Search for a dark photon and an invisible dark Higgs boson in $\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and missing energy final states with the Belle II experiment

Abstract

The dark photon $A^{\prime }$ and the dark Higgs boson $h^{\prime }$ are hypothetical particles predicted in many dark sector models. We search for the simultaneous production of $A^{\prime }$ and $h^{\prime }$ in the dark Higgsstrahlung process $e^{+}{e}^{-}\to A^{\prime }{h}^{\prime }$ with $A^{\prime }\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $h^{\prime }$ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019. With an integrated luminosity of 8.34 fb${}^{-1}$, we observe no evidence for signal. We obtain exclusion limits at 90% Bayesian credibility in the range of 1.7--5.0 fb on the cross section and in the range of $1.7\times 10^{-8}$--$200\times 10^{-8}$ on the effective coupling ${\epsilon }^2\times \hat{I}{\pm }_D$ for the $A^{\prime }$ mass in the range of 4.0 GeV/$c^2$ $<M_{A^{\prime }}<9.7$ GeV/$c^2$ and for the $h^{\prime }$ mass $M_{h^{\prime }}<M_{A^{\prime }}$, where $\epsilon$ is the mixing strength between the standard model and the dark photon and $\hat{I}{\pm }_D$ is the coupling of the dark photon to the dark Higgs boson. Our limits are the first in this mass range.

Accepted for publication in Physical Review Letters