World news – New estimate of the magnetic field strength of muon agrees with the standard model of particle physics


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April 7, 2021

from Pennsylvania State University

A new estimate of the strength of the magnetic field around the muon – a subatomic particle similar to but heavier than an electron – bridges the gap between theoretical and experimental measurements and brings it in line with the Standard Model that guided the particle has physics for decades.

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An article describing the research of an international team of scientists will appear in Nature on April 8, 2021.

Twenty years ago, physicists in an experiment at Brookhaven National Laboratory discovered a discrepancy between measurements of « magnetic moment » « of the muon – the strength of its magnetic field – and theoretical calculations of what that measurement should be, giving the lure increased possibility of physical particles or forces that are still undiscovered. The new finding reduces this discrepancy, suggesting that the muon’s magnetism is unlikely to be mysterious at all. To achieve this result, the researchers simulated every aspect of their calculations from scratch, rather than relying on experimental data – a task that requires tremendous supercomputing.

« Most phenomena in nature can be solved by the so-called Explaining the « standard model » of particle physics, said Zoltan Fodor, professor of physics at Penn State and head of the research team. « We can predict the properties of particles extremely accurately from this theory alone. So if theory and experiment do not match, we can look forward to having found something new, something that goes beyond the Standard Model. »

For a discovery of new physics beyond the Standard Model, there is consensus among physicists that the difference between theory and measurement must reach five sigmas – a statistical measure that has a probability of about 1 in 3.5 million.

In the case of the muon the measurements of its magnetic field deviate by about 3.7 sigma from the existing theoretical predictions. Fascinating, but not enough to explain the discovery of a new violation of the rules of physics. So the researchers set out to improve both measurements and theory, hoping either to reconcile theory and measurement, or to bring the sigma to a level that would allow the explanation of a discovery of new physics. « The existing theory for estimating the strength of the magnetic field of the muon was based on experimental electron-positron annihilation measurements, » said Fodor. « To have a different approach, we used a fully verified theory that was completely independent of experimental measurements. We started with more basic equations and built the entire estimate from scratch. »

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

« If our calculations are correct and the new measurements don’t change history, we don’t seem to need new physics to explain the muon’s magnetic moment – it follows the rules of the Standard Model, « said Fodor. « While the prospect of new physics is always tempting, it is also exciting to see how theory and experiment are aligned. It shows the depth of our understanding and opens up new possibilities for exploration. »

« Our result should be from other groups are being checked and we are expecting them, « said Fodor. « Furthermore, our finding means that there is a tension between the previous theoretical results and our new one. This discrepancy should be understood. Also, the new experimental results could be close to the old or closer to the previous theoretical calculations. We have many years the excitement ahead of us.  »

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