ANUBIS: Proposal to search for long-lived neutral particles in CERN service shafts

TL;DR

This work tackles the LLP discovery gap at the LHC by introducing ANUBIS, a transverse detector concept that reuses the PX14 service shaft near ATLAS/CMS to search for neutral long-lived particles with $m_{\text{LLP}} \gtrsim 1$ GeV and lifetimes up to $c\tau \approx 10^{6}$ m. It outlines a four-TS RPC-based tracking system in an air-filled volume of about $15{,}000$ m$^3$, achieving high-precision space-time vertexing ($\delta t \lesssim 0.5$ ns, $\delta \alpha \lesssim 0.01$ rad) and leveraging ATLAS integration for background vetoes and triggering. Sensitivity studies for the benchmark $h \to ss$ show reach to ${\rm Br}(h\to ss) \sim 10^{-5}$ (background-free) or $\sim 10^{-4}$ (conservative) over a broad $c\tau$ range, surpassing or rivaling other proposals in relevant regions. The approach is cost-effective (roughly ${\cal O}(10)$ MCHF) and adaptable to ATLAS/CMS, with demonstrators (proANUBIS) advancing feasibility and informing background and detector development.

Abstract

Long-lived particles are predicted by many extensions of the Standard Model and have been gaining interest in recent years. In this Letter we present a competitive proposal that substantially extends the sensitivity in lifetime by instrumenting the existing service shafts above the ATLAS or CMS experiments with tracking stations. For scenarios with electrically neutral long-lived particles with $m \gtrsim 1$~GeV, the lifetime reach is increased by 2-3 orders of magnitude compared to currently operating and approved future experiments at the LHC. A detector design proposal is outlined along with projected costs.

Figures (7)

• Figure 1: Schematic view of the UX15 cavern with the ATLAS detector and the access shafts PX16, PM15, and PX14 (from left to right). The beamline is indicated by the solid thick line, while the dimensions of the cavern and the main shaft are shown by the arrows. Potential positions of the ANUBIS tracking stations spaced about 18.5 m apart along the 18 m wide PX14 shaft are indicated by the ellipses. Representative trajectories of LLPs falling within the ANUBIS acceptance are indicated by dashed lines. Figure adapted from Ref. Aad:2008zzm.
• Figure 2: (a) View from the surface down the 92 m deep and 18 m wide main shaft PX14 of the ATLAS Experiment. The approximate location of the interaction region is indicated by the cross. Photo courtesy of CERN. (b) One of the four tracking stations (TS) of ANUBIS in the $(x,z)$ plane. The shaft walls and the ATLAS cavern pipework are shown in gray, the TS in blue, the support structure of the TS in orange. The TS is made up from $1\times1~\text{m}^2$ units shown in Fig. \ref{['fig:anubis_ts_1m_unit']} indicated by the squares.
• Figure 3: (a) Cross section of the PX14 ATLAS shaft in the $(y,z)$ plane with two representative TSs. The shaft walls are indicated in hatched gray, the TSs in shaded blue, and the fiducial region in checkered orange. The layers of both TSs are shown in solid blue, and a potential LLP decay into a pair of muons is sketched in red. (b) Side-view in the $(x,y)$ plane of one of the $1\times1~\text{m}^2$ RPC module cell units composing the ANUBIS tracking stations. The unit consists of two layers A and B (shaded blue) with a triplet of detection layers each.
• Figure 4: Projected sensitivity of ANUBIS, CODEX-b Gligorov:2017nwh, and MATHUSLA Chou:2016lxi for LLPs for Higgs decays $h \to ss$ with $m_s=5, 10$ and $40$ GeV at the HL-LHC with $\sqrt s = 14$ TeV. The projection bands 'ANUBIS 4 events' and 'ANUBIS 50 events' are given for the 'background-free' and 'conservative' background cases requiring 4 and 50 signal events, respectively, as motivated in Section \ref{['sec:bkg']}. The upper (lower) boundary of the projection bands corresponds to the 'shaft-only' ('shaft+cone') configurations defined in the text. The grey parameter space is excluded by the existing constraints on invisible Higgs decays Sirunyan:2018owyAaboud:2019rtt. The region excluded by a dedicated Higgs portal LLP search with ATLAS is shown for reference Aaboud:2018aqj.
• Figure 5: Geometry of the proposed ANUBIS tracking stations (shown in orange) in the ATLAS installation pit together with an example LLP trajectory from a Higgs decay that would be reconstructed if it decays in the detector volume (blue) and the ATLAS detector with the beam axis shown in red.