Proposal for a shared transverse LLP detector for FCC-ee and FCC-hh and a forward LLP detector for FCC-hh

This paper proposes the design of two dedicated long-lived particle (LLP) detectors, DELIGHT and FOREHUNT, for the Future Circular Collider (FCC) facility, featuring a novel shared transverse detector for both FCC-ee and FCC-hh runs to optimize costs and significantly enhance sensitivity to elusive new physics.

The Gist

Imagine the world of particle physics as a giant, high-speed racetrack where scientists smash tiny particles together to see what happens. For decades, the main goal has been to find "heavy" new particles, like looking for a giant elephant in a room. But so far, the elephants haven't shown up. Now, scientists are changing their strategy: they are looking for "ghosts"—tiny, light, and very shy particles that don't interact much with anything and might hang around for a while before disappearing. These are called Long-Lived Particles (LLPs).

The problem is that the giant detectors currently used to catch these particles (like the ones at the Large Hadron Collider) are designed to catch the "elephants." They are like a massive net that catches everything immediately. If a "ghost" particle travels a few meters before vanishing, the main net often misses it because it's looking right at the starting line.

This paper proposes a new strategy for the future Future Circular Collider (FCC), a massive new particle accelerator being planned. The authors suggest building two specialized "ghost-hunting" tools: one to the side of the track and one straight ahead.

1. The "Side-Kick" Detector: DELIGHT

Think of the main particle collision point as a fountain spraying water in all directions. The main detectors are right under the fountain, getting soaked immediately. But some "ghost" particles are like water droplets that travel a bit further out before evaporating.

The authors propose building a dedicated detector called DELIGHT (Detector for Long-lived particles at high energy of 100 TeV) placed 25 meters to the side of the collision point.

• The "Shared" Innovation: Here is the clever part. The FCC will run in two phases: first with electrons (FCC-ee) and later with protons (FCC-hh). Usually, you would build a different detector for each phase. The authors propose building one single detector that serves both phases. It's like building a house that can be used as a summer home and then, years later, converted into a winter home without tearing it down. This saves money and resources.
• The "Core" Version: They realized that building a massive 100-meter cube might be too expensive or difficult. So, they optimized the design to find the "sweet spot." They found that a smaller, 50-meter cube (called core-DELIGHT) placed at the right distance would catch almost as many ghosts as the giant version, making it a more practical "minimal viable product."

2. The "Forward" Detector: FOREHUNT

While DELIGHT looks to the side, the authors also propose a detector called FOREHUNT (Forward experiment for hundred TeV) to look straight ahead down the beam pipe.

• The Analogy: Imagine throwing a ball. Sometimes, the ball goes straight forward with incredible speed. The main detectors are too far to the side to see it, and the side detector (DELIGHT) might miss it if it's moving too fast in a straight line. FOREHUNT is like a catcher standing right in front of the thrower, waiting for those high-speed, straight-line "ghosts."
• The Hybrid Idea: Placing a giant detector right next to the collision point is dangerous and technically hard because of the intense energy and radiation. The authors suggest a hybrid approach: a small, tough detector close to the start line to catch fast ghosts, and a larger detector further down the track (about 1 km away) to catch slower ghosts that take longer to arrive. This combination covers all the bases.

3. Why This Matters

The paper argues that if we wait until the FCC is built to think about where to put these detectors, we might make the same mistake we made with the current LHC: putting them in suboptimal spots because of space constraints.

• The "Real Estate" Argument: The authors urge the builders of the FCC to reserve the space for these detectors now, even if they don't have the money to build them immediately. It's like buying a plot of land next to a future highway; if you wait until the highway is built, you might not be able to get a good spot anymore.
• The Goal: By placing these detectors in the perfect spots (optimized for distance and size), they can catch "ghost" particles that the main detectors and other proposed experiments will miss. This could be the key to finding the new physics that has been hiding in plain sight.

Summary

In short, this paper is a blueprint for building specialized, optimized "ghost traps" for the next generation of particle accelerators.

