Searching for Long-lived Particles: A Compact Detector for Exotics at LHCb

TL;DR

The paper proposes CODEX-b, a compact, shielded detector element near the LHCb interaction point to search for displaced decays of long-lived particles (LLPs). It outlines a practical 10×10×10 m fiducial box instrumented with RPC tracking, foreground shielding, and a shield-veto system to achieve a low-background environment for GeV-scale LLPs produced at IP8, with potential triggerless readout interfaces. Two benchmark portals are analyzed: Higgs decays to dark photons ($h \to \gamma_d \gamma_d$) and Higgs portal-driven B decays ($B \to X_s \varphi$), demonstrating reach improvements over existing LHCb limits and competitive sensitivity with SHiP and MATHUSLA in relevant regions of parameter space. The study shows that CODX-b can reconstruct LLP velocity with $\beta$-resolution better than 1% and, with RPC timing, perform partial mass measurements, highlighting CODEX-b’s role as a complementary, cost-effective LLP search channel that complements the broader LLP program across LHC experiments.

Abstract

We advocate for the construction of a new detector element at the LHCb experiment, designed to search for displaced decays of beyond standard model long-lived particles, taking advantage of a large shielded space in the LHCb cavern that is expected to soon become available. We discuss the general features and putative capabilities of such an experiment, as well as its various advantages and complementarities with respect to the existing LHC experiments and proposals such as SHiP and MATHUSLA. For two well-motivated beyond Standard Model benchmark scenarios -- Higgs decay to dark photons and $B$ meson decays via a Higgs mixing portal -- the reach either complements or exceeds that predicted for other LHC experiments.

Figures (8)

• Figure 1: Layout of the LHCb experimental cavern UX85 at point 8 of the LHC cavern, overlaid with the CODEX-b apparatus.
• Figure 2: (Top) Energy dependence of muon number flux before propagation through shielding (blue), and of muon number flux incident on the fiducial volume (green) after propagation through shielding. (Bottom) Energy dependence of neutral kaon (blue) and neutron (green) shield vetoable and irreducible fluxes. The vetoable fluxes are specified with a rejection factor $\varepsilon_{\text{veto}} = 10^{-5}$ already applied.
• Figure 3: CODEX-b reach for $B\rightarrow X_s\varphi$ in the $s^2_\theta$--$m_\varphi$ plane. Solid (dashed) blue line assumes 100% (Tab. \ref{['tab:effsummary']}) tracking efficiency. Dot-dashed line indicates the reach for $\mathcal{L} = 1\, \text{ab}^{-1}$.
• Figure 4: Inclusive CODEX-b $B \to X_s \varphi$ reach (solid lines). The shaded regions (dashed lines) indicate current LHCb limits (300$\,$fb$^{-1}$ projection) from $B \to K(\varphi \to \mu\mu)$, rescaled to the inclusive process using the ratio of Eq. \ref{['eqn:BRIC']} and the theory predictions for the exclusive branching ratio Batell:2009jfKrnjaic:2015mbs, and assuming $\text{Br}[\varphi \to \mu\mu] \simeq 30\%$ and $10\%$ for $m_\varphi = 0.5$ GeV and $1$ GeV, respectively. Approximate current PDG:2016 and Belle II projected BelleIIreport limits from $B \to K^{(*)}\nu\bar{\nu}$ precision measurements are also shown (gray shading and dashed line).
• Figure 5: Higgs decay to dark photon reach, using the CODEX-b fiducial volume alone and with the muon shadow '$\mu$Sh'. The $\gamma_{\text{d}} \to \mu\mu$ branching ratio is taken from $e^+e^-$ data Meade:2009rb. Also shown is the CODEX-b reach with $\mathcal{L} = 1$ ab$^{-1}$ and a larger box, should DELPHI be removed. The approximate reach for MATHUSLA (gray dotted), rescaled from Chou:2016lxi, and $h \to \text{invisibles}$ is also shown (horizontal gray dashed) CMS:2013xfa.