image: Representation of the calculation of the hadronic vacuum polarization effect on muon magnetism. The muon (μ) spins like a top, turning into a tiny magnet surrounded by a magnetic field. It follows a trajectory along which it interacts with the magnet from the "muon g-2" experiment, as well as with virtual particles from the quantum vacuum state. Thus, it polarizes the hadronic vacuum, leading to modification of its magnetic moment. The background of 0s and 1s, along with the square tiling, represent the supercomputer calculation, which is one of the approaches described here.
Muons, particles akin to electrons, have kepts physicists' heads spinning for more than a decade, because an experimental measurement of their magnetic properties (1) disagrees with theory. Could this be caused by unknown particles or forces?
A new theoretical calculation of this parameter, involving CNRS physicists and published in the journal Nature, has reduced the discrepancy with the experimental measurement. The debate nevertheless continues.
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For over 10 years, measurement of the magnetic properties of the muon (an ephemeral cousin of the electron) has exhibited disagreement with theoretical predictions. This suggests a possible gap in the standard model of particle physics (2), possibly providing a glimpse of a more exotic physics. The first results of Fermilab's "Muon g-2" experiment, which measures one of these properties known as the muon "magnetic moment," will be revealed on 7 April 2021.
While France is not directly participating in this experiment, a CNRS team (3) played a decisive role in calculating the theoretical prediction used as a reference,(4) without which no conclusion would have been possible. To determine the effect of hadronic vacuum polarization, which currently limits the accuracy of calculations, the team used measurements made with electron-positron colliders. This exact approach, which depends exclusively on the precision of these measurements, has been developed and improved by this team for 20 years, leading to the disagreement with the experimental measurement of the muon's magnetic moment.
A different method was recently used by a team including CNRS researchers(5), whose result for the calculation of this contribution is being published in the journal Nature. This result notably reduces the discrepancy with the current experimental value. Thus, the standard model may yet have the last word! To achieve this result, the scientists calculated this contribution ab initio, which is to say using the standard model's equations with no additional parameter. With approximately one billion variables involved, multiple massively parallel European supercomputers (6) were needed to meet this great challenge. This is the first time an ab initio calculation has rivalled the precision of the reference approach, which predicts values for the muon magnetic moment that differ from the measured value to a greater degree.
To settle the matter once and for all, scientists will have to wait for the results of this new theoretical calculation to be confirmed by other teams, and determine what causes the differences between the two theoretical approaches. CNRS teams are currently working together to meet this challenge. They hope to obtain, by combining approaches, a new theoretical reference prediction that is accurate enough to decide the fate of the standard model in coming years, which will see the publication of the final results from Fermilab's "Muon g-2" experiment, as well as those from another experiment with similar objectives in Japan.