Decay Rate and Hyperfine Structure of the bound mu+ mu- system

Results are obtained for the decay rate of ortho and para states and for the hyperfine structure of the dimuonic system mu+ mu- (dimuonium). We calculate next--to-leading order radiative corrections. It is shown that previously reported results for the decay rate of ortho states are incomplete. The decay rate is strongly influenced by the electronic vacuum polarization in the far time-like asymptotic region and thus allows for a test of QED in a previously unexplored kinematic regime.