Exploring the lifetime frontier with a beam-dump experiment at CiADS

This paper proposes a cost-effective beam-dump experiment at the CiADS facility to search for long-lived particles, specifically demonstrating that a 5-year run could probe currently unexcluded parameter spaces for dark photons with masses around 100 MeV and kinetic mixing between $10^{-9}$ and $10^{-8}$, while also noting similar potential at the nearby HIAF.

The Gist

Imagine a massive particle accelerator in Huizhou, China, called CiADS. Its main job is like a high-powered industrial furnace: it smashes protons into a block of copper to help scientists figure out how to clean up radioactive nuclear waste. But while the scientists are busy with their "waste cleanup" work, the authors of this paper have a secret side project in mind. They want to turn this machine into a cosmic detective.

Here is the story of their proposal, the CiADS-BDE (Beam-Dump Experiment), explained in everyday terms.

The Setup: The "Backyard" Detective

Think of the accelerator as a giant cannon firing protons (tiny, fast particles) into a thick block of copper (the "beam dump").

• The Crash: When the protons hit the copper, it's like a high-speed car crash. It creates a shower of debris. Most of this debris is ordinary stuff, but the scientists suspect that hidden in this chaos might be Long-Lived Particles (LLPs).
• The Mystery: These LLPs are like "ghosts." They are predicted by theories that go beyond our current understanding of physics (called Beyond-the-Standard-Model). They are very light, interact very weakly with normal matter, and can travel a long distance before they finally "pop" and turn into something we can see.
• The Trap: The scientists propose placing a special detector 10 meters behind the copper block. Because these ghost particles are born from the crash, they zoom forward in a straight line (like a bullet). The detector is waiting in the "forward lane" to catch them.

The Target: The "Dark Photon"

To test their idea, the authors focus on a specific suspect called the Dark Photon.

• The Analogy: Imagine our visible world is a radio station playing music (Standard Model physics). The "Dark Sector" is a secret radio station playing a different frequency that we can't hear. The Dark Photon is a tiny bridge or a translator that allows the two stations to talk to each other.
• The Clue: If a Dark Photon is created in the copper crash, it will fly through the shielding, enter the detector, and decay (break apart) into an electron and a positron (a particle and its anti-particle twin). Finding this pair of twins appearing out of nowhere, far away from the crash site, would be the smoking gun.

The Shield: Keeping the Noise Down

One of the biggest challenges in particle physics is background noise. Imagine trying to hear a whisper in a stadium full of cheering fans.

• The Problem: Cosmic rays (particles from space) and neutrinos (ghostly particles that pass through everything) constantly hit detectors, creating false alarms.
• The Solution: The space between the copper block and the detector is huge. The authors plan to fill this space with thick layers of lead and other materials to act as a soundproof wall. They also use a "veto" system (like a security guard) to spot and ignore any particles that look like they came from space rather than the crash.
• The Result: Their calculations suggest that with these shields, the "noise" will be almost completely silent, leaving a clear path to hear the whisper of the Dark Photon.

The Detective's Toolkit

The detector itself is a cylinder filled with liquid scintillator (a special glowing liquid).

• How it works: When a Dark Photon decays into an electron and a positron inside the liquid, they create a flash of light. The detector is designed to catch this flash, measure the energy, and determine the direction.
• The Filter: The scientists have set strict rules to ignore fake signals: the two particles must have enough energy and must be moving apart at a specific angle. This ensures they are looking at a real "Dark Photon" event, not a random glitch.

The Results: What Can They Find?

The authors ran simulations to see what this experiment could achieve over 5 years of operation.

• The Sweet Spot: They found that the CiADS machine is uniquely good at finding Dark Photons that are light (around 100 to 800 times heavier than an electron) but have a very weak connection to our world.
• New Territory: Current experiments have already ruled out many possibilities, but this setup could explore a "no-man's-land" of parameters that no one has checked yet. It's like searching for a specific type of fish in a lake that other divers haven't looked at.
• The Neighbor: They also looked at a nearby facility called HIAF (which uses heavier ions and higher energy). While HIAF has fewer particles per second, its higher energy allows it to hunt for heavier Dark Photons (over 1 GeV), extending the search to even larger "ghosts."

Why This Matters (According to the Paper)

The authors emphasize that this experiment is cost-effective.

• No New Cannon: They don't need to build a new accelerator. They are using the beam that is already being built for the nuclear waste project.
• Minimal Gear: The detector is relatively simple compared to the massive machines at CERN.
• The Goal: By catching these particles, they hope to prove that there is a "Dark Sector" of the universe we haven't seen yet, potentially explaining the nature of Dark Matter.

In short, the paper proposes turning a nuclear waste cleanup machine into a highly sensitive, low-cost, and quiet "ghost hunter" to find a specific type of invisible particle that could unlock the secrets of the universe.

Technical Summary

Technical Summary: Exploring the Lifetime Frontier with a Beam-Dump Experiment at CiADS

Problem Statement
The search for Beyond-the-Standard-Model (BSM) physics has increasingly focused on Long-Lived Particles (LLPs), particularly in the sub-GeV to GeV mass range, following the absence of new heavy fundamental particles at the LHC. While high-energy colliders like the LHC and dedicated forward experiments (e.g., FASER, SHiP) target LLPs, there remains a need to explore the low-mass, high-intensity frontier. The China initiative Accelerator Driven System (CiADS), currently under construction in Huizhou, China, is designed primarily for nuclear waste transmutation using a high-power proton beam. During its beam commissioning phase, the facility will generate a high flux of protons on target (POT) striking a beam dump. This paper addresses the opportunity to utilize this existing infrastructure to search for LLPs, specifically dark photons, without requiring a dedicated proton beam or significant additional instrumentation.

