US scientists place record limit on muon’s electric signal in search for new physics

Can a particle hide a secret in the tiniest of wobbles? A new analysis hints at this possibility. Recently, physicists at the US Department of Energy’s Fermi National Accelerator Laboratory have used the Muon g-2 experiment to look for one of the most elusive properties of the muon—its electric dipole moment (EDM). They did not find one, but the absence is itself a useful result. Using just 25 percent of the experiment’s data, the collaboration has placed the strongest direct limit yet on the muon EDM. This finding shows that if the property exists, it is smaller than current instruments can see. The result adds to a growing effort to probe muons for signs of physics beyond the Standard Model. An EDM is a property that would give a particle an intrinsic electric polarity, linking the direction of its spin to its electric behavior. For a fundamental particle such as the muon, that would be far more than an oddity. A measurable EDM would violate important symmetries of nature and could point to physics beyond the Standard Model. It could also offer clues to one of cosmology’s biggest puzzles: why the universe contains vastly more matter than antimatter. The Standard Model predicts a muon EDM of roughly 10⁻³⁸ e·cm — far too small for current experiments to detect. The new Fermilab limit remains about 19 orders of magnitude above that prediction, so the result does not rule out the tiny EDM expected by the Standard Model. Instead, it sharply narrows the range in which a much larger, new-physics contribution could be hiding. “The primary measurement of the g-2 experiment is sensitive to new physics unrelated to the matter-antimatter asymmetry. The electric dipole moment search gives us this sensitivity,” Gavin Hesketh, one of the researchers from University College London, said. That makes the new result interesting even though it is a null measurement. The collaboration found a value of dμ = (−0.35 ± 0.19(stat) ± 0.34(sys)) × 10⁻¹⁹ e·cm, consistent with zero, and set an upper limit of |dμ| < 1.1 × 10⁻¹⁹ e·cm at 95% confidence. The limit is about 1.5 times tighter than the previous direct limit from Brookhaven National Laboratory, which was about 1.8 × 10⁻¹⁹ e·cm. Turning a magnetic experiment into an EDM detector The clever part is that Fermilab did not need to build an entirely new experiment. “If you set it all up and you tune all of the parameters of the experiment to measure the magnetic dipole moment as well as possible, there’s also, coincidentally, some sensitivity to the electric dipole moment,” Joe Price, one of the researchers who led the EDM analysis, said. The Muon g-2 apparatus uses a 50-foot-diameter superconducting storage ring to hold positive muons — technically anti-muons — traveling at nearly the speed of light. The muons circulate hundreds of times before decaying. As they move through the magnetic field, their spins behave somewhat like tiny gyroscopes, wobbling in a measurable way. The experiment was originally designed to measure the muon’s magnetic dipole moment, the property behind its famous g-2 measurement. However, this same motion contains another possible signal. A muon EDM would introduce a tiny vertical component to the wobble. Researchers can look for this signal by measuring the average vertical direction of the positrons produced when the muons decay. The muons’ extraordinary speed helps, too. Their normal lifetime is only 2.2 microseconds, but relativistic time dilation stretches it by nearly 30 times inside the experiment. This gives researchers more time to detect the subtle EDM-related motion and enhances the relevant electromagnetic effects. Two tracking detectors inside the storage ring were crucial. Each tracker contains 32 layers of aluminum-coated Mylar straws that record the paths of charged particles. When a muon decays, it produces a positron — the antimatter counterpart of an electron. Positrons themselves are an active subject of fundamental physics research, including experiments that have recently observed wave behavior in positronium. By tracking these positrons, researchers could determine their vertical decay angles and search for the tiny signal expected from an EDM. The trackers were not originally built solely for this purpose. However, when the collaboration turned on the experiment, researchers found that the beam did not meet specifications. The trackers helped locate the beam and map its profile. “The trackers were essential in mapping the profile of the beam. Without them, we could not have extracted g-2 or the EDM from the data. They really turned out to be an absolute necessity,” Brendan Casey, a senior researcher from Fermilab, said. A stronger constraint, not the final word Only 25 percent of Fermilab’s data went into this first EDM result. The data used for the analysis were collected in 2019 and 2020, and the collaboration’s improved tracking and analysis methods allowed it to set a stronger direct limit than Brookhaven despite using an experiment originally designed for a different measurement. The result adds to a broader Fermilab effort to search for rare muon behavior that could reveal physics beyond the Standard Model. It becomes the most sensitive direct search for a muon EDM so far — and only the third direct search worldwide in roughly five decades. The experiment also shows why a null result can be scientifically valuable. A confirmed EDM would have pointed toward new physics and potentially helped explain the matter-antimatter asymmetry. Other precision experiments are tackling the same mystery from different directions, including DUNE’s search for clues to why matter dominates over antimatter. Fermilab’s result will provide a benchmark for newer experiments being developed in Japan and Switzerland, which aim for substantially greater sensitivity. Meanwhile, the collaboration can continue extracting information from its remaining data. The hunt is therefore not over. The first Fermilab search has simply pushed the boundary of what physicists know the muon is hiding. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Rupendra Brahambhatt is an experienced writer, researcher, journalist, and filmmaker. With a B.Sc (Hons.) in Science and PGJMC in Mass Communications, he has been actively working with some of the most innovative brands, news agencies, digital magazines, documentary filmmakers, and nonprofits from different parts of the globe. As an author, he works with a vision to bring forward the right information and encourage a constructive mindset among the masses.