New precise measurement of the $e^+e^- \rightarrow π^+π^-(γ)$ cross section with BABAR

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 from hadronic vacuum polarization, in particular through cross section measurements of hadronic final states from $e^+e^-$ collisions. After a first measurement in 2009 of the largest input that comes from the $e^+e^- \rightarrow π^+π^-(γ)$ cross section, we present preliminary results from a new study on 460 ${\rm fb}^{-1}$ of BABAR data, involving a blind and independent procedure. The results of the two analyses are shown to be consistent.

Figures (3)

• Figure 1: The $\pi^+\pi^-$ contribution to $a_\mu$ in the energy range $0.6 < \sqrt{s} < 0.88$ GeV obtained from multiple experiments cmd (a) and comparison of dispersive predictions (preliminary new results in red) with the direct experimental measurement (vertical band) and lattice-based results (b).
• Figure 2: Angular fit results in two $m_{\pi\pi}$ bins, close to threshold (a) and at the $\rho$ peak (b). The quantities $f_{XX\gamma}\ (X=\pi,\mu,K,e)$ give the fitted fractions of the relevant processes ($KK\gamma$ becomes negligible beyond 0.4 ${\rm GeV}/c^2$).
• Figure 3: The $e^+e^- \rightarrow \pi^+\pi^-(\gamma)$ cross section measured in this work as a function of the reduced energy $\sqrt{s^\prime}$ (a) and the ratio of this measurement to the 2009 B A B A R result (b).