The Breakthrough Prize in Fundamental Physics, colloquially known as the “Oscar of Science”, goes to “Muon g-2”, a conglomerate of three international research collaborations that performed their groundbreaking measurements at the American Fermi National Accelerator Laboratory (Fermilab) and the Brookhaven National Laboratory as well as CERN in Switzerland. Over the course of more than 60 years, researchers in the collaborations worked to measure the subtle wobble of the muon as precisely as possible. While scientists from the Mainz Institute of Physics were already central to the last experiment at CERN in the 1970s, the group of Professor Dr. Martin Fertl from the PRISMA⁺⁺ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) provided core contributions to the latest experiment at Fermilab.

The Breakthrough Prize Foundation citation recognizes the awardees’ “multi-decade, groundbreaking contributions to the measurement of the muon’s anomalous magnetic moment, pushing the boundaries of experimental precision and igniting a new era in the quest for physics beyond the Standard Model.”

Since 1959, researchers in the collaborations conducted experiments, first at CERN, then Brookhaven and finally at Fermilab, to measure the subtle wobble of the muon, a subatomic particle not yet fully understood. Its internal magnetism (or “g”) could point towards interactions with as-yet undiscovered other particles – if it was measured precisely enough.

The Muon g-2 experiment measures the rotation frequency of the "internal compass needle" of the muons in a magnetic field as well as the magnetic field itself. This is then used to determine the anomalous magnetic moment aμ = (g-2/2), the tiny deviation from the simplest theoretical prediction g=2. Hence the collaboration’s name: Muon g-2. The muon beam was generated at the Fermilab muon campus especially for the experiment – and exhibited a previously unattained purity. To carry out the measurement, the Muon g-2 collaboration repeatedly sent this beam of muons into a superconducting magnetic storage ring with a diameter of 14 meters, where they circulated on average around 1,000 times at almost the speed of light. The third and final measurement result of the latest experiment reached a precision of 127 parts per billion (ppb), exceeding expectations.

Measuring magnetic fields with extreme precision

Martin Fertl and his working group at the PRISMA⁺⁺ Cluster of Excellence contributed extreme precision measurements of the magnetic field in the muon storage ring over the entire measurement period of several years. Several hundred nuclear magnetic resonance magnetometers, developed and built by Martin Fertl, were installed in the walls of the vacuum chambers surrounding the magnetic storage ring. “With the help of further calibration systems, we determined the magnetic field in the muon storage ring to better than 70 ppb, an unprecedented accuracy,” said Fertl. “To determine the magnetic field with such small uncertainty, we had to investigate and understand countless small effects. For example, in dedicated measurement campaigns we discovered that the magnet changes its magnetic field minimally even days after being switched on,” reported Dr. René Reimann, a former post-doctoral researcher in Professor Fertl’s research group, now a scientist at the TU Dortmund.

The success of the Muon g-2 experiment is deeply rooted in its international composition. “Highly automatized processes allowed collaboration members based in Europe to work the night shifts at Fermilab and keep the experiment running day and night, even throughout the international Covid pandemic”, added Hassan Qureshi, a graduate student in Fertl’s research group. The award of the Breakthrough Prize in Fundamental Physics 2026 to the Muon g-2 collaborations on April 18, 2026, came as a total surprise to most members of the collaboration, according to Fertl. “We are deeply humbled to be awarded in recognition of the decades-long adventure to explore the fundamental properties of the muon, only possible through intense international collaboration, with one commonly shared goal: to test the Standard Model of Particle Physics ever more precisely and reveal the marvels of Nature,” summarized Fertl.