Search For The Dark Photon In B-0 -> A ' A ', A ' -> E(+)E(-), Mu(+)Mu(-), And Pi(+)Pi(-) Decays At Belle

We present a search for the dark photon A in the B-0 -> A ' A ' decays, where A ' subsequently decays to e(+)e(-), mu(+)mu(-), and pi(+)pi(-). The search is performed by analyzing 772 x 10(6) B (B) over bar events collected by the Belle detector at the KEKB e(+)e(-) energy-asymmetric collider at the Upsilon(4S) resonance. No signal is found in the dark photon mass range 0.01 GeV/c(2) <= m(A ') <= 2.62 GeV/c(2), and we set upper limits of the branching fraction of B-0 -> A ' A ' at the 90% confidence level. The products of branching fractions, B(B-0 -> A ' A ) x B(A '-> e(+)e(-))(2) and B(B-0 -> A ' A ') x B(A '-> mu(+)mu(-))(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-0 -> 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 m(h ') similar or equal to m(B0) +/- 40 MeV/c(2) and 10(-1)-1 at mh ' similar or equal to m(B0) +/- 3 GeV/c(2).