year would have 20 times the data that Brookhaven collected — easily pushing the experiment past the gold-standard 5 sigma threshold. Already, many theorists cite the g-2 anomaly as circumstantial evidence that supersymmetry could resolve problems in the standard model. Another route beyond the standard model that would be consistent with the anomaly is a new hidden sector filled with ‘dark forces’.
The $200-million Mu2e experiment, which aims to begin taking data in 2019, will sift through many trillions of muons to see if any happen to spontaneously morph into their cousins, electrons — something that is almost entirely forbidden under the standard model. But physicists nonetheless have reason to suspect that it might happen occasionally because neutrinos, which are within the same family of particles called leptons, have been found to morph into different flavours of each other. “It’s going to look into this great mystery about why we have these different families of particles in the leptons,” says co-spokesman James Miller, of Boston University in Massachusetts.