We present a measurement of $B({\pi }^0\to e^{+}{e}^{-}\gamma )/B({\pi }^0\to \gamma \gamma )$, the Dalitz branching ratio, using data taken in 1999 by the E832 KTeV experiment at Fermi National Accelerator Laboratory. We use neutral pions from fully reconstructed $K_L$ decays in flight; the measurement is based on $\sim 60$ thousand $K_L\to {\pi }^0{\pi }^0{\pi }^0\to \gamma \gamma \gamma \gamma e^{+}{e}^{-}\gamma$ decays. We normalize to $K_L\to {\pi }^0{\pi }^0{\pi }^0\to 6\gamma$ decays. We find $B({\pi }^0\to e^{+}{e}^{-}\gamma )/B({\pi }^0\to \gamma \gamma )$ $(m_{e^{+}{e}^{-}}>15\mathrm{MeV}/c^2)=[3.920\pm 0.016(\text{stat})\pm 0.036(\text{syst})]\times {10}^{-3}$. Using the Mikaelian and Smith prediction for the $e^{+}{e}^{-}$ mass spectrum, we correct the result to the full $e^{+}{e}^{-}$ mass range. The corrected result is $B({\pi }^0\to e^{+}{e}^{-}\gamma )/B({\pi }^0\to \gamma \gamma )=[1.1559\pm 0.0047(\text{stat})\pm 0.0106(\text{syst})]\%$. This result is consistent with previous measurements, and the uncertainty is a factor of 3 smaller than any previous measurement.

• Received 6 February 2019

DOI:https://doi.org/10.1103/PhysRevD.100.032003

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Published by the American Physical Society