Search for the dark photon and the dark Higgs boson at Belle

TL;DR

This study searches for dark photon and dark Higgs boson particles at Belle, setting new limits on their properties and interactions, but finds no evidence of their existence within the explored mass ranges.

Contribution

First comprehensive search for dark photon and dark Higgs boson at Belle covering wider mass ranges with improved limits and first constraints on certain final states.

Findings

No significant dark photon or dark Higgs signal observed.

Set new upper limits on production cross sections and coupling parameters.

First experimental constraints on the $3(\pi^+\pi^-)$ and $2(e^+e^-)X$ final states.

Abstract

The dark photon, $A^\prime$, and the dark Higgs boson, $h^\prime$, 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 Higgs-strahlung channel, $e^+e^- \rightarrow A^\prime h'$, with $h^\prime \rightarrow A^\prime A^\prime$. We investigate ten exclusive final-states with $A^\prime \rightarrow e^+e^-$, $\mu^+\mu^-$, or $\pi^+\pi^-$, in the mass ranges $0.1$~GeV/$c^2$~$< m_{A^\prime} < 3.5$~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/$c^2$~$< m_{A^\prime} < 3.5$~GeV/$c^2$ and $2.2$~GeV/$c^2$~$< m_{h'} < 10.5$~GeV/$c^2$. Using the entire $977\,\mathrm{fb}^{-1}$ data set collected by Belle, we observe no significant signal. We obtain individual and combined 90$\%$ confidence level upper limits on the branching fraction times the Born cross section, $\cal B \times \sigma_{\mathrm{Born}}$, on the Born cross section, $\sigma_{\mathrm{Born}}$, and on the dark photon coupling to the dark Higgs boson times the kinetic mixing between the Standard Model photon and the dark photon, $\alpha_D \times \epsilon^2$. 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 $m_{A^\prime}<$ 1 GeV/$c^2$, we exclude values of the mixing parameter, $\epsilon$, above $\sim 8 \times 10^{-4}$.