Search for the dark photon in B0 \u2192 A\u2032A\u2032, A\u2032 \u2192 e+e\u2212, \u03bc+\u03bc\u2212, and \u03c0+\u03c0\u2212 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 Boverline{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 ≤ mA′ ≤ 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}^0to A^{prime }A^{prime}right)times mathrm{mathcal{B}}{left(Aprime to {e}^{+}{e}^{-}right)}^2 and mathrm{mathcal{B}}left({B}^0to A^{prime }A^{prime}right)times mathrm{mathcal{B}}{left(Aprime 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}^0to A^{prime }A^{prime}right) are of the order of 10−8–10−5. From the upper limits of mathrm{mathcal{B}}left({B}^0to 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}_{hprime}simeq {m}_{B^0} ± 40 MeV/c2 and 10−1–1 at {m}_{hprime}simeq {m}_{B^0} ± 3 GeV/c2.