Search for the decay $K_L^0 \rightarrow 3\gamma$

TL;DR

This paper reports a search for the rare decay $K_L^0 ightarrow 3\,\gamma$ using the E391a detector, setting new upper limits on its branching ratio and improving previous constraints by a factor of 3.2.

Contribution

The study provides the first upper limit on the decay $K_L^0 ightarrow 3\gamma$ assuming parity-violation and also discusses results assuming parity-conservation.

Findings

No events observed in the signal region.

Upper limit on branching ratio set at $7.4\times10^{-8}$.

Improved sensitivity by a factor of 3.2 over previous results.

Abstract

We performed a search for the decay $K_L^0 \rightarrow 3\gamma$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-violation, we obtained the single event sensitivity to be $(3.23\pm0.14)\times10^{-8}$, and set an upper limit on the branching ratio to be $7.4\times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-conservation were also presented in this paper.