Search for the Rare Decays KL→π0π0μ+μ- and KL→π0π0X0→π0π0μ+μ-

The KTeV E799 experiment has conducted a search for the rare decays, K_L→π⁰π⁰μ⁺μ^- and K_L→π⁰π⁰X⁰→π⁰π⁰μ⁺μ^-, where the X⁰ is a possible new neutral boson that was reported by the HyperCP experiment with a mass of (214.3±0.5)MeV/c². We find no evidence for either decay. We obtain upper limits of Br(K_L→π⁰π⁰X⁰→π⁰π⁰μ⁺μ^-)<1.0×10^-10 and Br(K_L→π⁰π⁰μ⁺μ^-)<9.2×10^-11 at the 90% confidence level. This result rules out the pseudoscalar X⁰ as an explanation of the HyperCP result under the scenario that the d¯sX⁰ coupling is completely real.