Search for a light pseudoscalar particle in the decay $K^0_L \to \pi^0 \pi^0 X$

TL;DR

This study searched for a hypothetical light pseudoscalar particle in kaon decays, motivated by previous experimental hints, but found no evidence and set upper limits on its production rate.

Contribution

The paper provides the first experimental search for the particle suggested by HyperCP, establishing new upper limits on its production in kaon decays.

Findings

No evidence for the particle was observed.

An upper limit of 2.4 x 10^-7 was set on the branching ratio.

Constraints were placed on the particle's mass region from 194.3 to 219.3 MeV/c^2.

Abstract

We performed a search for a light pseudoscalar particle $X$ in the decay $K_L^0->pi0pi0X$, $X->\gamma\gamma$ with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/$c^2$ was suggested by the HyperCP experiment. We found no evidence for $X$ and set an upper limit on the product branching ratio for $K_L^0->pi0pi0X$, $X->\gamma\gamma$ of $2.4 \times 10^{-7}$ at the 90% confidence level. Upper limits on the branching ratios in the mass region of $X$ from 194.3 to 219.3 MeV/$c^2$ are also presented.