Search for a light pseudoscalar particle in the decay KL0→π0π0X

We performed a search for a light pseudoscalar particle X in the decay K{sub L}{sup 0}{yields}{pi}{sup 0}{pi}{sup 0}X, X{yields}{gamma}{gamma} with the E391a detector at KEK. Such a particle with a mass of 214.3 MeV/c{sup 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{sub L}{sup 0}{yields}{pi}{sup 0}{pi}{sup 0}X, X{yields}{gamma}{gamma} of 2.4x10{sup -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{sup 2} are also presented.