Search for long-lived particles using displaced vertices of oppositely charged leptons in 140 fb⁻¹ of pp collisions at √𝒔 = 13 TeV with the ATLAS detector

A search is presented for long-lived particles decaying into an oppositely charged lepton pair, 𝜇⁺ 𝜇⁻, 𝑒⁺ 𝑒⁻, or 𝑒^± 𝜇^∓, that form a vertex within the inner tracking system of the ATLAS detector at the Large Hadron Collider, displaced from the primary proton–proton interaction region. The analysis uses the 140 fb⁻¹ of Run-2 data collected at √𝑠 = 13 TeV by the ATLAS experiment in 2015–2018. The results of the analysis are interpreted in the context of three benchmark models covering masses from 0.1 to 2.2 TeV and a range of mean proper lifetimes times the speed of light from 1 to 10 000 mm. The first model is a generic 𝑍′ boson pair-produced by a new heavy scalar, with the 𝑍′ decaying into lepton pairs. The remaining two models are 𝑅-parity violating supersymmetric models in which the lightest neutralino 𝜒˜⁰₁ decays into ℓ⁺ ℓ ′⁻ 𝜈 (ℓ, ℓ′ = 𝑒, 𝜇). The models differ by the mode of production of the 𝜒˜⁰₁ , which can be produced via the decay of pairs of gluinos or of pairs of charginos and neutralinos (𝜒˜^±₁ 𝜒˜⁰₁, 𝜒˜^±₁ 𝜒˜⁰₂, or 𝜒˜⁰₂ 𝜒˜⁰₁). Although each benchmark sample includes pair-produced LLPs, only a single vertex is required to be reconstructed. No dilepton displaced vertex candidate is observed and the results are presented as upper limits on the production cross-sections. This analysis sets leading limits on the production cross-sections for multiple models, including parameter space that has never been directly probed.