LAYCAST: LAYered CAvern Surface Tracker at future electron-positron colliders

This paper proposes LAYCAST, a novel detector concept installed on the cavern walls and ceiling of future electron-positron colliders like CEPC and FCC-ee, to significantly extend the sensitivity for detecting various long-lived particles beyond the reach of main experiments by leveraging detailed simulations and effective background suppression.

The Gist

Imagine you are trying to catch a ghost. But this isn't a spooky ghost; it's a "Long-Lived Particle" (LLP)—a tiny, mysterious piece of matter predicted by theories beyond our current understanding of physics. The problem is, these ghosts are shy. They are created in high-energy collisions, but instead of vanishing instantly like normal particles, they travel a bit, hide, and then decay (disappear) somewhere far away from the main action.

The standard way to catch them is to build a giant, high-tech camera (the Main Detector) right next to the collision point. But if the ghost travels too far before disappearing, it slips right past the camera's lens, and we never see it.

This paper proposes a clever new solution: LAYCAST.

The Big Idea: The "Cave Wall" Camera

Think of the particle collider (like the future CEPC or FCC-ee) as a massive, underground cavern.

• The Main Detector: This is a giant, expensive, high-tech camera sitting right in the center of the room, right where the particles collide. It's great at catching things that happen immediately.
• The Problem: If a "ghost" particle travels 10, 20, or 50 meters away before vanishing, the Main Detector misses it.
• The LAYCAST Solution: Instead of just looking at the center, the authors propose wrapping the ceiling and walls of the entire cavern in a special, thin, sensitive net made of plastic scintillators (a material that flashes when hit by particles).

The Analogy:
Imagine a room where a firecracker goes off in the middle.

• The Main Detector is a high-speed camera right next to the firecracker. It sees the initial explosion perfectly.
• LAYCAST is like painting the walls and ceiling of the whole room with glow-in-the-dark paint. If a piece of the firecracker flies out, travels across the room, and then explodes into a shower of sparks on the far wall, the paint lights up. Even though the explosion happened far away, the wall tells us it was there.

Why is this special?

1. It's a "360-Degree" Net: Most other proposed detectors are like looking through a telescope in just one direction (forward). LAYCAST covers almost the entire room (except the floor, which is too crowded with heavy machinery). This means it can catch ghosts flying in any direction.
2. It's the "In-Between" Zone: It fills the gap between the main camera and the far walls. It's designed specifically for particles that live just a little too long for the main camera, but not long enough to escape the building entirely.
3. It's a "Veto" System: The Main Detector acts as a bouncer. If a particle hits the Main Detector, it gets flagged. If a particle misses the Main Detector and then hits the wall detector, we know it's a special, long-lived traveler.

What are they looking for?

The paper simulates four specific types of "ghosts" they hope to catch:

• The Dark Higgs: A hidden cousin of the famous Higgs boson.
• The Heavy Neutrino: A heavy, invisible sibling to the neutrinos we know.
• The Light Neutralino: A candidate for Dark Matter that is very light and long-lived.
• The Axion: A particle proposed to solve a puzzle about why the universe doesn't behave exactly as expected.

The "Noise" Problem (Background)

In a busy cavern, there is a lot of "noise." Normal particles (like neutral kaons) can also fly out and hit the walls, creating false alarms.

• The Paper's Fix: The authors show that by combining the Main Detector's data with the Wall Detector's data, they can filter out the noise. It's like having a security guard (Main Detector) check your ID, and if you didn't get stopped by the guard but are seen on the wall camera, that is the suspicious person. They calculated that this "double-check" system reduces the background noise to almost zero.

The Bottom Line

The authors ran millions of computer simulations (like a video game) to see how well this idea works. They found that LAYCAST could see new physics that the Main Detector alone would miss, and it could see things that other proposed "far-away" detectors would miss too.

In simple terms:
If the Main Detector is a microscope looking at the center of a pond, LAYCAST is a net stretched across the entire surface of the pond. It catches the ripples that travel too far for the microscope to see, potentially revealing new secrets about the universe that have been hiding in plain sight.

