Dark Matter May Betray Itself by Arriving Late at the LHC

Hunting for dark matter at the Large Hadron Collider has long been a game of invisible footprints: particles that fly through the detectors without leaving a trace, betraying themselves only through the energy they carry away. But a new theoretical study suggests that one of the most elusive candidates in particle physics could be caught by a completely different kind of clue, one measured not in space but in time. The research, published in The European Physical Journal C by Renjie Wang of the Institute of High Energy Physics in Beijing, proposes exploiting the nanosecond-level timing detectors being installed for the High-Luminosity LHC to catch a class of dark matter particles that existing searches are poorly equipped to see. The idea turns the HL-LHC’s precision clocks into a fourth dimension of particle tracking, and in simulated studies it delivers a staggering result: essentially zero background events survive the selection.

The target of the proposed search is the minimal scotogenic model, an elegant extension of the Standard Model first proposed by Ernest Ma in 2006. In this framework, neutrino masses and the dark matter particle are linked in a single loop of quantum processes. The model adds a set of particles that are odd under a symmetry called Z2, including an inert scalar doublet and heavy right-handed neutrinos. When the lightest of these new fermions plays the role of dark matter, the observed abundance of dark matter in the universe, corresponding to a relic density of about 0.12 in standard cosmological units, can be naturally explained if it co-annihilates with a charged partner scalar whose mass lies only a few gigaelectronvolts above its own.

That compressed mass regime is precisely where the LHC struggles most. When the charged inert scalar and the dark matter fermion are separated by only about 2 GeV, the scalar decays into a dark matter particle plus a single very soft charged lepton, typically carrying a transverse momentum near 2 GeV. This sits right at the edge of standard track reconstruction, which at CMS requires transverse momenta of roughly 2 GeV or more. In the detector’s 3.8 tesla magnetic field, such a low-momentum lepton curves with a gyration radius of about 1.8 meters, exceeding the tracker’s radius and degrading conventional reconstruction. Meanwhile, the charged parent itself travels visibly through the inner detector as a heavy particle before abruptly vanishing, leaving what physicists call a disappearing track: a stub of hits that terminates with no matching continuation.

Existing searches were not built for this topology. ATLAS’s displaced-lepton searches typically demand lepton momenta above 30 to 60 GeV, a factor of fifteen or more beyond what this signal produces. Displaced-vertex searches require two or more tracks at a common vertex, impossible when the decay yields only one soft charged particle. Disappearing-track and soft-track analyses retain only partial, unoptimized acceptance. Wang’s simplified truth-level estimates quantify this gap dramatically: for a benchmark with a 200 GeV charged scalar and a 2 GeV mass splitting, the ATLAS disappearing-track search would retain roughly 8.5 percent of the signal, the CMS version about 3.3 percent, and a soft displaced-track search a vanishing 0.01 percent, while displaced-vertex searches capture none at all.

The proposed remedy adds time as an independent observable, capitalizing on a unique capability of the HL-LHC era. Both CMS and ATLAS are installing precision timing layers, the CMS MIP Timing Detector in the barrel and the ATLAS High-Granularity Timing Detector in the endcaps, each offering about 30 picoseconds of timing resolution for minimum-ionizing particles. These devices were primarily conceived to disentangle the roughly 200 simultaneous proton-proton collisions expected per bunch crossing. But the study shows they can do far more. The 2 GeV mass splitting makes the charged scalar long-lived: with a Yukawa coupling of roughly nine parts in ten million, its decay length reaches about 300 millimeters, corresponding to a lifetime of one nanosecond. The parent travels macroscopic distances, between roughly 100 and 1000 millimeters, before decaying.

That journey creates a measurable delay. Because the parent scalar moves at only about 35 percent of the speed of light, the decay products arrive at the timing layer tens to hundreds of picoseconds late relative to a prompt particle born at the collision point. For the 200 GeV benchmark, the expected delay is about 700 picoseconds, once a small helical path-length correction in the magnetic field is included, more than twenty times the detector’s timing resolution. By contrast, every prompt Standard Model background, from W boson decays to b-hadron semileptonic decays, produces arrival delays below about 0.2 picoseconds, a separation of more than three orders of magnitude. The signal and background thus occupy cleanly distinct regions of a two-dimensional map of momentum versus timing delay.

The proposed signal region, labeled SR-4DT, combines six requirements: missing transverse energy above 105 GeV at trigger level and 150 GeV in the final selection, a leading jet above 100 GeV from initial-state radiation, a delayed timing hit in the MTD barrel with a delay exceeding 200 picoseconds, a mandatory low-momentum disappearing-track stub between 0.7 GeV and 300 millimeters transverse displacement that points to the timing hit, a veto on any prompt lepton above 25 GeV, and compatibility with the hard-scatter primary vertex. Crucially, the analysis never demands full lepton reconstruction, sidestepping the tracking threshold that cripples conventional approaches. The stub-hit pointing requirement, with an angular window of 0.1 in radius, suppresses accidental coincidences by roughly a factor of a thousand or more.

