Probing pair production of long-lived scalars via an off-shell standard-model-like Higgs boson at the LHC

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We study the collider phenomenology of a long-lived scalar particle S that arises from Higgs mixing in a broad class of Standard-Model (SM) extensions. When the mixing angle is sufficiently small, S becomes long-lived, while its pair production via the Higgs portal can remain sizable. We focus on the production channel g g → h * → S S at the LHC, mediated by an off-shell SM-like Higgs boson. This mechanism provides a complementary probe of S in the mass region above the kinematic threshold of the conventional on-shell decay h → S S , thereby extending the accessible parameter space to heavier scalars. The long-lived S particles can decay inside the inner detector, leading to displaced vertices (DVs) accompanied by jets. We perform a detailed Monte Carlo simulation and reinterpret an existing recast of an ATLAS search for DV-plus-jets signatures in this scenario. We also consider a modified analysis strategy based on the same search to assess potential improvements in sensitivity. We find that the current ATLAS search already excludes a significant region of the parameter space, reaching scalar masses up to m S ∼ 230 GeV for a benchmark h S S coupling λ v of 246 GeV. The modified analysis and projections to the high-luminosity LHC further extend the sensitivity to wider regions of the mass–lifetime parameter space.