Muon g-2 experiment places new constraints on a forbidden property of muons

Scientists working on the Muon g-2 experiment at Fermilab have announced a new constraint on a rare property of muons known as the electric dipole moment (EDM). The findings, based on an analysis of 25 percent of the data collected during the experiment’s run, represent the most sensitive direct search for a muon EDM to date. Published on the arxiv preprint server, the study marks the first such search conducted at Fermilab and only the third in the last five decades globally. The Muon g-2 collaboration, which has been operating at Fermilab since 2008, previously focused on measuring the magnetic dipole moment of the muon, a property described by the letter g. That work culminated in a major announcement last year regarding the muon’s magnetic anomaly. Now, using the same infrastructure and data, researchers have turned their attention to another potential property of the muon: its EDM. The experiment involves sending beams of anti-muons into a superconducting magnetic storage ring, where they travel at near-light speeds. These particles decay after their lifetimes are extended by relativistic effects, allowing scientists to study their behavior in detail. Detectors surrounding the ring capture the decay products, providing insights into the muons' motion and interactions with magnetic and electric fields. The EDM is a measure of the separation of positive and negative charges within a particle. If present, it would suggest a violation of certain fundamental symmetries in physics, potentially offering clues to the matter-antimatter imbalance observed in the universe. According to the latest results, if a muon EDM exists, it must be smaller than the current detection threshold of the experiment. Joe Price, a co-lead of the EDM analysis from the University of Liverpool, explained that the setup used to measure the magnetic dipole moment inadvertently provides some sensitivity to the EDM. “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,” he said. The Muon g-2 experiment builds upon an earlier version conducted at Brookhaven National Laboratory, which ended in 2001. The Fermilab iteration includes improvements that enhance precision and reduce systematic errors. The facility’s high-energy proton accelerator produces the muon beams necessary for the experiment, while advanced tracking systems record the particles' trajectories. The search for EDMs plays a crucial role in particle physics. Detecting even a small EDM in a fundamental particle could provide evidence of new physical phenomena beyond the Standard Model. The Standard Model itself predicts that the EDM of a muon would be too small to be measured with current technology. Thus, any nonzero value found would signal the existence of unknown forces or particles influencing the muon’s behavior. Gavin Hesketh, another co-lead of the EDM analysis from the University of Liverpool, emphasized that the primary focus of the g-2 experiment is on new physics unrelated to the matter-antimatter asymmetry. However, the EDM measurement offers a complementary avenue to explore unexplained phenomena in the universe. The experiment’s ability to probe the EDM stems from the extreme conditions under which muons are studied. Their relativistic speeds amplify both their lifetimes and the electric fields they experience, increasing the likelihood of detecting even minute EDM effects. This enhancement allows researchers to test theories that predict deviations from the Standard Model. With the current data, the team has established tighter limits on the muon EDM, setting the stage for further investigations as more data becomes available. Future studies will rely on the full dataset collected during the experiment’s operation, which promises to refine these constraints even further. Researchers anticipate that upcoming analyses will continue to push the boundaries of what can be measured with the Muon g-2 apparatus. As the collaboration prepares to release more detailed findings, physicists around the world await the implications of these results for our understanding of fundamental particles and the laws governing them.