Upper Limit on the Branching Ratio for the Decay $\pi^0 \to \nu \bar\nu$

TL;DR

This paper reports an experimental search for the rare decay of neutral pions into neutrino-antineutrino pairs, setting a new upper limit on its branching ratio, which constrains theories involving weakly interacting particles.

Contribution

The study provides the first experimental upper limit on the decay $ ext{ extpi}^0 o u ar{ u}$ using data from the E949 detector, improving constraints on such rare processes.

Findings

Upper limit on $ ext{ extpi}^0 o u ar{ u}$ branching ratio is $2.7 imes 10^{-7}$ at 90% CL.

The result constrains models predicting invisible or weakly interacting decay modes of the $ ext{ extpi}^0$.

The limit applies also to unknown weakly interacting particles beyond the Standard Model.

Abstract

A sample of kinematically identified $K^+ \to \pi^+ \pi^0$ decays obtained with the E949 detector was used to search for the helicity-suppressed decay $\pi^0 \to \nu \bar{\nu}$ resulting in an upper limit of $2.7\times10^{-7}$ at 90% confidence level.The upper limit is also applicable to $\pi^0$ decays into unknown weakly interacting particles.