A Track That Vanishes, Then a Vertex Appears: New Collider Signature Could Reveal How Neutrinos Get Their Mass

Neutrinos are the only known fundamental particles whose mass cannot be explained by the Standard Model, and the mechanism behind it remains one of particle physics’ most stubborn open questions. Now a theoretical study published in The European Physical Journal C proposes that the world’s most powerful particle accelerator may be able to watch that mechanism in action — not through conventional decay chains, but through a striking double signature in which one particle’s track simply vanishes inside the detector and, moments later, a second invisible particle materialises into a pair of leptons far from the collision point.

The work, by Renjie Wang of the Institute of High Energy Physics in Beijing, focuses on the scotogenic model, one of the most elegant extensions of the Standard Model. In this framework, neutrinos acquire their tiny masses radiatively — through quantum loops rather than direct couplings — thanks to a hidden sector of new particles protected by an exact symmetry. That same symmetry also supplies a natural dark matter candidate, making the model unusually economical: one structural addition explains neutrino mass and dark matter at once.

The key insight is a numerical coincidence with profound experimental consequences. To reproduce the observed sub-electronvolt neutrino masses, the Yukawa couplings linking the hidden sector to ordinary leptons must be extraordinarily small, of order one thousandth. Such feeble interactions make the heavy neutral fermion known as N2 exceptionally long-lived: rather than decaying promptly, it survives for tens to hundreds of millimetres, travelling a macroscopic distance through the detector before it decays. Measuring that flight path would, in effect, be measuring the very coupling that generates neutrino mass.

Wang’s study explores a particularly challenging corner of the model’s parameter space, called the compressed-spectrum regime, in which a charged scalar particle and N2 are separated by only about 200 MeV — roughly the mass of a proton. In this regime, a second, even tinier Yukawa entry, of order one millionth, renders the charged scalar long-lived as well. The result is a nested cascade unique in collider physics: the charged scalar crosses the inner tracker as a high-momentum track that abruptly disappears when it decays into N2 plus a lepton too soft to be detected, and then the neutral N2 continues invisibly for another macroscopic distance before decaying into a pair of electrons or muons at a displaced vertex.

This nested disappearing-track-plus-displaced-vertex topology offers two decisive experimental handles. The first is spatial: because N2 is emitted at the endpoint of the vanishing track, the vector connecting that endpoint to the displaced vertex must align with the original track direction to within a few milliradians. No known Standard Model process produces a high-momentum charged track that disappears and is then followed, in the same flight direction, by a displaced dilepton vertex. The second handle is temporal: the two slow, sequential flights accumulate a combined time-of-flight delay of roughly 200 to 800 picoseconds, resolvable by the CMS MIP Timing Detector, a precision timing layer planned for the High-Luminosity LHC upgrade.

Together, these requirements suppress the expected Standard Model background to between one hundred-thousandth and one thousandth of an event over the full High-Luminosity LHC dataset of 3000 inverse femtobarns. Crucially, the discovery reach is robust: even if the background were underestimated by four orders of magnitude and reached ten events, a five-sigma discovery would still require only about twenty signal events. At the study’s benchmark point — an N2 mass of 150 GeV with proper decay lengths of 300 and 189 millimetres for the charged scalar and N2 respectively — the analysis yields roughly 1212 signal events, an overwhelming margin.

The analysis was performed with a deliberately conservative, reconstruction-level strategy. Every observable entering the selection, from the timing reference to the displaced-vertex position, is built from quantities a real detector would measure, with only one truth-level input: the requirement that the vanishing track traverse all four barrel pixel layers, decaying at a radius between 160 and 300 millimetres. Two complementary Level-1 trigger paths — one based on an energetic initial-state radiation jet plus missing energy, the other on a displaced muon — ensure the rare events can be recorded at all, since no hard charged object appears at the primary collision point.

Scanning a grid of 56 benchmark points across the two-dimensional plane of the two decay lengths, the study finds that all 40 points with measurable disappearing tracks exceed both the five-sigma discovery threshold and the 95 percent confidence exclusion limit, with peak sensitivity of about 1635 signal events at decay lengths of 150 and 60 millimetres. The remaining 16 points, where the charged scalar decays too early to leave a reconstructable track stub, fall into a blind zone that only a dedicated displaced-vertex search could address. No existing ATLAS or CMS analysis simultaneously targets this correlated cascade: standalone disappearing-track searches assume the downstream particle is effectively stable, while standalone displaced-vertex searches impose no upstream track requirement.

The deeper significance lies in what the measurement would reveal. Because the N2 decay length is controlled by the same Yukawa couplings that generate sub-electronvolt neutrino masses, the covered region probes couplings between roughly six ten-thousandths and four thousandths — precisely the range associated with the neutrino mass scale. Mapping the nested cascade would therefore turn the High-Luminosity LHC into a direct collider probe of the interaction responsible for neutrino mass, complementing neutrino oscillation experiments and cosmological observations.

Challenges remain before the strategy can be implemented in practice. The dominant residual background arises from pileup combinatorics at the HL-LHC’s expected 200 simultaneous collisions per bunch crossing, and a definitive estimate will require data-driven control regions or full pileup-overlay simulation with four-dimensional tracking. Tau-lepton decay channels of N2, which would demand more complex vertex reconstruction, are deferred to future work. Nevertheless, the study provides a concrete roadmap for adding the scotogenic nested cascade to the Run 4 search programme — and a vivid reminder that the next neutrino-mass clue may not come from a deep underground detector, but from a track that vanishes in the dark of the CMS pixel barrel.

Subject of Research: A collider search strategy for long-lived particles in the scotogenic neutrino mass model at the High-Luminosity LHC

Article Title: Nested disappearing-track plus displaced-vertex topology at the HL-LHC: a dual long-lived-particle signature of the scotogenic model

Article References: Wang, R. (2026). Nested disappearing-track plus displaced-vertex topology at the HL-LHC: a dual long-lived-particle signature of the scotogenic model. The European Physical Journal C, 86(10), Article 1187. https://doi.org/10.1140/epjc/s10052-026-16416-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16416-2

Keywords: scotogenic model, neutrino mass, long-lived particles, disappearing track, displaced vertex, HL-LHC, CMS MIP Timing Detector, dark matter, Yukawa coupling, particle physics, beyond the Standard Model, Nested