MUonE: a direct study of the hadronic vacuum polarization and the muon magnetic anomaly*

The gyromagnetic ratio 𝑔 for an elementary point-particle differs from the Dirac value of 𝑔 = 2 by the “magnetic anomaly” 𝑎 = (𝑔 − 2)/2 ≈ 10⁻³, due to couplings to virtual particles in the vacuum. The muon, through its greater mass, probes significantly deeper into the high-mass excitations of the vacuum than does the electron. Efforts to measure 𝑎_𝜇 , the muon magnetic anomaly, with high precision studying stored muon decays, have culminated in the Fermilab E989 experiment.  The E989 𝑎_𝜇 results to date, as well as the world average, differ from the standard model predictions by just over 5σ, although the first lattice QCD results to reach comparable precision indicate that the discrepancy may be markedly smaller and not statistically significant.  The main uncertainty in the theoretical analysis of 𝑎_𝜇 comes from the leading order of the hadronic vacuum polarization (HVP) term.

The MUonE collaboration has proposed to measure the leading-order HVP term directly, using muonic Bhabha scattering, a radically different method with completely different systematics from the stored muon 𝑔−2 measurements.  The experiment will be carried out at the CERN SPS.  This talk will discuss the goals, method, current status and future plans for the MUonE experiment.