1. DELIGHT is a side-view detector that can be shared between two different types of particle collisions to save money.
2. FOREHUNT is a forward-view detector, potentially split into a "near" and "far" team, to catch particles flying straight ahead.
3. The main message is: Plan ahead. Don't wait until the last minute to decide where to put these detectors, or we might miss the most exciting discoveries.

Technical Summary

Technical Summary: Proposal for Shared Transverse and Forward LLP Detectors at the FCC

Problem Statement
As the particle physics community has largely explored conventional TeV-scale new physics avenues, attention has shifted toward elusive light and weakly interacting long-lived particles (LLPs). While general-purpose collider detectors (e.g., ATLAS, CMS) have some sensitivity to displaced signatures, their effectiveness diminishes in highly displaced scenarios. Dedicated LLP detectors are required to probe these regions, but their potential is often limited by suboptimal positioning and dimensions constrained by existing infrastructure (as seen at the LHC). The Future Circular Collider (FCC) offers a unique opportunity to integrate dedicated detectors into the design phase. However, the feasibility of constructing separate, large-scale detectors for both the lepton collider (FCC-ee) and the hadron collider (FCC-hh) raises concerns regarding cost and sustainability. The paper addresses the need to optimize detector placement and dimensions to maximize sensitivity to LLPs while exploring the possibility of a shared detector concept to minimize costs and resource usage.

Methodology
The authors utilize the dark Higgs model as a benchmark scenario, characterized by a scalar particle $\phi$ mixing with the Standard Model Higgs. The study focuses on two production modes:

1. $B \to K\phi$ for $m_\phi \in [0.3, 4.5]$ GeV.
2. $h \to \phi\phi$ for $m_\phi \in [6, 60]$ GeV.

The methodology involves:

• Benchmarking against existing/future main detectors: The authors first calculate the projected reach of the HL-LHC (CMS muon spectrometer) and the FCC-ee (IDEA detector) for the $h \to \phi\phi$ process. They simulate signal and background events, applying specific cuts on displaced vertices (e.g., transverse displacement, number of charged particles, invariant mass) to determine the upper limits on the branching fraction $\text{Br}(h \to \phi\phi)$.
• Optimization of Transverse Detectors (DELIGHT): The authors propose DELIGHT, a transverse detector placed 25 m from the interaction point (IP). They perform a systematic optimization study varying the detector's distance ($D$), surface area ($A$), and length ($L$). They calculate the efficiency gain ($G_{det}^T$) relative to the FCC-ee IDEA detector across nine benchmark points in the $(m_\phi, c\tau)$ parameter space. This leads to the identification of a "minimal" configuration, core-DELIGHT ($50 \times 50 \times 50$ m$^3$), which balances performance with construction feasibility.
• Shared Detector Concept: The study evaluates the feasibility of using the same physical detector for both FCC-ee and FCC-hh runs, leveraging the fact that both colliders share the same experimental cavern and IP locations. The authors analyze background mitigation strategies, including shielding (lead/concrete) and timing constraints, to handle the vastly different background environments (e.g., high pile-up at FCC-hh vs. cleaner environment at FCC-ee).
• Optimization of Forward Detectors (FOREHUNT): For forward physics, the authors propose FOREHUNT (Forward experiment for hundred TeV). They optimize cylindrical detector geometries (inner radius, outer radius, length, distance from IP) to maximize acceptance for LLPs produced in $B$-meson decays. They compare various configurations against the LHC's FASER experiment and propose a hybrid-FOREHUNT concept combining a near detector (200 m) and a far detector (1 km) to balance sensitivity to short and long lifetimes.
• Background Analysis: The paper estimates backgrounds from high-energy muons, neutrinos, and secondary interactions. It uses Geant4 simulations to model hadronic shower leakage and muon energy loss through shielding to determine necessary shielding thicknesses (e.g., 5 m of lead for FCC-hh).

