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

TL;DR

The study delivers a precise measurement of the rare decay $\pi^0\rightarrow e^+e^-$ using the full KTeV-E799-II dataset, leveraging tagged $\pi^0$ from $K_L\rightarrow 3\pi^0$ decays and normalizing to the Dalitz channel $\pi^0\rightarrow e^+e^-\gamma$. By defining the signal with $x_D>0.95$ and applying radiative corrections, the authors extract $B(\pi^0\rightarrow e^+e^-, x_D>0.95)=(6.44\pm0.25\pm0.22)\times10^{-8}$, and after removing the radiative tail, obtain $B^{\text{no-rad}}=(7.48\pm0.29\pm0.25)\times10^{-8}$, which significantly exceeds the unitary bound of $4.69\times10^{-8}$. The result provides a stringent test of long-distance QED dynamics and constrains $\chi$PT and VMD models for pseudoscalar decays, with implications for related processes such as $\eta\rightarrow\mu^+\mu^-$ and $K_L\rightarrow\mu^+\mu^-$. Overall, the measurement demonstrates robust agreement with standard-model predictions within the evaluated uncertainties and improves the precision of rare $\pi^0$ decay studies.

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.

Figures (4)

• Figure 1: The KTeV spectrometer in E799-II configuration.
• Figure 2: Dalitz $x_D \equiv (m_{e^+e^-}/m_{\pi^0})$ for fully-reconstructed Dalitz decay candidates after normalization mode analysis cuts. Monte Carlo prediction using the world average form factor slope parameter is overlaid.
• Figure 3: Positron-electron invariant mass for $\pi^0\rightarrow e^+e^-$ signal candidates passing all other cuts. The points with error bars are data; the solid histogram is background MC.
• Figure 4: Ratio of data to MC distribution of $m_{e^+e^-}$ in the sideband region below the signal peak, where the MC was normalized to the number of fully-reconstructed Dalitz decays. The dotted line indicates the ratio over the entire region.