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 p?p → 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.