New BABAR studies for the data-driven predictions of the muon g − 2

Using the full B A B A R data sample, a dedicated measurement of additional radiation in e + e − → μ + μ − γ and e + e − → π + π − γ initial-state-radiation events is presented. Results are obtained at next-to-leading and next-to-next-to-leading order, with one and two additional photons, respectively, for radiation from the initial and final states. The comparison with predictions from Phokhara and AfkQed Monte Carlo generators reveals discrepancies for the former in the one-photon rates and angular distributions. While this disagreement has a negligible effect on the e + e − → π + π − ( γ ) cross section measured by B A B A R , the impact on other cross-section measurements may be larger than uncertainties assigned. The findings shed a new light on the long-standing deviation between the muon g − 2 measurement and the Standard Model prediction using the data-driven dispersive approach.