Search for the Dark Photon and the Dark Higgs Boson at Belle

Abstract

The dark photon A' and the dark Higgs boson h' are hypothetical constituents featured in a number of recently proposed dark sector models. Assuming prompt decays of both dark particles, we search for their production in the so-called Higgstrahlung channel e#+#e#-# -˃ A'h', with h' -˃ A'A'. We investigate ten exclusive final states with A' -˃ e#+#e#-#, mu#+#mu#-#, or pi#+#pi#-# in the mass ranges 0.1 GeV/c#2# ˂ m#A'# ˂ 3.5 GeV/c#2# and 0.2 GeV/c#2# ˂ m#h'# ˂ 10.5 GeV/c#2#. We also investigate three inclusive final states 2#e#+#e#-##X, 2#mu#+#mu#-##X, and #e#+#e#-###mu#+#mu#-##X, where X denotes a dark photon candidate detected via missing mass, in the mass ranges 1.1 GeV/c#2# ˂ m#A'# ˂ 3.5 GeV/c#2# and 2.2 GeV/c#2# ˂ m#h'# ˂ 10.5 GeV/c#2#. Using the entire 977 fb#-1# data set collected by Belle, we observe no significant signal. We obtain individual and combined 90% credibility level upper limits on the branching fraction times the Born cross section, B x sigma#Born#, on the Born cross section sigma#Born#, and on the dark photon coupling to the dark Higgs boson times the kinetic mixing between the standard model photon and the dark photon, alpha#D# x epsilon#2#. These limits improve upon and cover wider mass ranges than previous experiments. The limits from the final states 3#pi#+#pi#-## and 2#e#+#e#-##X are the first placed by any experiment. For alpha#D# equal to 1/137, m#h'# ˂ 8 GeV/c#2#, and m#A'# ˂ 1 GeV/c#2#, we exclude values of the mixing parameter epsilon above similar to 8 x 10#-4#.