The Role of Mesons in Muon g-2

TL;DR

This paper discusses how meson physics impacts the calculation of the hadronic contributions to the muon g-2 anomaly, emphasizing the importance of improved meson data and modeling to refine Standard Model predictions and identify potential new physics.

Contribution

It highlights the critical role of meson physics in reducing hadronic uncertainties in muon g-2 calculations and suggests that better meson data can significantly enhance the search for new physics.

Findings

Hadronic uncertainties limit muon g-2 accuracy.

Improved meson data can reduce uncertainties by a factor of two.

Refined calculations could increase the significance of the muon g-2 deviation.

Abstract

The muon anomaly $a_\mu=(g_\mu-2)/2$ showing a persisting 3 to 4 $\sigma$ deviation between the SM prediction and the experiment is one of the most promising signals for physics beyond the SM. As is well known, the hadronic uncertainties are limiting the accuracy of the Standard Model prediction. Therefore a big effort is going on to improve the evaluations of hadronic effects in order to keep up with the 4-fold improved precision expected from the new Fermilab measurement in the near future. A novel complementary type experiment planned at J-PARC in Japan, operating with ultra cold muons, is expected to be able to achieve the same accuracy but with completely different systematics. So exciting times in searching for New Physics are under way. I discuss the role of meson physics in calculations of the hadronic part of the muon g-2. The improvement is expected to substantiate the present deviation $\Delta a_\mu^{\rm New \ Physics}=\Delta a_\mu^{\rm Experiment}- \Delta a_\mu^{\rm Standard \ Model}$ to a 6 to 10 standard deviation effect, provided hadronic uncertainties can be reduce by a factor two. This concerns the hadronic vacuum polarization as well as the hadronic light-by-light scattering contributions, both to a large extent determined by the low lying meson spectrum. Better meson production data and progress in modeling meson form factors could greatly help to improve the precision and reliability of the SM prediction of $a_\mu$ and thereby provide more information on what is missing in the SM.