ANUBIS header

Welcome to the webpage of the ANUBIS effort, which foresees instrumenting the shaft of the ATLAS experiment with precise, yet cheap tracking detectors. Below, the physics goals, sensitivity, and detector concept of ANUBIS are discussed, followed by an overview of ongoing efforts and opportunities to contribute. The ANUBIS evolution timeline is briefly presented in the context of the LHC/PBC ecosystem. For those interested in more details after going through this website, selected pointers to ANUBIS documents are provided at the end.

Physics case

The particle nature of Dark Matter, which accounts for 4/5 of the Universe, is one of the biggest questions in physics today. Many extensions of the Standard Model with Dark Matter candidates also predict other new long-lived particles, i.e., particles with macroscopic lifetimes τ > 100 ps. The Large Hadron Collider (LHC) provides an unprecedented possibility to search for such long-lived particles (LLP) in a controlled laboratory environment.

The main LHC detectors like ATLAS are doing a fantastic job searching for long-lived particles, however, due to their finite size, their sensitivity to electrically neutral LLPs with decay lengths > 10 m is limited. For very light LLP masses < 1 GeV produced at a similar scales, the FASER detector provides complementary sensitivity. The ANUBIS (AN Underground Belayed In-Shaft, 1909.13022) detector will probe LLPs with decay lengths > 10 m and masses < 1 GeV with an unprecedented sensitivity. In a nutshell, sensitivity to large decay lengths is established by instrumenting a large decay volume (e.g., the ATLAS cavern + service shaft) that is transverse to the LHC beamline to probe potentially heavy LLPs produced at the electroweak scale and above.

ANUBIS detector concept

Sensitivity to New Physics

For the above scenarios, i.e., electrically neutral LLPs with masses > 1 GeV produced at the electroweak scale or above, ANUBIS improves the LHC sensitivity by 2-3 orders of magnitude at large decay lengths > 10 m compared to currently operating and approved future experiments. As a concrete example, this is shown for a representative benchmark model in the Neutral Naturalness scenario, where the Higgs boson acts as a portal between the Standard Model and the Dark Sector with long-lived scalar states s. This scenario can be probed by searching for Higgs →ssff decays where f represents SM fermions (and their charge conjugates).

ANUBIS sensitivity

The central advantage of the ANUBIS proposal is that it provides a competitive projected sensitivity for very moderate costs. One important factor to achieve this is to rely on existing infrastructure, practically avoiding any need for civil engineering, for instance by "recycling" the PX14 service shaft of ATLAS that represents a close-to-ideal, almost perfectly projective decay volume for LLPs. The installation costs can be further reduced by using the existing infrastructure (for instance the crane, that can carry up to 270 t). Another key factor to reduce costs is a smart detector concept, as outlined below.

ANUBIS shaft

In addition to dramatically reduced costs, ANUBIS features a combination of unique advantages:

  • Due to its proximity to ATLAS and the low rate of candidate LLP events, a central feature of ANUBIS is to trigger the readout of the ATLAS detector. This is key not only to dramatically reduce backgrounds by several orders of magnitude, but also allows to probe more complex scenarios where the LLPs are produced in association with other SM particles, e.g., long-lived axion-like particles produced in association with a Z boson or R-parity violating SUSY;
  • The above is especially relevant given that ATLAS is a general-purpose detector capable of capturing all relevant features of candidate events, including the missing transverse momentum;
  • With its precise timing detectors and a large tracking volume, ANUBIS will be particularly sensitive to LLPs moving with a velocity β just O(1%) below unity;
  • The above feature is particularly interesting for electrically charged LLPs that may otherwise look very similar to muons in the ATLAS detector;
  • In a scenario where ATLAS and CMS find an excess in a generic search for Dark Matter like the mono-jet analysis, ANUBIS will be in a prime position to have a closer look at candidate events, since LLPs decaying outside the ATLAS detector volume will register as missing transverse momentum.

ANUBIS Detector Concept

An important challenge for the costs of the ANUBIS detector is the sheer active area to be instrumented to define a large active volume. We identified the Resistive Plate Chamber (RPC) technology as the best fit for purpose. This technology provides an excellent timing resolution of <200 ps per tracking station and a good spatial resolution of about 0.1 cm, at a very moderate cost in the low four-digit range per square metre. An important practical advantage is that the specification of ANUBIS RPC chambers are very similar to BIS-7 from the ATLAS Phase-I upgrade (installed for Run 3), and the future Phase II upgrade chambers (installed for HL-LHC).

Costs will be further reduced through a modular design of the detector.

ANUBIS concept

Current efforts

The efforts currently focus on further refining the strong physics case for ANUBIS and to explore Run 3 opportunities, as outlined below.

Detalied simulation of ANUBIS

We are currently working on a more detailed simulation of the ANUBIS detector. After an initial round of studies of the basic ANUBIS detector geometry (see proposal, first presentation at Physics Beyond Colliders, and recent talk at the VIIIth LLP workshop), we are aiming to further refine the ANUBIS detector geometry using MC simulations. Here, we focus on both SM processes and on other New Physics scenarios, using basic detector geometries. The imperative next step is to implement a full Geant4 based model of the ANUBIS detector, together with the PX14 shaft, the ATLAS cavern, and the ATLAS detector. This will help to leverage the ANUBIS geometry optimisation studies to the next level. Finally, we are working on the validation and performance studies of the BIS-7/8 PRCs, which use a very similar technology to that foreseen for ANUBIS, using MC simulations.

The proANUBIS prototype

An important component of any simulations of zero-background (or very low background) searches is to juxtapose the projections based on simulation with measurements. For this, we foresee constructing proANUBIS, a 1 x 1 x 1 m3 prototype of one tracking station unit module for ANUBIS based on BIS-7/8 RPC technology or possibly a Phase II upgrade technology, as shown in the sketch below. The primary goal of one proANUBIS prototype is to measure the particle fluxes in various positions inside the ATLAS cavern during Run 3 of the LHC and to study the performance in detail. Here, the focus is on the detector occupancy rate due to cavern background radiation and a solid trigger rate estimate. The ultimate goal is to correlate the measurements with the results from simulations elaborated above. Another proANUBIS prototype will be used for general performance studies, and to confirm the long-term stability of the detector response and radiation hardness when using eco-gases.


Collaboration on ANUBIS

We welcome contributions to ANUBIS and new proponents of the ANUBIS idea!

  • If you are an experimentalist and would like to collaborate on the experimental aspects of ANUBIS (detailed background simulation studies, detector R&D or detector electronics), there are plenty of opportunities to do so, and we would be happy to share further details about existing studies, code, and methods.
  • If you are a theorist, there are equally many opportunities to contribute. Primarily, we would be very interested in sensitivity studies in the context of models endorsed by the Physics Beyond Colliders effort. We are happy to share the detector model (Mathematica-based) for further studies.

Interested, even mildly? Please reach out to us through or (experiment) or (pheno).

Further information

Further information on ANUBIS can be found below, ranging from phenomenology/sensitivity studies performed by other groups, to (selected) presentations on ANUBIS at workshops and conferences.

ANUBIS on arXiv

There are several phenomenology/sensitivity studies involving ANUBIS that were performed by other groups:

Our ANUBIS proposal has also stimulated further work on future colliders:

Presentations on ANUBIS

Selected presentations on ANUBIS at workshops and seminars are provided below:

ANUBIS Workshops (protected)

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Topic revision: r18 - 2021-12-12 - OlegBrandt
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