ANUBIS Experiment Overview

Updated 28 December 2025

The ANUBIS experiment is an underground detector designed to search for long-lived, electrically neutral particles with decay lengths from meters to kilometers, opening new windows in BSM physics.
It employs high-precision resistive plate chamber (RPC) tracking integrated with ATLAS triggers to reconstruct displaced vertices with centimeter and nanosecond resolutions across unused underground spaces.
Simulations and prototype tests demonstrate robust background suppression and significant sensitivity to exotic Higgs decays, heavy neutral leptons, and RPV neutralinos, making ANUBIS a key asset for the HL-LHC era.

ANUBIS is a proposed large-volume, underground experiment dedicated to the detection of long-lived, electrically neutral particles (LLPs) with meter-to-kilometer-scale decay lengths, targeting physics scenarios not accessible to the primary LHC detectors. Strategically located above the ATLAS interaction point in the existing PX14 or PX16 service shafts and the UX1 cavern ceiling at CERN, ANUBIS leverages high-precision resistive plate chamber (RPC) tracking to reconstruct displaced vertices from LLP decays occurring in otherwise unused air-filled spaces. Its integration with the ATLAS trigger, extensive acceptance at large transverse angles, and engineered low-background environment allow the experiment to probe a broad class of models in new lifetime regimes, making it a cornerstone of the "Transverse Physics Facility" vision for the High-Luminosity LHC era (Bauer et al., 2019, Brandt et al., 4 Apr 2025, Collaboration, 30 Oct 2025, Reymermier et al., 16 Dec 2025).

1. Scientific Motivation and Physics Scope

ANUBIS is motivated by several compelling classes of Beyond the Standard Model (BSM) theories that generically predict neutral LLPs with macroscopic lifetimes (τ ≳ 10⁻¹⁰ s) and GeV to TeV masses. These theories include:

Higgs portal models: h→ss (s scalar LLP), with proper lifetimes covering cτ ∼ 10⁻¹–10⁵ m relevant for partially or fully invisible Higgs decays.
Sterile neutrinos/heavy neutral leptons (HNLs): minimal or left–right symmetric extensions of the Seesaw mechanism; production via W/Z/h decays or meson decays, with mixings |U|² ≲ 10⁻⁹–10⁻¹¹.
Dark photon/Z′ and baryogenesis portals: LLPs decaying to charged leptons or light hadrons.
R-parity–violating light neutralinos: with displaced hadronic or leptonic final states (Dreiner et al., 2020).

ANUBIS uniquely targets the intermediate "lifetime frontier" (cτ ∼ 0.1–10⁴ m), where standard LHC detectors lose efficiency and both forward and surface-based detectors lack acceptance (Collaboration, 30 Oct 2025, Mitsou, 2021).

2. Detector Architecture and Layout

The baseline design employs a modular array of RPC tracking planes instrumenting the ATLAS cavern ceiling (UX1), complemented by additional stations at the bottoms of the PX14/PX16 shafts:

Ceiling Configuration: Two main tracking stations (TSs), each comprising triplet RPC layers separated by ∼1 m, are suspended from the UX1 ceiling ∼23 m vertically above the ATLAS interaction point. Each provides O(100 m²) of sensitive area, yielding almost 2 sr acceptance in pseudorapidity |η|≲0.5–1.0 (Reymermier et al., 16 Dec 2025, Collaboration, 30 Oct 2025, Shah, 2024).
Shaft Deployments: Additional TSs in the PX14/PX16 shaft bottoms supplement coverage at higher angles; four-shaft configurations scale up total detector area to ∼1600 m² (Brandt et al., 4 Apr 2025).
RPC Technology: 1 mm gas-gap, double-gap, high-pressure laminate (HPL) "BIS78" RPCs as developed for the ATLAS muon upgrade: spatial resolution σₓ ~ O(1 cm) (strip pitch 2.5 cm), time resolution σₜ ∼ 0.4–1 ns, layer hit efficiency ≥98% (Aielli et al., 15 Dec 2025, Aielli et al., 20 Dec 2025, Shah et al., 20 Jun 2025).
DAQ and Integration: All channels are digitized and time-stamped with the 40 MHz LHC clock; event building and selection are integrated into the ATLAS high-level trigger, permitting event-level correlation via bunch-crossing ID (Reymermier et al., 16 Dec 2025, Aielli et al., 20 Dec 2025).

