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Abstract

We present a measurement of 𝐡(πœ‹0→𝑒+π‘’βˆ’π›Ύ)/𝐡(πœ‹0→𝛾𝛾), 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 𝐾𝐿 decays in flight; the measurement is based on ∼60 thousand πΎπΏβ†’πœ‹0πœ‹0πœ‹0→𝛾𝛾𝛾𝛾𝑒+π‘’βˆ’π›Ύ decays. We normalize to πΎπΏβ†’πœ‹0πœ‹0πœ‹0β†’6𝛾 decays. We find 𝐡(πœ‹0→𝑒+π‘’βˆ’π›Ύ)/𝐡(πœ‹0→𝛾𝛾)(π‘šπ‘’+π‘’βˆ’>15  MeV/𝑐2)=[3.920Β±0.016(stat)Β±0.036(syst)]Γ—10βˆ’3. Using the Mikaelian and Smith prediction for the 𝑒+π‘’βˆ’ mass spectrum, we correct the result to the full 𝑒+π‘’βˆ’ mass range. The corrected result is 𝐡(πœ‹0→𝑒+π‘’βˆ’π›Ύ)/𝐡(πœ‹0→𝛾𝛾)=[1.1559Β±0.0047(stat)Β±0.0106(syst)]. This result is consistent with previous measurements, and the uncertainty is a factor of 3 smaller than any previous measurement.

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