LAUSR

The tripletlike QED processes γ+li→lj+lj-+li with i≠j and i=e, μ, j=e, μ, τ has been investigated as the reactions where a dark photon A′ is produced as a virtual state with a subsequent decay into a lj+lj- pair. This effect arises due to the so-called kinetic mixing and is characterized by a small parameter e describing the coupling strength relative to the electric charge e. In these processes, the dark photon appears as a resonance with a Breit-Wigner propagator in the lj+lj- system. The distributions over the invariant mass of the produced lj+lj- pair are calculated, and the value of the parameter e, as a function of the dark photon mass, is estimated for a given number of measured events in the specific experimental conditions. Assuming a standard deviation of σ=2 (corresponding to ≈95% confidence limit) and a number of measured events equal to 104, we obtain the following limits for the parameter space of the dark photon: e2≤10-7 (γ+μ-→e++e-+μ-) and e2≤10-7÷6×10-8 (γ+e-→μ++μ-+e-) in the A′ mass region 520