The design allows for rapid lowering/raising of detector planes for shaft access and operational flexibility, with no new cavern construction required (Bauer et al., 2019, Bronsard et al., 2024).

3. Detection Principle and Background Suppression

The experiment exploits the characteristic displaced-vertex (DV) topology:

Signal: Neutral LLPs produced at ATLAS traverse the calorimeters and inner detectors (providing passive and active vetoes against SM backgrounds), and may decay in the ∼10–30 m open volume between ATLAS and the cavern ceiling or in-shaft region (Shah, 2024).
Reconstruction: At least two separated hits in distinct RPC planes with O(cm) spatial and O(ns) timing resolution suffice to reconstruct the vertex and back-propagate the trajectory to the IP (Aielli et al., 15 Dec 2025, Aielli et al., 20 Dec 2025).
Backgrounds: Cosmic-ray muons, beam-induced neutrons and kaons, and random track coincidences constitute the dominant challenges. Timing coincidence with the LHC bunch crossing, pointing constraints (Δθ < 10 mrad to the IP), passive shielding, and topological cuts (minimum vertex opening angle, multiplicity) suppress these by 10⁴–10⁶, yielding O(10²) or fewer background events in the full HL-LHC run (Collaboration, 30 Oct 2025, Reymermier et al., 16 Dec 2025).
Prototype Validation: The proANUBIS demonstrator (1.08–2 m² area, triplet–singlet–doublet stack) has demonstrated per-layer efficiency >95%, track spatial resolution ∼7 mm, timing ∼1 ns, cosmic + collision-induced flux ∼10⁻³ s⁻¹cm⁻², and successful LHC/ATLAS synchronization, validating simulation-based background estimates and channel performance metrics (Aielli et al., 15 Dec 2025, Aielli et al., 20 Dec 2025).

4. Sensitivity Projections and Physics Reach

The discovery and exclusion power is quantified via detailed simulation for benchmark models:

Exotic Higgs Decays: For h→ss, m_s=10–60 GeV, with BR(h→ss) ∼ 10⁻⁶–10⁻⁵ and cτ ∼ 1–10³ m, ANUBIS achieves O(4–36) signal events at HL-LHC (3 ab⁻¹); for m_s=15 GeV and BR=0.1%, sensitivity covers cτ=0.11–4.0×10³ m (Collaboration, 30 Oct 2025).
Heavy Neutral Leptons: Type-I seesaw or left–right scenarios—mixing parameters |U_ℓ|² as low as 10⁻⁹ are probed for m_N=2–5 GeV; reach in the (m_N,|U_ℓ|²) plane extends well beyond prompt and far detector searches, filling essential coverage gaps (Reymermier et al., 16 Dec 2025, Hirsch et al., 2020, Vries et al., 2024).
Long-lived RPV Neutralinos: For 0.5–5 GeV neutralinos from meson decays, coupling sensitivities down to λ′/m̃² ∼ 10⁻¹¹ GeV⁻² are achievable, surpassing MAPP and rivaling MATHUSLA in the optimal cτ range (Dreiner et al., 2020).
Background Systematics: Data-driven rescaling of ATLAS Muon Spectrometer displaced-vertex searches, combined with dedicated in-situ background mapping by proANUBIS, yields robust N_bkg estimates (N_bkg ∼ 90–182 per 3 ab⁻¹ for typical geometries) (Collaboration, 30 Oct 2025, Reymermier et al., 16 Dec 2025).
Comparative Perspective: ANUBIS exceeds or matches CODEX-b and MATHUSLA in the cτ ~ few–10³ m window, and improves over current ATLAS/CMS DVs by 2–3 orders of magnitude in lifetime reach (Collaboration, 30 Oct 2025, Brandt et al., 4 Apr 2025, Mitsou, 2021).

