Session J01: Update on Muon g-2 Theory and Experiment

Experimental results for muon g-2

The Muon g-2 Experiment E989 at Fermilab aims at measuring the anomalous magnetic moment of the muon a_μ≡(g-2)/2 to an unprecedented precision of 140 parts per billion. In July 2023 the experiment finished data-taking and in August 2023 the Muon g-2 collaboration released an intermediate result based on 35% of the total dataset. In this talk we give an update of the progress toward releasing the final result and discuss the current situation of comparison with Standard Model prediction.   

The hadronic contributions to muon g-2 from lattice QCD

Fermilab announced a new experimental result for muon g-2 on August 10, 2023. The statistical uncertainty of the new result is smaller than the previous experiments by a factor of two and the central value is consistent. A result with even more statistics from Fermilab is expected in the coming years. For the Standard Model prediction, the two hadronic contributions, HVP (hadronic vacuum polarization) and HLbL (hadronic light-by-light) are the dominant sources of uncertainty. Currently, there is a tension between the lattice QCD calculations and the dispersion-relation based data-driven approach in the determination of the HVP contribution. We are trying to resolve this tension and reach a consensus on the theoretical determination of muon g-2. I will report the efforts from the lattice side, and also the progress of improving the overall precision of the theoretical prediction of muon g-2 from lattice.   

Hadronic contribution to muon g-2 using dispersive approach

The muon g-2, is one of the most precisely measured quantities in particle physics, both experimentally and theoretically. It represents a crucial test of the Standard Model (SM) and a sensitive probe for new physics, as any deviation between the experimental and theoretical values could indicate Beyond Standard Model physics contributions. Its SM evaluation is limited by current tensions in the evaluation of the leading order hadronic contributions (HVP). These have been tradionally computed by means of a dispersion relation technique involving experimental data measuring the cross section of electron positron annihilation into hadrons, e⁺e⁻→hadrons, (so-called "time-like” or “dispersive” approach). The calculation of HVP via time-like data is highly challenging, due to many hadron-production channels and different experiments with their own systematics and correlations between the involved datasets and a recent result from CMD3 experiment in Novosibirk is in significant discrepancy with previous measurements. In the last few years there has been a lot of progress on the first-principle calculation of HVP using lattice QCD  and a new precise evaluation from BMW collaboration is in closer agreement with the muon g-2 experimental value and with the value for HVP provided by CMD3 taken in isolation. I will review the status and the prospects of the HVP calculation using dispersive approach.