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

The BaBar collaboration
Lees, J.P. , Poireau, V. , Tisserand, V. , Garra Tico, J. , Grauges, E. , Palano, A. , Eigen, G. , Stugu, B. , Brown, David Nathan , Kerth, L.T.
Phys.Rev.D 86 (2012) 032013, 2012.

Abstract (data abstract)
A precise measurement of the cross section of the process $e^+e^- \to \pi^+\pi^-(\gamma)$ from threshold to an `energy of $3$ GeV is obtained with the initial-state radiation (ISR) method using $232$ fb$^{-1}$ of data collected with the BaBar detector at $e^+e^-$ center-of-mass energies near $10.6$ GeV. 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 $1.8$ GeV is $(514.1 \pm 2.2({\rm stat}) \pm 3.1({\rm syst}))\times 10^{-10}$.

Version 2 modifications: Since in particular the covariance matrix can be affected by instabilities due to numerical truncations, all values in all tables are now stored with the maximum precision available in Python on Linux x86_64, the formatter is set to 24 digits, but the actual number of meaningful digits is about 15 for double floating point numbers.

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