Search for a new light gauge boson in π0, η and η' decays

Upper limits for the production of new light gauge bosons in π0, η and η′ decays have been obtained with the Crystal Barrel detector. The kinematically well-constrained reactions p̅p → π0π0 P have been studied, where P ? π 0, η, η′ decays through the emission of a single photon recoiling against a missing state X. X can be a long-lived weakly interacting particle or it decays into ??̅. The resulting branching ratio upper limits (90 % C.L.) are: 6 × 10−5 for masses M X of the missing particle X lying between 65 MeV/c2 and 125 MeV/c2 (π0 decay), 6 × 10−5 for M X between 200 MeV/c2 and 525 MeV/c2 (gh decay), and 4 × 10−5 for M X between 50 MeV/c2 and 925 MeV/c2 (η′ decay). The π0-decay limit represents an improvement by a factor of 4 to 8 (depending on M X) when compared to the existing limit, whereas the η and η′ decay limits have been measured for the first time, thereby extending the M X range from 125 MeV/c2 up to 925 MeV/c2.