The anomalous magnetic moment of the muon has, for well over ten years now, provided an enduring hint for new physics, in the form of a tantalizing 3-4σ tension between Standard Model (SM) theory and experiment. It is currently measured to a precision of about 0.5 ppm, commensurate with the theoretical uncertainty in its SM prediction. With a plan to reduce the experimental uncertainty by a factor of four, two new experiments will shed new light on this tension: the E989 experiment at Fermilab, which started running in 2018, and the E34 experiment at J-PARC, which plans to start its first run in 2024.
However, without improvements on the theoretical side, the discovery potential of these efforts may be limited. To leverage the new experimental efforts at Fermilab and J-PARC and hence unambiguously discover whether or not new-physics effects contribute to this quantity, the theory errors must be reduced to the same level as the experimental uncertainties. In the SM, aμ is calculated from a perturbative expansion in the fine-structure constant α, which starts at O(α) with the Schwinger term and has been carried out up to and including O (α5). Its uncertainty, dominated by the unknown O(α6) term, is completely negligible. Electroweak corrections have been evaluated at full two- loop order, with dominant three-loop effects estimated from the renormalization group. Their uncertainty, mainly arising from nonperturbative effects in two-loop diagrams involving the light quarks, is still negligible compared to the experimental precision. The dominant sources of theory error are by far the hadronic contributions, in particular, the O(α2) HVP term and the O(α3) HLbL term. There are a number of complementary theoretical efforts underway to better understand and quantify the hadronic corrections, including using dispersive methods, lattice QCD, and effective field theories, as well as a number of different experimental efforts to provide inputs to dispersive, data- driven evaluations. The Muon g-2 Theory Initiative was created to facilitate interactions among these different groups, as well as between the theoretical and experimental g-2 communities.
The Initiative's activities are being coordinated by a Steering Committee that consists of theorists, experimentalists, and representatives from the Fermilab and J-PARC muon g-2 experiments:
- Gilberto Colangelo (University of Bern)
- Michel Davier (University of Paris-Saclay and CNRS, Orsay)
- Simon Eidelman (BINP, Novosibirsk)
- Aida X. El-Khadra (University of Illinois)
- Christoph Lehner (University of Regensburg and Brookhaven National Lab)
- Tsutomu Mibe (KEK)
- Andreas Nyffeler (University of Mainz)
- Lee Roberts (Boston University)
- Thomas Teubner (University of Liverpool)
The Steering Committee is co-chaired by Aida El-Khadra and Christoph Lehner. Its tasks are the long-term planning of the Theory Initiative as well as the planning and organization of the workshops that led to the writing of the White Paper. Given the precision goals and the potential impact, it is crucially important to have more than one independent method for each of the two hadronic corrections, each with fully quantified uncertainties. Fostering the development of such methods is a prime goal of the Initiative, as this will enable critical cross-checks, and, upon combination, may yield gains in precision, to maximize the impact of E989 and E34. To this end, several workshops were organized in 2017, 2018, and 2019.