Precise Measurement of the e+ e- --> pi+ pi- (gamma) Cross Section with the Initial-State Radiation Method at BABAR

TL;DR

This paper presents a highly precise measurement of the $e^+e^- o \pi^+\pi^-( ext{gamma})$ cross section using the ISR method at BABAR, significantly improving the accuracy of the hadronic contribution to the muon g-2.

Contribution

It provides the most precise cross section measurement of the $^+e^- o\u03c0^+^-( ext{gamma})$ process to date, refining the calculation of the muon magnetic anomaly.

Findings

Cross section measured with 0.5% systematic uncertainty.

The muon g-2 hadronic contribution is calculated as (514.1 ± 2.2(stat) ± 3.1(syst))×10^{-10}.

ISR luminosity agrees with QED predictions within 1.1%.

Abstract

A precise measurement of the cross section of the process $e^+e^-\to\pi^+\pi^-(\gamma)$ from threshold to an energy of 3GeV is obtained with the initial-state radiation (ISR) method using 232fb$^{-1}$ of data collected with the BaBar detector at $e^+e^-$ center-of-mass energies near 10.6GeV. The ISR luminosity is determined from a study of the leptonic process $e^+e^-\to\mu^+\mu^-(\gamma)\gamma_{\rm ISR}$, which is found to agree with the next-to-leading-order QED prediction to within 1.1%. The cross section for the process $e^+e^-\to\pi^+\pi^-(\gamma)$ is obtained with a systematic uncertainty of 0.5% in the dominant $\rho$ resonance region. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the measured $\pi\pi$ cross section from threshold to 1.8GeV is $(514.1 \pm 2.2({\rm stat}) \pm 3.1({\rm syst}))\times 10^{-10}$.