Probing ultra-compressed scotogenic dark matter at the HL-LHC via 4D spacetime tracking

Abstract The ultra-compressed scotogenic regime, in which a 2 GeV mass splitting between the charged inert scalar and the dark-matter fermion is motivated by and compatible with co-annihilation to the observed relic density for suitable choices of the remaining model parameters, produces decay leptons for which existing LHC displaced-lepton and disappearing-track searches retain only partial, unoptimised acceptance, leaving room for a dedicated timing-assisted strategy. We propose exploiting the 30 ps timing resolution of the HL-LHC precision timing detectors as a fourth observable, converting the macroscopic scalar decay length of 100–1000 mm into an arrival-time delay of 70–700 ps – more than three orders of magnitude above the sub-picosecond delays of all prompt Standard Model backgrounds. Operating at the level of a low- $$p_T$$ p T disappearing-track stub matched to a delayed timing hit, without requiring full lepton reconstruction, our 4D timing strategy achieves signal efficiencies of 12–36% across the probed mass range (14% at the 200 GeV benchmark) and zero background across $$4.20\times 10^6$$ 4.20 × 10 6 simulated prompt SM events, projecting 95% C.L. exclusion up to scalar masses of $$\simeq \!670$$ ≃ 670 GeV at 3000 fb $$^{-1}$$ - 1 under conservative background assumptions. The scalar lifetime is set by the Yukawa coupling y through $$c\tau \propto y^{-2}$$ c τ ∝ y - 2 , while sub-eV neutrino masses constrain only the product $$y^2\lambda _5$$ y 2 λ 5 ; the small couplings that place the charged scalar in the optimal timing window are therefore compatible with, though not uniquely fixed by, the radiative neutrino-mass mechanism, making the co-annihilation corridor – conventionally the most elusive regime – an accessible target for 4D spacetime tracking at the HL-LHC.