This proposal is a cost-effective, realistic way to turn the entire underground cavern into a giant particle catcher, giving us a much better chance of finding the "ghosts" of the new physics world.

Technical Summary

1. Problem Statement

Long-Lived Particles (LLPs) are a compelling signature of physics Beyond the Standard Model (BSM), arising in scenarios such as dark matter, neutrino mass generation, and supersymmetry. While the Large Hadron Collider (LHC) and its proposed far-detector experiments (e.g., FASER, MATHUSLA) have made significant progress, future electron-positron colliders like the Circular Electron Positron Collider (CEPC) and Future Circular Collider-ee (FCC-ee) offer a cleaner environment with lower backgrounds.

However, standard main detectors (MD) at these colliders are optimized for prompt decays and have limited geometric acceptance for LLPs that travel significant distances before decaying. Existing proposals for "far detectors" at CEPC/FCC-ee often assume macroscopic distances from the interaction point (IP) with shielding, or they propose hermetic detectors covering the entire cavern (like HECATE). The challenge is to design a detector that:

• Maximizes the geometric acceptance for LLPs decaying in the space between the main detector and the cavern walls.
• Accounts for realistic engineering constraints (e.g., load-bearing floors, cabling, fire safety) that prevent full 4$\pi$ coverage.
• Quantifies the sensitivity to various LLP models while realistically assessing Standard Model (SM) backgrounds, particularly from long-lived neutral kaons ($K_L$).

2. Methodology

The authors propose a new detector concept named LAYCAST (LAYered CAvern Surface Tracker) and perform a comprehensive phenomenological study using Monte Carlo (MC) simulations.

A. Experimental Setup (LAYCAST)

• Location: Installed on the ceiling and four vertical walls of the main experimental cavern, surrounding the main detector. The floor is excluded due to load-bearing requirements for the main detector and other facilities.
• Geometry: The fiducial volume is defined as the space between the outer surface of the cylindrical main detector and the inner surface of the cavern (modeled as a cuboid).
  • Cavern Dimensions: $40 \text{ m} \times 20 \text{ m} \times 30 \text{ m}$ (based on CEPC designs).
  • Main Detector: Cylindrical, radius $4.26 \text{ m}$, half-length $5.56 \text{ m}$.
  • IP Position: Approximately $5 \text{ m}$ above the cavern floor.
• Technology: Proposed to use plastic scintillators (e.g., EJ-200) with readout electronics similar to LHCb's Sci-Fi tracker. The design requires at least four layers for proper tracking and timing to distinguish cosmic rays and directionality.
• Background Suppression: Unlike far-detector concepts with shielding, LAYCAST relies on the Main Detector (MD) as a veto. Events must pass the MD (no activity or specific veto signatures) and then decay in the cavern volume. Timing and pointing information are used to reject cosmic rays and distinguish LLP decays from prompt SM processes.

B. Theoretical Models Studied

The study evaluates sensitivity to four benchmark LLP scenarios:

1. Exotic Higgs Decays ($h \to XX$): A light scalar boson $X$ produced at $\sqrt{s} = 240 \text{ GeV}$ (Higgs factory mode).
2. Heavy Neutral Leptons (HNLs): Produced via $Z \to \nu N$ at the Z-pole ($\sqrt{s} = 91.2 \text{ GeV}$).
3. R-Parity Violating (RPV) Neutralinos: The lightest neutralino $\tilde{\chi}^0_1$ produced via $Z \to \tilde{\chi}^0_1 \tilde{\chi}^0_1$ at the Z-pole.
4. Axion-Like Particles (ALPs): Produced via $e^-e^+ \to \gamma a$ at the Z-pole, coupling to photons and Z-bosons.

C. Simulation and Calculation

• Event Generation: Used PYTHIA8 for showering and MadGraph5 for scattering processes (specifically for ALPs).
• Signal Calculation: The number of signal events ($N_{signal}$) is calculated as:
  $$N_{signal} = N_{prod} \times \langle P[\text{LLP in f.v.}] \rangle \times \text{Br}(\text{visible})$$
  Where $\langle P[\text{LLP in f.v.}] \rangle$ is the average decay probability within the fiducial volume, calculated by integrating the exponential decay law over the specific geometry of the cavern and the main detector.
• Background Estimation: A dedicated estimate of the $K_L$ background from hadronic Z decays is performed. The authors demonstrate that the combined requirement of a "quiet" main detector (single photon or missing energy) and a displaced vertex in the far detector strongly suppresses this background.