Key Contributions

1. DELIGHT (Shared Transverse Detector): The paper proposes DELIGHT, a dedicated transverse detector optimized for both FCC-ee and FCC-hh. A novel aspect is the shared detector concept, where a single physical detector serves both collider runs, minimizing civil engineering costs and maximizing resource utilization.
2. Core-DELIGHT Configuration: Through optimization, the authors identify core-DELIGHT ($50 \times 50 \times 50$ m$^3$ at 26 m from the IP) as the minimal viable configuration that maintains sensitivity superior to general-purpose detectors across the benchmark parameter space.
3. FOREHUNT (Forward Detector): The paper proposes FOREHUNT for the forward region at FCC-hh, demonstrating that optimizing the detector's proximity to the IP and its dimensions significantly enhances sensitivity to light LLPs from $B$-meson decays compared to LHC-based forward detectors.
4. Hybrid-FOREHUNT: Recognizing practical constraints on placing large detectors very close to the IP, the authors propose a hybrid-FOREHUNT configuration (near + far detectors) that retains significant sensitivity while potentially reducing integration challenges and backgrounds.
5. Background Mitigation Strategies: The study provides quantitative estimates for neutrino and muon backgrounds and proposes specific shielding and veto strategies (e.g., scintillating layers, active vetoes) to ensure signal purity.

Results

• Sensitivity Gains: The shared DELIGHT detector significantly extends the reach for $\text{Br}(h \to \phi\phi)$ compared to the HL-LHC and even the FCC-ee IDEA detector. For instance, at FCC-hh, DELIGHT can probe branching fractions down to $\sim 10^{-8}$. Compared to the FCC-ee IDEA detector, DELIGHT improves sensitivity by factors of 7 to 14 for specific mass/lifetime benchmarks.
• Optimization Limits: The study finds that moving the detector beyond a certain distance or reducing its dimensions below the "core" configuration results in a loss of sensitivity relative to the main detector (IDEA), establishing a lower bound on the necessary size and proximity.
• Background Feasibility:
  • Neutrinos: At FCC-ee, neutrino backgrounds are calculated to be negligible ($N_{events} \ll 1$). At FCC-hh, an estimated $\mathcal{O}(1000)$ neutrino events are expected, requiring dedicated discrimination techniques.
  • Muons: Without additional shielding, penetrating muons would overwhelm the detector. The study shows that a 5 m lead shield at FCC-hh reduces the 10 GeV muon flux from ~50 MHz to ~10 MHz, making the detector viable.
• Forward Physics: The FOREHUNT-C configuration (idealized, close to IP) offers a gain of $\sim 4 \times 10^5$ over FASER. The hybrid configuration offers a gain four times lower than FOREHUNT-C but remains superior to smaller, distant configurations.

Significance and Claims
The paper claims that the proposed DELIGHT and FOREHUNT detectors are essential for unlocking the full physics potential of the FCC in the LLP sector.

• Sustainability and Cost-Effectiveness: The shared transverse detector concept is presented as a novel and sustainable approach, utilizing the same interaction points for both lepton and hadron colliders to minimize costs and maximize resource utilization.
• Strategic Planning: The authors argue that even if immediate funding for construction is not available, CERN must reserve the necessary space (caverns) for these optimized detectors during the FCC design phase. They cite the LHC experience, where the lack of foresight led to suboptimal placement of LLP detectors, as a cautionary tale.
• Physics Reach: The detectors are claimed to probe "previously uncharted regions" of the new physics parameter space, particularly for light, weakly coupled, and highly displaced scenarios that general-purpose detectors cannot access.
• Feasibility: The paper asserts that the civil engineering challenges (excavating large caverns) are comparable to ongoing projects like DUNE and Hyper-Kamiokande, making the proposal technically feasible.

In conclusion, the paper advocates for a proactive, optimized design strategy for the FCC that integrates dedicated LLP detectors, emphasizing that the physical placement and dimensions of these detectors are critical variables that must be resolved now to ensure the facility's ability to discover new physics.