Measurement of the rare decay pi0 -> e+ e-

TL;DR

This paper reports a precise measurement of the rare decay pi0 -> e+ e- using Fermilab data, providing results consistent with Standard Model predictions and improving understanding of rare meson decays.

Contribution

The study presents the first precise measurement of the pi0 -> e+ e- decay branching ratio using the full KTeV dataset, surpassing previous experimental limits.

Findings

Measured branching ratio B[(pi0 -> e+ e-), (m_e+e-/m_pi0)^2 > 0.95] = 6.44 ± 0.25(stat) ± 0.22(syst) x10^(-8)

Observed 794 candidate events with an expected background of 52.7 ± 11.2 events

Result is consistent with Standard Model predictions and above the unitarity bound.

Abstract

The branching ratio of the rare decay pi0 -> e+ e- has been measured precisely, using the complete data set from the KTeV E799-II experiment at Fermilab. We observe 794 candidate pi0 -> e+ e- events using K0_L -> 3pi0 as a source of tagged pi0's. The expected background is 52.7 +- 11.2 events, predominantly from high e+ e- mass pi0 -> e+ e- gamma decays. We have measured B[(pi0 -> e+ e-), (m_e+e-/m_pi0)^2 > 0.95] = 6.44 +- 0.25(stat) +- 0.22(syst) x10^(-8), which is above the unitary bound from pi0 -> gamma gamma and within the range of theoretical expectations from the standard model.