The muon g − 2 anomaly confronts new physics in e^± and μ^± final states scattering

Abstract

The 4.2σ discrepancy between the standard model prediction for the muon anomalous magnetic moment a_μ and the experimental result is accompanied by other anomalies. A crucial input for the prediction is the hadronic vacuum polarization ${a}_{\mu}^{\mathrm{HVP}}$ inferred from σ_had = σ(e⁺e⁻ → hadrons) data. However, the two most accurate determinations of σ_had from KLOE and BaBar disagree by almost 3 σ. Additionally, the combined data-driven result disagrees with the most precise lattice determination of ${a}_{\mu}^{\mathrm{HVP}}$ by 2.1 σ. We show that all these discrepancies could be accounted for by a new boson produced resonantly around the KLOE centre of mass energy and decaying promptly yielding e⁺e⁻ and μ⁺μ⁻ pairs in the final states. This gives rise to three different effects: (i) the additional e⁺e⁻ events will affect the KLOE luminosity determination based on measurements of the Bhabha cross section, and in turn the inferred value of σ_had; (ii) the additional μ⁺μ⁻ events will affect the determination of σ_had via the (luminosity independent) measurement of the ratio of π⁺π⁻γ versus μ⁺μ⁻γ events; (iii) loops involving the new boson would contribute directly to the prediction for a_μ. We discuss in detail this possibility, and we present a simple model that can reconcile the KLOE and BaBar results for σ_had, the data-driven and the lattice determinations of ${a}_{\mu}^{\mathrm{HVP}}$, the predicted and measured values of a_μ, while complying with all phenomenological constraints.