Fermilab's Muon Department at edge of Intensity Frontier

Rendering of the proposed Fermilab Muon Campus, which will support the experiments Muon g-2 and Mu2e. Image courtesy of Muon Department/FESS

Fermilab's antiproton source is preparing to host a new tenant—the Muon Department.

The Muon Department was established to support and develop the infrastructure for Muon g-2, Mu2e and future muon experiments as part of Fermilab's move to the Intensity Frontier.

"We're very excited to start something new again," said Jerry Annala, head of the Muon Department. "We're still in the early stages, but we are moving ahead with two exciting projects and are working with the experimenters to transition into muon research."

As part of the Intensity Frontier initiative, Fermilab is inaugurating experiments that hunt for physical anomalies and look for discrepancies between the Standard Model's predictions and experimental measurements. Scientists working on Muon g-2 and Mu2e have chosen to scrutinize muons, one specimen of the subatomic world.

"Muons are special," said Chris Polly, the Muon g-2 project manager. "They are light enough to be produced copiously, yet heavy enough that we can use them experimentally to uniquely probe the accuracy of the Standard Model."

The Muon g-2 experimenters will examine the precession, or wobble, of muons that are subjected to a magnetic field. The main goal is to test the Standard Model's predictions of this value by measuring the precession rate experimentally. If there is an inconsistency, it could mean the Standard Model is incomplete or wrong.

"An analogous experiment at Brookhaven National Laboratory showed that there is a discrepancy between the predicted value and the experimentally observed value," Polly said. "They measured the precession with 3 sigma certainty, which made the discrepancy tantalizing, but not decisive. We want to take this experiment to the next level and make an extremely precise measurement that will either verify or refute the Standard Model. If the discrepancy remains it could be interpreted as evidence of an undiscovered class of particles interacting with muons or that the muons themselves are more complicated than we previously thought."

The Mu2e experiment will also use an intense beam of muons but will examine a property outside the understanding of the Standard Model: the possibility of a muon-to-electron conversion.

"We know that a muon can convert into a muon neutrino, an anti-electron neutrino and an electron," Mu2e co-spokesperson Jim Miller said. "But we want to see if a muon can cleanly convert into an electron without generating any neutrinos in the process. If this were observed it would be momentous because it would signal the presence of some new physics not currently included in the Standard Model. The Mu2e experiment is so sensitive that it can spot several electron conversions out of 10¹⁶ muons."

The Standard Model is silent on the prospect of lepton conversions, such as neutrino oscillation or muon-to-electron conversions. Theorists have patched the Standard Model to include the shape-shifting behavior of neutrinos, but there is no real understanding of why or how these transformations happen.

Muon-to-electron conversions go beyond the Standard Model entirely and enter the realm of supersymmetry, an emerging theory that relates particles based on their spin.

"For charged leptons, many new ideas that go beyond the Standard Model, like supersymmetry, predict that as many as one muon for every 10¹⁵ muons should convert into an electron," Mu2e co-spokesperson Bob Bernstein said. "In fact, most of the supersymmetric models predict that we should have seen conversions already! The question we really should be asking is why haven't we seen them yet."

Both the Muon g-2 and Mu2e experiments are still in the planning phase and several years from taking data. Currently, the researchers are working with engineers to develop the infrastructure for the Muon Campus. Later, they will collaborate to compare results and experimental insight.

"There is a lot of overlap between Muon g-2 and Mu2e," Bernstein said. "Muon research at Fermilab is a community with a lot of common interest."

The Muon Department is at the forefront of the accelerator design effort and will provide support and assistance to all current and future muon experiments. "The Muon Department is critical for the success of the muon program," Polly said. "The experts in that group really understand the equipment and know how to transform the complex to meet the needs of these demanding experiments."

—Sarah Charley