Measurement of hadronic cross sections via initial state radiation at BABAR

TL;DR

The BABAR experiment is refining measurements of hadronic cross sections via initial state radiation to improve predictions of the muon's anomalous magnetic moment, identifying generator shortcomings that may resolve past experimental discrepancies.

Contribution

This work presents a new measurement of the $e^+e^- ightarrow ext{hadrons}$ cross section with improved uncertainties and investigates radiative effects revealing Monte Carlo generator issues.

Findings

Identified shortcomings in the Phokhara Monte Carlo generator.

Uncovered potential explanations for longstanding experimental discrepancies.

Enhanced the precision of hadronic cross section measurements.

Abstract

The BABAR experiment participates to the global endeavor for a precise prediction of the anomalous magnetic moment of the muon by evaluating the contribution of hadronic processes to the vacuum polarization. After its last result published in 2009 and 2012, BABAR is preparing a new independent measurement of the $e^+e^- \rightarrow \pi^+\pi^-(\gamma)$ cross section via initial state radiation, with full data statistics and improved uncertainties. A first milestone was reached with the recent study of additional radiations in $e^+e^- \rightarrow \pi^+\pi^-(\gamma)$ and $e^+e^- \rightarrow \mu^+\mu^-(\gamma)$, which uncovered shortcomings of the Phokhara Monte Carlo generator in one-photon rates and angular distributions. This has practically no effect on the previous BABAR measurement, but could explain longstanding discrepancies with other experiments.