The muon g − 2 anomaly confronts new physics in e$^{±}$ and μ$^{±}$ final states scattering
Résumé
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.