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The ANUBIS detector and its sensitivity to neutral long-lived particles

ANUBIS Collaboration

TL;DR

The paper presents the ANUBIS detector concept, evaluating two transverse layouts (shaft and ceiling) for an air-filled decay volume adjacent to ATLAS at the HL-LHC. It develops a data-driven background framework, dominated by SM LLP hadronic interactions, and demonstrates how ATLAS shielding and veto strategies can suppress backgrounds to near-background-free levels, enabling strong LLP sensitivity. Using a Higgs-mediated BC5 benchmark with $h\to ss$ and $s\to bb$, it shows that ANUBIS can probe $\mathcal{B}r(h\to ss)$ down to the $10^{-5}$–$10^{-6}$ range, with a $c\tau$ reach up to $10^3$–$10^4$ m (and potentially $10^4$–$10^5$ m with improvements), significantly extending HL-LHC LLP coverage and complementing forward detectors. The ceiling layout is identified as optimal, and a proANUBIS demonstrator is underway to validate backgrounds and inform future optimizations, underscoring ANUBIS's role in the diverse LLP search program at the HL-LHC.

Abstract

Long-lived particles (LLPs), i.e., particles with macroscopic lifetimes $τ>10$~ns, appear in various extensions of the Standard Model (SM) that address fundamental questions like the particulate nature of dark matter or baryogenesis. The ANUBIS detector will achieve unprecedented sensitivity to such models compared to existing and approved experiments by instrumenting a large decay volume adjacent to the ATLAS experiment at the High-Luminosity LHC with tracking detectors. This paper outlines the proposed detector layouts for ANUBIS, explores their physics potential with a scalar LLP model, and identifies the preferred layout, comparing it to other experiments. The potential background contributions to ANUBIS are estimated using a data-driven method, and the topology of potential background events is studied using Monte Carlo simulations. Overall, ANUBIS is expected to probe branching ratios down to $\mathcal{O}$(10$^{-6})$ for exotic decays of the Higgs boson to scalar long-lived particles with masses of 10, 15, 40, and 60 GeV and proper lifetimes of $cτ=2.4,\, 3.0,\, 12$, and 18 m, respectively. Moreover, for branching ratios of 0.1\% of the Higgs boson into long-lived scalars with a mass of 15 GeV, ANUBIS can probe a $cτ$ range between $1.1\times10^{-1}$~m and $4.0\times10^3$~m.

The ANUBIS detector and its sensitivity to neutral long-lived particles

TL;DR

The paper presents the ANUBIS detector concept, evaluating two transverse layouts (shaft and ceiling) for an air-filled decay volume adjacent to ATLAS at the HL-LHC. It develops a data-driven background framework, dominated by SM LLP hadronic interactions, and demonstrates how ATLAS shielding and veto strategies can suppress backgrounds to near-background-free levels, enabling strong LLP sensitivity. Using a Higgs-mediated BC5 benchmark with and , it shows that ANUBIS can probe down to the – range, with a reach up to – m (and potentially – m with improvements), significantly extending HL-LHC LLP coverage and complementing forward detectors. The ceiling layout is identified as optimal, and a proANUBIS demonstrator is underway to validate backgrounds and inform future optimizations, underscoring ANUBIS's role in the diverse LLP search program at the HL-LHC.

Abstract

Long-lived particles (LLPs), i.e., particles with macroscopic lifetimes ~ns, appear in various extensions of the Standard Model (SM) that address fundamental questions like the particulate nature of dark matter or baryogenesis. The ANUBIS detector will achieve unprecedented sensitivity to such models compared to existing and approved experiments by instrumenting a large decay volume adjacent to the ATLAS experiment at the High-Luminosity LHC with tracking detectors. This paper outlines the proposed detector layouts for ANUBIS, explores their physics potential with a scalar LLP model, and identifies the preferred layout, comparing it to other experiments. The potential background contributions to ANUBIS are estimated using a data-driven method, and the topology of potential background events is studied using Monte Carlo simulations. Overall, ANUBIS is expected to probe branching ratios down to (10 for exotic decays of the Higgs boson to scalar long-lived particles with masses of 10, 15, 40, and 60 GeV and proper lifetimes of , and 18 m, respectively. Moreover, for branching ratios of 0.1\% of the Higgs boson into long-lived scalars with a mass of 15 GeV, ANUBIS can probe a range between ~m and ~m.

Paper Structure

This paper contains 15 sections, 8 equations, 14 figures, 2 tables.

Table of Contents

  1. Introduction
  2. The ANUBIS project and its layout
  3. Backgrounds
  4. Background sources
  5. Background reduction using ATLAS
  6. Background predictions for ANUBIS
  7. Signature of hadronic interactions in ANUBIS
  8. Sensitivity Studies
  9. Signal Model
  10. Signal sample simulation
  11. Event Selection
  12. Detector geometry simulations
  13. Sensitivity calculation
  14. Results
  15. Summary and outlook

Figures (14)

  • Figure 1: A schematic representation of the coverage of different experiments to the proper lifetime $c\tau$, and partonic centre-of-mass energy, $\sqrt{\hat{s}}$.
  • Figure 2: The layout of the underground cavern at LHC Point 1, UX14, featuring the ATLAS experiment and the PX14 and PX16 service shafts above it. The two proposed configurations of the ANUBIS detector are shown in orange, where the shaded area illustrates the potential ceiling configuration. This includes two circular stations at the bottom of the two access shafts. The shaft configuration is indicated by the four circular stations within the PX14 service shaft, and have a solid orange line.
  • Figure 3: The two layers of the tracking stations of the ANUBIS detector for the ceiling configuration are shown inside the UX1 cavern including the ATLAS detector in (a) the $(x,y)$ plane and (b) the $(y,z)$ plane. The active volume is indicated as a shaded yellow region and the acceptance boundaries in $\eta$ and $\phi$ are highlighted as dashed blue lines. The drawings of the ATLAS detector and the cavern courtesy of ATLAS.
  • Figure 4: Display of a representative background candidate event featuring a hadronic interaction induced by a $n$ inside the ATLAS Cavern. Dashed lines represent the LLP trajectory, while solid lines show the trajectories of the ensuing jet of 4 charged final-state particles. Cross-sections of the ATLAS cavern and service shafts in the $zy$- (top left) and $xy$- (top right) planes, with dotted lines representing ATLAS' vertexing limit. The $xz$ intersection points of the final-state, charged particles with ANUBIS' tracking stations inside the shaft (bottom) are shown as crosses.
  • Figure 5: The impact of successive selection requirements and background events from $K_{L}$ and $n$ interactions within the active volume of the ANUBIS detector. Note that the final two bins are non-cumulative: events passing the final event-level selection are considered independently for the 'ceiling' and 'shaft' scenarios respectively.
  • ...and 9 more figures