Search for a Dark Higgs Boson Produced in Association with Inelastic Dark Matter at the Belle II Experiment
Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles 𝜒 1 and 𝜒 2 , with 𝑚(𝜒 2 ) >𝑚(𝜒 1 ), is induced by a dark Higgs field and a corresponding dark Higgs boson ℎ′. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a 365 fb −1 data sample collected at Belle II, which operates at the SuperKEKB 𝑒 + 𝑒 − collider, we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎(𝑒 + 𝑒 − → ℎ′𝜒 1 𝜒 2 ), and the product of branching fractions ℬ(𝜒 2 → 𝜒 1 𝑒 + 𝑒 − ) ×ℬ(ℎ′ → 𝑥 + 𝑥 − ), where 𝑥 + 𝑥 − indicates 𝜇 + 𝜇 − , 𝜋 + 𝜋 − , or 𝐾 + 𝐾 − , as functions of ℎ′ mass and lifetime at the level of 10 −1 fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 10 −5 and on the dark photon kinetic mixing parameter at the level of 10 −3 . This is the first search for dark Higgs bosons in association with inelastic dark matter.