Search for the dark photon in B-0 -> A ' A ', A ' -> e(+)e(-), mu(+)mu(-), and pi(+)pi(-) decays at Belle

Abstract

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)BB<overbar></mml:mover> 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<mml:mfenced close=")" open="(">B0 -> A ' A '</mml:mfenced>xB<mml:mfenced close=")" open="(">A ' -> e+e-</mml:mfenced>2 and B<mml:mfenced close=")" open="(">B0 -> A ' A '</mml:mfenced>xB<mml:mfenced close=")" open="(">A ' -> mu+mu-</mml:mfenced>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<mml:mfenced close=")" open="(">B0 -> A ' A '</mml:mfenced> are of the order of 10(-8)-10(-5). From the upper limits of B<mml:mfenced close=")" open="(">B0 -> A ' A '</mml:mfenced>, 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 ' similar or equal to mB0 +/- 40 MeV/c(2) and 10(-1)-1 at mh ' <mml:mo>similar or equal to mB0 +/- 3 GeV/c(2).<fig id="Figa" position="anchor"><graphic position="anchor" specific-use="HTML" mime-subtype="JPEG" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="MediaObjects/13130_2021_15453_Figa_HTML.jpg" id="MO1"></graphic