Search for a light pseudoscalar particle in the decay K_{L};{0}-->pi;{0}pi;{0}X.

We performed a search for a light pseudoscalar particle X in the decay K 0 L → π 0 π 0 X, X → γγ 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 0 L → 7r°7r°X, X → yy of 2.4 X 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.