Delayed displaced vertices at the HL-LHC: A novel window into the scotogenic mechanism

We propose a novel search strategy for the origin of neutrino mass at the High-Luminosity LHC (HL-LHC) exploiting the 4D spacetime structure of delayed displaced vertices. In the two-inert-doublet scotogenic model (2IDM3N), the heavy neutral fermion N 2 is a naturally long-lived particle (LLP) whose proper decay length c τ N 2 ∼ 10 –3000 mm is directly linked to the smallness of neutrino masses via the one-loop Yukawa coupling y ∼ O ( 10 − 3 ) . Electroweak pair production of the inert scalars (e.g. p p → η ˜ ± η ˜ 0 , η ˜ + η ˜ − ) provides at least one hard prompt lepton for single-lepton triggering, while the subsequent decay N 2 → N 1 + ℓ + ℓ − produces a displaced vertex with a pair of leptons carrying a large time-of-flight delay Δ t ∼ 300 ps relative to prompt particles. We demonstrate that a single cut Δ t > 200 ps on the arrival time at the CMS MIP Timing Detector eliminates the dominant material-conversion background ( γ → e + e − ), rendering the search effectively background-free within the idealized detector simulation employed here. Full detector-level simulation at the benchmark point m N 2 = 150 GeV , m N 1 = 100 GeV , c τ N 2 = 189 mm yields a signal efficiency of ε = 3.46 % and an expected yield of ∼ 10 800 signal events at 3000 fb − 1 , with negligible expected background ( ≲ 1 event) based on analytic timing arguments and simplified detector modelling. The HL-LHC sensitivity covers the scotogenic parameter space corresponding to 10 mm ≲ c τ N 2 ≲ 3000 mm for 120 GeV ≲ m N 2 ≲ 300 GeV .