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

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 ${\pi }^0\to {\gamma }^{\ast }{\gamma }^{\ast }$, 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.

Comments: 9 pages, 3 figures; journal version