Search for the Rare Decays

The KTeV E799 experiment has conducted a search for the rare decays K L → π 0 π 0 µ + µ − and K L → π 0 π 0 X 0 → π 0 π 0 µ + µ − , where the X 0 is a possible new neutral boson that was reported by the HyperCP experiment with a mass of (214.3 ± 0.5) MeV/ c 2 . We find no evidence for either decay. We obtain upper limits of Br( K L → π 0 π 0 X 0 → π 0 π 0 µ + µ − ) < 1 . 0 × 10 − 10 and Br( K L → π 0 π 0 µ + µ − ) < 9 . 2 × 10 − 11 at the 90% confidence level. This result rules out the pseudoscalar X 0 as an explanation of the HyperCP result under the scenario that the ¯ d s X 0 coupling is completely real.