5. Technology Development and R&D Achievements

ANUBIS has driven significant R&D in environmentally sustainable RPC operation:

Low-GWP Gas Mixtures: Standard CERN mixtures (95.2% R-134a, 4.5% i-C4H10, 0.3% SF₆) have high GWP; test campaigns have shown that up to 30% CO₂ substitution reduces GWP by ∼30–40% while maintaining ≥98% efficiency, and that all-hydrocarbon/CO₂ mixtures (GWP≪100) are viable at somewhat high voltage (Shah et al., 20 Jun 2025).
QA/QC and Electronics: Front-end boards achieve threshold <0.3 mV, time jitter <400 ps, and multi-hit capability; cosmic tests validated channel integrity and noise performance (Aielli et al., 15 Dec 2025).
Mechanical Integration: Rigid steel frames, robust gas/power distribution, modular RPC panel construction, and shielding optimized for ∼100 kHz/m² particle environments have been proven in proANUBIS (Aielli et al., 15 Dec 2025, Aielli et al., 20 Dec 2025).

6. Experimental Roadmap and Future Prospects

The full-scale deployment is synchronized with ATLAS/LHC upgrade schedules:

Schedule: Large-area module prototyping (2025), CERN LHCC technical review (2025), installation during LS3/LHC shutdown (2026–27), data-taking in Run 4 (2028 onward) (Aielli et al., 20 Dec 2025, Brandt et al., 4 Apr 2025).
Upgrades: Enhanced DAQ firmware for real-time clustering and sub-ns timing; machine-learning algorithms for advanced background rejection, lowering p_T thresholds, and multivariate signal–background discrimination; addition of fast-timing silicon or Micromegas layers is under study (Reymermier et al., 16 Dec 2025, Shah, 2024).
Synergy: ANUBIS is designed to operate in close symbiosis with ATLAS, supplementing mainstream detector coverage and allowing prompt-object triggers alongside displaced-event signatures. The combined TPF approach (with MATHUSLA, CODEX-b) ensures parameter space continuity for BSM lifetime and mass coverage at the HL-LHC (Brandt et al., 4 Apr 2025).
Environmental Impact: The shift to low-GWP gases in large RPC systems, validated through ANUBIS R&D, sets precedents for ATLAS, CMS, and future collider detectors (Shah et al., 20 Jun 2025).

7. Comparative Assessment and Community Significance

ANUBIS occupies a unique position in the global LLP search program:

Lifetime and Mass Coverage: It bridges the cτ∼10–10³ m, m ≳ 1–100 GeV region, where neither prompt DVs (ATLAS/CMS/SHiP) nor surface/forward detectors (FASER, MATHUSLA) can provide optimal sensitivity.
Background Mitigation: Use of a deep-underground, air-filled decay volume and precise LHC-timestamping permits essentially zero-background or low-background operation.
Cost and Feasibility: Repurposing existing shafts and infrastructure yields O(20–30 MCHF) construction and modest operating cost; full integration with ATLAS DAQ reduces overhead (Brandt et al., 4 Apr 2025, Bauer et al., 2019).
Operational Flexibility: Modular design and staged deployment allow for progressive coverage expansion, in situ performance validation, and prompt response to LHC schedule constraints.

In conclusion, the ANUBIS experiment exemplifies a technically mature, high-impact approach for LLP discovery at the HL-LHC, built upon robust R&D, strategic infrastructure integration, and a design philosophy tightly coupled to the evolving frontiers of BSM phenomenology (Reymermier et al., 16 Dec 2025, Collaboration, 30 Oct 2025, Brandt et al., 4 Apr 2025, Aielli et al., 15 Dec 2025, Aielli et al., 20 Dec 2025, Shah, 2024, Shah et al., 20 Jun 2025, Bauer et al., 2019).