SEARCH FOR SHORT-LIVED NEUTRAL BOSONS IN ORTHOPOSITRONIUM DECAY

We searched for an exotic decay of orthopositronium into a single phot on and a short-lived neutral boson PHI0 in hitherto unexplored mass re gion below 300 keV/c2. A high-resolution measurement of the associated photon energy spectrum was carried out with a germanium detector to d iscriminate the expected sharp peak of this decay from the broader pea k of the pick-off annihilations. No evidence of such decay was observe d, resulting in the upper-limits on the branching ratio of 3.0 x 10(-4 ) (M(PHI)0 < 500 keV/c2 ). This clearly rules out the possibility of t his exotic decay mode explaining the reported discrepancy in the ortho positronium decay rate.