Measurement of electromagnetic transition form factors in two-photon collisions at BESIII

Abstract

The muon anomalous magnetic moment, $a_μ= (g-2)_μ/2$, shows a sizeable discrepancy between Standard Model (SM) predictions and direct measurements, hinting at either New Physics or limitations in modeling low-energy strong interactions. As the uncertainty in the SM predictions currently exceeds the experimental uncertainty by a substantial margin, further improvements in theoretical precision are essential to clarify the origin of this discrepancy. The SM theoretical uncertainty is dominated by hadronic processes. The hadronic light-by-light (HLbL) scattering contribution relies on highly accurate data for hadron production via two-photon fusion. Operating at $\sqrt{s}=3.773\,\text{GeV}$ with over $20\,\text{fb}^{-1}$ of integrated luminosity, the BESIII experiment is ideally suited to probe these processes at small momentum transfers. By precisely measuring channels like $e^+e^-\to e^+ e^-π^0$ and $e^+e^-\to e^+ e^-π^0π^0$, BESIII provides the critical experimental input necessary to sharpen the SM prediction and clarify the true nature of the $a_μ$ anomaly.