Measurement of the branching ratio of pi^0 -> e^+e^- using K_L -> 3 pi^0 decays in flight

TL;DR

This paper reports the first significant measurement of the rare decay pi^0 -> e^+e^- using K_L decays, providing a precise branching ratio that exceeds the unitarity lower bound, based on data from Fermilab's KTeV experiment.

Contribution

The study presents the first significant observation and measurement of the pi^0 -> e^+e^- decay branching ratio using in-flight K_L decays, improving understanding of rare meson decays.

Findings

Observed 275 candidate events for pi^0 -> e^+e^-

Measured branching ratio BR = (6.09 ± 0.40 ± 0.24) x 10^{-8}

Result exceeds the unitarity lower bound for this decay.

Abstract

The branching ratio of the rare decay pi^0 -> e^+e^- has been measured in E799-II, a rare kaon decay experiment using the KTeV detector at Fermilab. The pi^0's were produced in fully-reconstructed K_L -> 3 pi^0 decays in flight. We observed 275 candidate pi^0 -> e^+e^- events, with an expected background of 21.4 +- 6.2 events which includes the contribution from Dalitz decays. We measured BR(pi^0 -> e^+e^-, x>0.95) = (6.09 +- 0.40 +- 0.24) times 10^{-8}, where the first error is statistical and the second systematic. This result is the first significant observation of the excess rate for this decay above the unitarity lower bound.