New calculation confirms Standard Model of particle physics

The magnetic second of the muon is a vital precision parameter for placing the Standard Model of particle physics to the take a look at. After years of labor, the analysis group led by Professor Hartmut Wittig of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) has calculated this amount utilizing the so-called lattice quantum chromodynamics methodology (lattice QCD methodology).

Their consequence agrees with the most recent experimental measurements, in distinction to earlier theoretical calculations.

After the experimental measurements had been pushed to ever increased precision in recent times, consideration had more and more turned to the theoretical prediction and the central query of whether or not it deviates considerably from the experimental outcomes and thus offers proof for the existence of recent physics past the Standard Model.

The anomalous magnetic second is an intrinsic property of elementary particles such because the electron or its heavier brother, the muon. Calculating this amount with enough accuracy throughout the framework of the Standard Model is a gigantic problem.

With the one exception of gravity, all basic interactions contribute to the anomalous magnetic second. In specific, the contributions of the sturdy interplay, which describes the forces between the fundamental constructing blocks of protons and neutrons, the quarks, trigger nice difficulties for physicists.

The essential supply of uncertainty within the theoretical calculation of the anomalous magnetic second of the muon is the contribution of the so-called hadronic vacuum polarization (HVP). Traditionally, this contribution has been decided utilizing experimental knowledge—that is known as the “data-driven” methodology.

In reality, over a few years, this method supplied a major deviation from the experimental measured worth and thus additionally one of the vital promising indications of the existence of recent physics.

Result of the PRISMA+ Cluster of Excellence

Wittig’s group has now revealed a brand new consequence for the HVP contribution as a preprint within the open entry archive arXiv, which was obtained utilizing the complementary methodology of lattice QCD.

“Our work confirms earlier proof suggesting a transparent divergence between the data-driven methodology and lattice QCD calculations,” says Wittig. “At the identical time, we now have to conclude from our consequence that the Standard Model has as soon as once more been confirmed, as a result of our consequence agrees with the experimental measurement.”

In 2020, the “Muon g-2 Theory Initiative”—a global group of 130 physicists with sturdy participation from Mainz—revealed a reference worth for the theoretical prediction of the anomalous magnetic second of the muon throughout the framework of the Standard Model, which relies on the data-driven methodology.

This really confirmed a transparent deviation from the brand new direct measurements of this amount, which have been carried out at Fermilab close to Chicago since 2021.

However, because the publication of recent outcomes from the CMD-3 experiment in Novosibirsk in February 2023, this reference worth has come into query, because the Standard Model prediction varies drastically relying on which knowledge set is used.

In order to beat the disadvantages of the data-driven methodology, Wittig’s group has centered on calculations utilizing the lattice QCD methodology, which permits the contributions of the sturdy interplay to be calculated numerically utilizing supercomputers. The benefit of such an method is that, not like the worth revealed in 2020, it offers outcomes that don’t require experimental knowledge.

Agreement with the experimental imply worth

Wittig’s group centered on calculating the contribution of the HVP, which offers the most important contribution of the sturdy interplay to the anomalous magnetic second of the muon. In their current work, the group has discovered a brand new worth for the muon’s anomalous magnetic second that’s in keeping with the present experimental imply and much from the 2020 theoretical estimate.

“After years of labor on decreasing the uncertainties of our calculations and overcoming the computational challenges related to performing such lattice QCD calculations, we now have obtained the HVP contribution with an general accuracy of just under 1% and a superb stability between statistical and systematic uncertainties,” says Wittig. “This permits us to reassess the validity of the Standard Model.”

Even if the brand new consequence as soon as once more confirms the Standard Model, there are nonetheless many puzzles. Where the distinction between the lattice QCD and the data-driven methodology comes from and the way the results of the CMD-3 experiment ought to be evaluated will not be but absolutely understood.

“We nonetheless have an extended strategy to go to realize our long-term objective of decreasing the overall error to round 0.2%. No matter the way you take a look at it, we won’t get round the truth that there are discrepancies within the anomalous magnetic second of the muon that should be defined. There continues to be rather a lot for us to know,” concludes Wittig.