New estimate of the strength of the magnetic field of muon corresponds to the standard model of particle physics

A new estimate of the strength of the magnetic field around the muon – a sub-atomic particle similar to, but heavier than, an electron – narrows the gap between theory and experimental measurements and compares it with the standard model that led physics for decades.

A paper describing the research by an international team of scientists appeared in the journal on April 8, 2021 Nature.

Twenty years ago, in an experiment at the Brookhaven National Laboratory, physicists found a contradiction between the measurements of the muon’s “magnetic moment” – the strength of its magnetic field – and the theoretical calculations of what the measurement should have been, what the seductive increases possibility of physical particles or forces not yet discovered. The new finding narrows this gap, suggesting that the muon’s magnetism is probably not mysterious at all. Instead of relying on experimental data, researchers have simulated every aspect of their calculations from the ground up – a task that requires massive supercomputer power.

“Most phenomena in nature can be explained by what we call the ‘standard model’ of particle physics,” said Zoltan Fodor, a professor of physics at Penn State and a leader of the research team. “We can predict the properties of particles extremely accurately based on this theory alone, so if theory and experiment do not match, we can get excited that we may have found something new, something outside the standard model.”

For the discovery of new physics outside the standard model, there is consensus among physicists that the difference of opinion between theory and measurement should reach five sigma – a statistical measure equal to a probability of about 1 in 3.5 million.

In the case of the muon, measurements of the magnetic field deviated by about 3.7 sigma from the existing theoretical predictions. Interesting, but not enough to explain a discovery of a new break in the physical rules. Therefore, researchers decided to improve the measurements as well as the theory in the hope of reconciling theory and measurement, or to raise sigma to a level that allows the discovery of new physics.

“The existing theory for estimating the strength of the muon’s magnetic field is based on experimental electron-positron destruction measurements,” Fodor said. “To get a different approach, we used a fully verified theory that was completely independent of the reliance on experimental measurements. We started with fairly basic equations and built the whole estimate from the ground up.”

The new calculations required hundreds of millions of processing hours at various supercomputer centers in Europe and brought the theory back into line with the measurement. However, the story is not over yet. New, more accurate experimental measurements of the muon’s magnetic moment are expected soon.

“If our calculations are correct and the new measurements do not change the story, it seems that we do not need any new physics to explain the muon’s magnetic moment – it follows the rules of the standard model,” Fodor said. “While the prospect of new physics is always enticing, it is also exciting to see theories and experiments align. It shows the depth of our understanding and offers new opportunities for exploration.”

The excitement is far from over.

“Our result needs to be crossed by other groups and we expect that,” Fodor said. “Furthermore, our finding means that there is a tension between the previous theoretical results and our new ones. This contradiction must be understood. In addition, the new experimental results may be close to the old ones or closer to the previous theoretical calculations. We have many years of excitement ahead of us. ”