Measurement of the $\mathrm e^+\mathrm e^-\rightarrow\mathrm\pi^+\mathrm\pi^-$ Cross Section between 600 and 900 MeV Using Initial State Radiation

TL;DR

This paper measures the $e^+e^- o \pi^+\pi^-$ cross section between 600 and 900 MeV using initial state radiation at BESIII, providing precise data crucial for understanding the muon g-2 anomaly.

Contribution

It presents the first high-precision measurement of the $e^+e^- o \pi^+\pi^-$ cross section in this energy range using initial state radiation, reducing uncertainties in the muon g-2 calculations.

Findings

Cross section measured with 0.9% systematic uncertainty.

Pion form factor $|F_\pi|^2$ extracted.

Contribution to muon g-2 calculated as $(368.2 imes 10^{-10})$.

Abstract

We extract the $e^+e^-\rightarrow \pi^+\pi^-$ cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb$^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor $|F_\pi|^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2)_\mu$. We find this value to be $a_\mu^{\pi\pi,\rm LO}(600-900\;\rm MeV) = (368.2 \pm 2.5_{\rm stat} \pm 3.3_{\rm sys})\cdot 10^{-10}$.