The simulated performance is remarkable. Across 4.2 million simulated prompt Standard Model background events, spanning W plus jet, Z plus jet and top pair production at a combined equivalent luminosity of 20.6 inverse femtobarns, not a single event survived the SR-4DT selection. Signal efficiencies range from 12 to 36 percent across the probed mass range, with 14 percent at the 200 GeV benchmark, translating to roughly 2,331 signal events at the full 3,000 inverse femtobarns of the HL-LHC program, fifty-four times above even the conservative exclusion threshold. Under deliberately conservative background assumptions derived from the finite simulated sample, the projected 95 percent confidence-level exclusion reaches charged scalar masses of about 670 GeV at a decay length of 300 millimeters; in the optimistic zero-background limit, the reach extends to roughly 1,100 GeV. The sensitivity holds across the full lifetime scan from 10 to 3,000 millimeters, with the yield peaking near 300 millimeters where both the timing delay and the stub reconstruction are optimal.

Wang is careful about the study’s limits. The simulation uses Delphes-level fast detector response with a parametrized timing layer, and instrumental and combinatorial backgrounds, including fake stubs, random timing hits, hadronic interactions in detector material and beam-induced background, were not modeled and would require full detector simulation to quantify. Physics arguments suggest each category is strongly suppressed, but the definitive residual background remains an open question for the experimental collaborations. Pile-up contamination is controlled by the mandatory stub-to-hit association, since out-of-time pile-up vertices will rarely point to a legitimate disappearing-track stub tied to the hard-scatter primary vertex.

If confirmed experimentally, the strategy would open a discovery window onto exactly the co-annihilation corridor that cosmology favors and collider searches have historically missed. The same small Yukawa coupling that renders the charged scalar long-lived automatically evades constraints from lepton-flavor-violating muon decays and direct-detection experiments, where the scotogenic candidate’s spin-independent cross section is loop-suppressed and consistent with the latest LUX-ZEPLIN limits. With the HL-LHC’s timing detectors coming online, dark matter may soon be caught not by where it goes, but by when it arrives.

The timing-based strategy also connects to a broader effort across collider physics to exploit temporal information in event reconstruction. Timing layers like the CMS MIP Timing Detector use silicon sensors with internal gain to timestamp minimum-ionizing particles, and their performance has been validated with test-beam measurements. What the new study illustrates is that such instrumentation, designed with pile-up separation in mind, can be repurposed as a discovery tool for long-lived particles whose macroscopic lifetimes imprint a characteristic time-of-flight signature.

From a theoretical standpoint, the compressed co-annihilation corridor occupies a special place in thermal dark matter calculations. When the mass gap between a particle and its charged companion shrinks to a few gigaelectronvolts, Boltzmann suppression of the heavier species weakens, and co-annihilation processes efficiently deplete the relic abundance without requiring large annihilation cross sections for the dark matter particle itself. This same kinematic compression, however, suppresses the visible energy release in collider decays, creating the tension between cosmological viability and experimental accessibility that motivates the timing approach.

The framework may generalize beyond the scotogenic model. Compressed spectra with macroscopic decay lengths and soft visible daughters arise in supersymmetric wino and higgsino scenarios, in simplified dark matter models, and in other radiative neutrino-mass constructions. A timing-assisted search requiring only a disappearing-track stub and a delayed hit is largely model-independent, depending mainly on the parent lifetime and the presence of a timing-visible charged daughter.

Verification will require the experimental collaborations to confront full detector simulation, including the combinatorial and instrumental backgrounds absent from this fast-simulation study. If the projected zero-background regime survives that scrutiny, the HL-LHC could deliver sensitivity to a class of dark matter models whose thermal production history makes them cosmologically favored yet collider-hidden.

Subject of Research: Timing-assisted collider search for ultra-compressed scotogenic dark matter at the High-Luminosity LHC

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

Article References: Wang, R. (2026). Probing ultra-compressed scotogenic dark matter at the HL-LHC via 4D spacetime tracking. The European Physical Journal C, 86(9), Article 1062. https://doi.org/10.1140/epjc/s10052-026-16304-9

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16304-9

Keywords: dark matter, scotogenic model, HL-LHC, disappearing tracks, precision timing, long-lived particles, neutrino masses, co-annihilation, CMS MTD, ATLAS HGTD, particle physics, displaced leptons