Measurement of additional radiation in the initial-state-radiation processes $e^+e^-\to \mu^+\mu^-\gamma$ and $e^+e^-\to \pi^+\pi^-\gamma$ at BABAR

TL;DR

This paper measures additional radiation in specific initial-state-radiation processes at BABAR, comparing results with theoretical predictions and revealing discrepancies that impact the understanding of muon magnetic moment calculations.

Contribution

First measurement of additional radiation at next-to- and next-to-next-to-leading order in these processes, providing new insights into radiation effects and generator accuracy.

Findings

Discrepancies found in one-photon rates and angular distributions from Monte Carlo generators.

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

Insights into the longstanding discrepancy affecting muon magnetic moment predictions.

Abstract

A dedicated 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. Comparison with predictions from Phokhara and AfkQed Monte Carlo generators is performed, revealing discrepancies in the one-photon rates and angular distributions for the former. This disagreement has a negligible effect on the BABAR measurement of the $e^+e^-\to\pi^+\pi^-(\gamma)$ cross section, but could affect other measurements significantly. This study sheds a new light on the longstanding discrepancy in this channel that affects the theoretical prediction of hadronic vacuum polarization contributions to the muon magnetic moment anomaly.