Search for the dark photon in B⁰ → A′A′, A′ → e⁺e⁻, μ⁺μ⁻, and π⁺π⁻ decays at Belle

Abstract

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 × 10⁶ $B\overline{B}$ 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, $\mathrm{\mathcal{B}}\left({B}^0\to A^{\prime }A^{\prime}\right)\times \mathrm{\mathcal{B}}{\left(A\prime \to {e}^{+}{e}^{-}\right)}^2$ and $\mathrm{\mathcal{B}}\left({B}^0\to A^{\prime }A^{\prime}\right)\times \mathrm{\mathcal{B}}{\left(A\prime \to {\mu}^{+}{\mu}^{-}\right)}^2$, 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 $\mathrm{\mathcal{B}}\left({B}^0\to A^{\prime }A^{\prime}\right)$ are of the order of 10⁻⁸–10⁻⁵. From the upper limits of $\mathrm{\mathcal{B}}\left({B}^0\to A^{\prime }A^{\prime}\right)$, 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 ${m}_{h\prime}\simeq {m}_{B^0}$ ± 40 MeV/c² and 10⁻¹–1 at ${m}_{h\prime}\simeq {m}_{B^0}$ ± 3 GeV/c².