One of the major cracks in modern physics may now have been closed: for years, scientists puzzled over why the measured values for the magnetic moment of muons did not match the calculations derived from the generally accepted Standard Model of particle physics. There appeared to be a discrepancy that could not be explained by known inaccuracies.

However, after years of work, the quantum physical calculation has now been refined – with significant participation from Austria: Prof. Anton Rebhan (Institute of Theoretical Physics, TU Wien, Vienna) was one of ten coordinators for the four main chapters of the project. Among the more than 200 authors of the theoretical work are three from TU Vienna, two from the University of Vienna and one from the University of Graz.

The improved calculations – with a precision of ten decimal places – agree with the known measurement data (within the expected accuracy), and the long-suspected discrepancy has been resolved. One week after their publication, FermiLab (USA) also provided improved, even more accurate experimental data in a seven-year experiment that has now been completed. This is one of the largest and most important confirmations of the Standard Model of particle physics to date.

The magnetic moment of muons

Muons are unstable elementary particles that are similar to electrons but have around 200 times more mass. They can be created, for example, when cosmic radiation hits our atmosphere, which is why we are constantly bombarded by muons on the Earth's surface. The strength of their interaction with magnetic fields is described by a number – the ‘magnetic moment’.

“The magnetic moment of muons is particularly interesting for particle physics because, due to the high mass of muons, it is sensitive to all fundamental forces of the Standard Model,” says Prof. Anton Rebhan from TU Wien. “You therefore need to have a very good understanding of the physics of different types of particles and how they interact in order to calculate the magnetic moment of muons so precisely.” This figure has therefore been considered an important test case for many years to investigate whether the Standard Model, the foundation of modern particle physics, is actually correct.

The apparent discrepancy

For years, however, it appeared that there was a problem: the discrepancy between the data from Fermilab, where the properties of muons were measured in the field of a large superconducting magnet, and the best available calculations was more than four standard deviations – much more than could be explained by ordinary measurement inaccuracy.

“But the magnetic moment of muons is not simply a number that can be derived from the formulas of the Standard Model in a simple calculation,” explains Anton Rebhan. “There is a long list of complicated effects that all influence the result. And it is scientifically very challenging to take them all into account and combine them correctly.” Quantum fluctuations play a central role here – random events that occur continuously in a vacuum and depend on all types of elementary particles that exist.

Some of the parameters needed for the calculation could not previously be determined mathematically and were taken from other experiments. For other parameters, one has to be satisfied with approximate calculations. In recent years, however, great progress has been made in this area – for example, with extremely complex computer simulations, the results of which can now be included in the calculation so that it is no longer necessary to rely on experimentally measured parameters.

Excellent agreement

Thanks to this improved theoretical work, earlier calculations have now been corrected – and the result agrees impressively well with the known measurement data. It provided the prediction for the final result of the experiment at Fermilab, which confirmed this agreement once again a week later (on June 3^rd).

And so it is now clear that theory and experiment agree excellently. It is another triumph for the Standard Model. “It shows that our Standard Model of particle physics is an extremely good description of reality,” says Anton Rebhan. “We can be really proud that we as a species have managed to achieve this level of precision. Who knows, maybe there is no other civilisation in our entire galaxy that has come this far.”

Original publication 

A Aliberti et al., The anomalous magnetic moment of the muon in the Standard Model: an update, opens an external URL in a new window

Press release from Fermilab., opens an external URL in a new window

Contact

Prof. Anton Rebhan
Institute for Theoretical Physics
TU Wien
+43 1 58801 13620
anton.rebhan@tuwien.ac.at