3. Key Contributions

1. Realistic Geometry Proposal: Unlike previous idealized "hermetic" proposals, LAYCAST explicitly accounts for the exclusion of the cavern floor and the specific placement of the main detector, providing a more accurate assessment of acceptance and sensitivity.
2. Background Analysis: The paper provides a rigorous, dedicated estimate of the $K_L$ background, showing that the MD$\times$FD (Main Detector $\times$ Far Detector) coincidence strategy reduces this background to negligible levels (often $<1$ event for the "Big" layout), validating the feasibility of the search.
3. Comprehensive Benchmarking: The study compares LAYCAST against:
   • The CEPC/FCC-ee Main Detector.
   • Proposed far-detector concepts (FD1, FD3, FD6) from previous literature.
   • LHC far detectors (CODEX-b, MATHUSLA, FASER2).
   • Existing experimental bounds.
4. Cost Estimation: Provides a preliminary cost estimate of < 3.6 MCHF per layer, suggesting the project is financially feasible compared to other large-scale detector upgrades.

4. Results

• Sensitivity Reach:
  • Light Scalars: LAYCAST can probe branching ratios $\text{Br}(h \to XX)$ down to $\sim 10^{-6}$ (assuming zero background) for proper decay lengths $c\tau_X$ between $0.1$ and $10$ meters, complementing the main detector which is sensitive to shorter lifetimes.
  • HNLs: LAYCAST extends the mass reach for HNLs up to $16\text{--}22 \text{ GeV}$ (depending on luminosity), surpassing the reach of the main detector and competing well with LHC far detectors for masses $> 3 \text{ GeV}$.
  • Neutralinos: The detector can probe coupling strengths $\lambda'_{112}/m^2_{\tilde{f}}$ down to $10^{-15} \text{--} 10^{-14} \text{ GeV}^{-2}$, significantly below current bounds.
  • ALPs: LAYCAST shows superior sensitivity to ALPs with masses $\lesssim 1 \text{ GeV}$ and coupling constants $C_{\gamma\gamma}/\Lambda$ down to $10^{-3} \text{--} 10^{-5} \text{ TeV}^{-1}$, particularly when compared to main detectors which suffer from higher backgrounds for long-lived particles.
• Background Impact: Even with a conservative assumption of 100 background events, LAYCAST retains sensitivity comparable to or better than background-free main detector searches for long decay lengths.
• Comparison with HECATE: LAYCAST achieves similar sensitivity to the HECATE proposal (which assumed a full 4$\pi$ hermetic detector) despite having a smaller fiducial volume (due to the missing floor), primarily because the realistic setup of LAYCAST (floor exclusion) was factored into the sensitivity calculation, whereas HECATE's idealized geometry overestimated its potential.

5. Significance

• Complementarity: LAYCAST fills a crucial "gap" in the search for LLPs. Main detectors are excellent for prompt or short-lived particles, while traditional far detectors (placed 50+ meters away) miss particles with intermediate lifetimes. LAYCAST, being immediately adjacent to the main detector but outside its volume, is uniquely positioned to detect LLPs with decay lengths in the meter-to-tens-of-meters range.
• Feasibility: By demonstrating that the detector can be installed on existing cavern infrastructure (walls/ceiling) without requiring massive new construction or shielding, and by proving that backgrounds are manageable, the paper makes a strong case for the inclusion of such a tracker in the design phases of CEPC and FCC-ee.
• Physics Potential: The proposed detector significantly expands the discovery potential for dark sectors, neutrino mass mechanisms, and supersymmetry at future lepton colliders, potentially probing parameter spaces inaccessible to both current LHC experiments and the main detectors of future colliders.

In conclusion, the paper presents LAYCAST as a technically viable, cost-effective, and highly sensitive solution for extending the LLP search capabilities of future electron-positron colliders, bridging the gap between main detectors and distant far-detector concepts.