Improved Standard-Model prediction for $\pi^0\to e^+e^-$

TL;DR

This paper provides an improved Standard-Model prediction for the rare decay of the neutral pion into an electron-positron pair, using a dispersive approach to better incorporate experimental data and theoretical constraints.

Contribution

The authors develop a dispersive method to accurately compute the pion transition form factor, leading to a more precise Standard-Model prediction for bc+e- decay rate.

Findings

Predicted branching ratio: 6.25(3) ^{-8}

Refined constraints on new physics models

Enhanced understanding of pion decay mechanisms

Abstract

We present an improved Standard-Model (SM) prediction for the dilepton decay of the neutral pion. The loop amplitude is determined by the pion transition form factor for $\pi^0\to\gamma^*\gamma^*$, for which we employ a dispersive representation that incorporates both space-like and time-like data as well as short-distance constraints. The resulting SM branching fraction, $\text{BR}[\pi^0\to e^+e^-]=6.25(3)\times 10^{-8}$ , sharpens constraints on physics beyond the SM, including pseudoscalar and axial-vector mediators.