Search for the dark photon in B ⁰ → A′A′, A′ → e ⁺ e ⁻, μ ⁺ μ ⁻, and π ⁺ π ⁻ decays at Belle

We present a search for the dark photon A′ in the B⁰ → A′A′ decays, where A′ subsequently decays to e⁺e⁻, μ⁺μ⁻, and π⁺π⁻. The search is performed by analyzing 772 × 10⁶BB¯ events collected by the Belle detector at the KEKB e⁺e⁻ energy-asymmetric collider at the ϒ(4S) resonance. No signal is found in the dark photon mass range 0.01 GeV/c² ≤ m_A′ ≤ 2.62 GeV/c², and we set upper limits of the branching fraction of B⁰ → A′A′ at the 90% confidence level. The products of branching fractions, ℬ(B0→A′A′)×ℬ(A′→e+e−)2 and ℬ(B0→A′A′)×ℬ(A′→μ+μ−)2, have limits of the order of 10⁻⁸ depending on the A′ mass. Furthermore, considering A′ decay rate to each pair of charged particles, the upper limits of ℬ (B→ A′ A′) are of the order of 10⁻⁸–10⁻⁵. From the upper limits of ℬ (B→ A′ A′) , we obtain the Higgs portal coupling for each assumed dark photon and dark Higgs mass. The Higgs portal couplings are of the order of 10⁻²–10⁻¹ at mh′≃mB0 ± 40 MeV/c² and 10⁻¹–1 at mh′≃mB0 ± 3 GeV/c². [Figure not available: see fulltext.]