New BaBar studies of high-order radiation and the new landscape of data-driven HVP predictions of the muon g-2

TL;DR

This paper presents advanced measurements of initial and final state radiation in electron-positron collisions, revealing discrepancies with theoretical models and impacting the precision of muon g-2 predictions.

Contribution

It introduces next-to- and next-to-next-to-leading order radiation measurements in BaBar data, highlighting generator discrepancies and their effects on HVP-based muon g-2 calculations.

Findings

Discrepancies found between data and Phokhara generator in one-photon rates.

Negligible impact on BaBar's pi+pi- cross section measurement.

Significant systematic effects estimated for KLOE and BESIII muon g-2 analyses.

Abstract

A measurement of additional radiation in $e^+e^- \to \mu^+\mu^- \gamma$ and $e^+e^- \to \pi^+\pi^- \gamma$ initial-state-radiation events is presented using the full $BaBar$ data sample. For the first time results are presented at next-to- 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 the predictions from Phokhara and AfkQed generators reveals discrepancies for the former in the one-photon rates and angular distributions. While this disagreement has a negligible effect on the $e^+e^- \to \pi^+\pi^- (\gamma)$ cross section measured by $BaBar$, the impact on the KLOE and BESIII measurements is estimated and found to be indicative of significant systematic effects. The findings shed a new light on the longstanding deviation among the muon $g-2$ measurement, the Standard Model prediction using the data-driven dispersive approach for calculation of the hadronic vacuum polarization (HVP), and the comparison with lattice QCD calculations.