New calculation confirms the standard model of particle physics

The muon magnetic moment is an important precision parameter for testing the standard model of particle physics. After years of work, the research group led by Professor Hartmut Wittig from the PRISMA+ Center of Excellence at the Johannes Gutenberg University Mainz (JGU) calculated this quantity using the so-called lattice quantum chromodynamics method. (lattice QCD method).

Their result is in agreement with the latest experimental measurements, contrary to previous theoretical calculations.

After experimental measurements have been pushed towards ever higher precision in recent years, attention has increasingly turned to theoretical prediction and the central question of whether it deviates significantly from experimental results and thus provides proof of the existence of new physics beyond the Standard Model. .

The anomalous magnetic moment is an intrinsic property of elementary particles like the electron or its heavier brother, the muon. Calculating this quantity with sufficient precision within the standard model is a huge challenge.

With the sole exception of gravity, all fundamental interactions contribute to the anomalous magnetic moment. In particular, contributions from the strong interaction, which describes the forces between the building blocks of protons and neutrons, quarks, pose great difficulties for physicists.

The main source of uncertainty in the theoretical calculation of the muon’s anomalous magnetic moment is the contribution of the so-called hadronic vacuum polarization (HVP). Traditionally, this contribution has been determined using experimental data: this is what we call the “data-driven” method.

In fact, for many years this technique provided a significant deviation from the experimentally measured value and therefore also constitutes one of the most promising indications of the existence of new physics.

Result of the PRISMA+ Center of Excellence

Wittig’s group has now published a new result for the HVP contribution as a preprint in the open access archive arXivwhich was obtained using the complementary lattice QCD method.

“Our work confirms previous evidence suggesting a clear divergence between the data-driven method and lattice QCD calculations,” says Wittig. “At the same time, we must conclude from our result that the standard model has once again been confirmed, because our result agrees with the experimental measurement.”

In 2020, the “Muon g-2 Theory Initiative” — an international group of 130 physicists with strong participation from Mainz — published a reference value for the theoretical prediction of the anomalous magnetic moment of the muon in the framework of the Standard Model, which is based on the data-driven method.

This actually shows a clear deviation from new direct measurements of this quantity, carried out since 2021 at Fermilab near Chicago.

However, since the publication of new results from the CMD-3 experiment in Novosibirsk in February 2023, this reference value has come into question, as the Standard Model’s prediction varies significantly depending on the dataset used.

To overcome the drawbacks of the data-driven method, Wittig’s group focused on calculations using the lattice QCD method, which allows the strong interaction contributions to be calculated numerically using supercomputers. The advantage of such an approach is that, unlike the value published in 2020, it provides results that do not require experimental data.

Agreement with experimental mean value

Wittig’s group focused on calculating the contribution of the HVP, which makes the largest strong interaction contribution to the muon’s anomalous magnetic moment. In their recent work, the team found a new value for the muon’s anomalous magnetic moment that is consistent with the current experimental average and far from the 2020 theoretical estimate.

“After years of work to reduce the uncertainties in our calculations and overcome the computational challenges associated with performing such lattice QCD calculations, we obtained the HVP contribution with an overall accuracy of just under 1% and a good balance between statistical and systematic uncertainties,” says Wittig. “This allows us to reassess the validity of the standard model.”

Although the new result once again confirms the standard model, many puzzles remain. The origin of the difference between lattice QCD and the data-driven method and how the result of the CMD-3 experiment should be evaluated are not yet fully understood.

“We still have a long way to go to reach our long-term goal of reducing the total error to around 0.2%. No matter how you look at it, we can’t get around the fact that it There are discrepancies in the muon’s magnetic timing anomalies that need to be explained. We still have a lot to understand,” concludes Wittig.