Methodology
The authors propose a Beam-Dump Experiment (CiADS-BDE) located 10 meters downstream from the CiADS beam dump. The experimental setup leverages the strong forward boost of particles produced in the dump.

• Detector Design: The proposed detector is a cylindrical liquid-scintillator detector (similar to the SHiNESS concept) loaded with gadolinium. It features a length of 1 meter and two potential radii for study: 0.1 m and 1 m. The design employs Large Area Picosecond Photodetectors (LAPPDs) to distinguish Cherenkov and scintillation light, enabling vertex reconstruction and directionality determination.
• Signal Channel: The study focuses on the dark photon ($\gamma'$) as a benchmark LLP model. The search signature is the decay of a dark photon into an electron-positron pair ($e^+e^-$) within the detector's fiducial volume.
• Production Mechanisms: Signal production is calculated via two primary channels:
  1. Meson Decays: Rare decays of neutral mesons ($\pi^0$ and $\eta$) produced in the dump into a photon and a dark photon ($\pi^0/\eta \to \gamma \gamma'$).
  2. Proton Bremsstrahlung (PB): Direct production via proton-copper collisions ($pCu \to \gamma' X$). The authors utilize a specific splitting kernel and form factors suitable for low-energy proton beams, avoiding the relativistic approximations used in higher-energy contexts.
• Background Estimation: Backgrounds are estimated using GEANT4 simulations. Major sources include cosmic rays, neutrino charged-current (CC) and neutral-current (NC) interactions, and intrinsic $\bar{\nu}_e$ flux. The authors apply kinematic cuts (electron/positron energy $>17$ MeV, opening angle $>30^\circ$) and a muon veto system to suppress backgrounds.
• Operational Parameters: Sensitivity studies assume a 5-year operation period. The primary beam energy considered is 600 MeV (initial commissioning) and 2 GeV (upgrade scenario) for CiADS, with a comparative study for the nearby High Intensity Heavy-ion Accelerator Facility (HIAF) at 9.3 GeV.

Key Contributions

• Feasibility Study: The paper provides a comprehensive feasibility study for a beam-dump experiment utilizing the CiADS facility, demonstrating that the beam dump's by-products can be effectively used for LLP searches.
• Background Suppression Strategy: The authors detail a strategy to achieve negligible background levels (less than 1 event per year) through the combination of a muon veto, energy thresholds, and angular cuts, leveraging the available space between the dump and the detector for shielding.
• Sensitivity Projections: Detailed numerical results are presented for the sensitivity to the dark photon kinetic mixing parameter ($\epsilon$) as a function of mass ($m_{\gamma'}$). The study accounts for both detector radii (0.1 m and 1 m) and different production modes.
• Comparative Analysis: The paper extends the analysis to the HIAF facility, comparing the sensitivity reach of a lower-energy, high-intensity setup (CiADS) against a higher-energy, lower-duty-factor setup (HIAF).

Results

• CiADS-BDE Performance:
  • For a 600 MeV proton beam, the sensitivity is limited to lower masses.
  • For a 2 GeV proton beam with a 1 m detector radius, the experiment can probe currently unexcluded parameter space. Specifically, it can exclude kinetic mixing values of $\epsilon \sim \mathcal{O}(10^{-9} - 10^{-8})$ for dark photon masses between 120 MeV and 800 MeV.
  • The Proton Bremsstrahlung (PB) channel is found to be particularly sensitive to heavier dark photons with smaller kinetic mixing compared to meson decays.
  • With a smaller detector radius (0.1 m), the reach is reduced but still extends into unexcluded regions for the PB channel.
• HIAF-BDE Performance:
  • Despite having a duty factor roughly three orders of magnitude lower than CiADS, the HIAF's higher beam energy (9.3 GeV) allows it to probe dark photon masses beyond 1 GeV.
  • For masses between 300 MeV and 1.2 GeV, HIAF-BDE can exclude $\epsilon$ values of $\mathcal{O}(10^{-8} - 10^{-7})$.
• Background Levels: The study concludes that with the proposed cuts and veto systems, the total background at CiADS-BDE is expected to be essentially vanishing (negligible).

Significance and Claims
The authors claim that the CiADS-BDE represents a cost-effective approach to exploring the lifetime frontier. Since the experiment utilizes the proton beam required for the facility's primary beam commissioning and requires only minimal instrumentation, it avoids the high costs associated with dedicated accelerator facilities.
The paper asserts that the proposed experiment can access unique regions of parameter space for dark photons (specifically $\mathcal{O}(100)$ MeV masses and $\mathcal{O}(10^{-9} - 10^{-8})$ mixing) that are currently unexcluded by existing experiments like E137, NA62, and LHCb.
Furthermore, the authors emphasize that while the dark photon is used as a benchmark, the setup is versatile enough to explore other LLP scenarios and neutrino physics (oscillations, unitarity violation). They conclude that given the construction status of CiADS and the operation of HIAF, the timing is appropriate to plan and realize these experiments to fill gaps in the search for light, long-lived BSM particles.