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 KL→π0π0µ+µ- and KL→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(KL→π0π0X0 →π0π0µ+µ-) < 1.0 x 10-10 and Br(KL→ π0π0µ+µ-) < 9.2 x 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.