Optimization of muon suppression using sweeper magnets for the Forward Physics Facility at the HL-LHC

This paper demonstrates that an optimized multi-stage sweeper magnet system, simulated using a framework combining SIBYLL, BDSIM, and Geant4, can effectively reduce the forward muon background at the Forward Physics Facility from $3.8\times10^3$ to $1.5\times10^3~\mathrm{cm^{-2}}$ per $\mathrm{fb^{-1}}$, thereby mitigating a major challenge for neutrino detection at the HL-LHC.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed particle racetrack. When protons crash into each other at the front of the track, they don't just stop; they spray debris in all directions. Most of this debris flies sideways, but a tiny, intense beam of particles shoots straight forward, like a powerful laser beam.

Scientists want to build a special "camera" (called the Forward Physics Facility, or FPF) down the tunnel to catch a very rare type of particle in this beam: neutrinos. Neutrinos are ghost-like particles that barely interact with anything, making them incredibly hard to catch but full of secrets about the universe.

The Problem: The "Muons" Crowd
There's a major obstacle: the beam is also packed with muons. Think of muons as rowdy, high-energy fans at a concert who are constantly bumping into the VIP section (the neutrino detector).

• The Damage: These muons are so numerous and energetic that they create a "traffic jam" of tracks inside the detector. This jams up the camera, making it impossible to see the rare neutrinos.
• The Cost: Currently, the detector gets so clogged with muon tracks that scientists have to replace the entire camera film multiple times a year. For the next-generation experiment, they want to replace it only once a year to save money and effort.

The Solution: The "Sweeper" Magnets
To fix this, the researchers proposed installing giant magnets along the tunnel before the camera.

• The Analogy: Imagine the muons are charged balls rolling down a hallway, and the neutrinos are neutral, invisible ghosts. If you place a strong magnet in the hallway, it acts like a magnetic wind that blows the charged balls (muons) off to the side, out of the hallway. The ghosts (neutrinos), having no electric charge, don't feel the wind and keep rolling straight through to the camera.
• The Goal: The magnets need to sweep the muons away just enough so that the camera sees a clear path.

The Challenge: The "Bouncing" Effect
The researchers discovered a tricky physics problem. Even if the magnets push the muons away, the tunnel walls are made of rock. As muons bounce off the rock (a process called "multiple scattering"), some of them can ricochet back into the camera's path, like a billiard ball bouncing off a cushion and returning to the pocket.

• The Energy Factor: Low-energy muons are easier to push away but also easier to bounce off the walls. High-energy muons are harder to push but also harder to bounce. The team had to find the perfect balance of magnet strength and distance to stop both.

The Experiment: Testing Different Magnet Setups
The team used powerful computer simulations to test different ways to install these magnets. They looked at three main spots:

1. Deep in the LHC tunnel (370m away): This is the earliest chance to sweep the muons.
2. In a connecting tunnel (480m away): A middle ground.
3. Right at the camera entrance (627m away): The last line of defense.

The Results

• One Magnet is Enough (Mostly): They found that installing just one large, powerful magnet deep in the LHC tunnel was enough to reduce the muon crowd to a manageable level. It lowered the muon count from about 3,800 to 2,000 per unit of time, hitting the target needed to replace the detector only once a year.
• More is Better (But Diminishing Returns): By adding smaller magnets in the connecting tunnel and right at the camera entrance, they could push the number down even further to about 1,500.
• The Verdict: A "multi-stage" system (magnets at different points) works best. The first magnet does the heavy lifting, and the later magnets clean up the stragglers that managed to bounce back.

Conclusion
The paper concludes that by carefully designing a system of magnets that act like a "muon sweeper," scientists can clear the path for the neutrino camera. This ensures the detector won't get overwhelmed by background noise, allowing them to study the universe's most elusive particles without having to constantly rebuild their equipment. The study proves that with the right magnetic "wind," we can clear the crowd and let the ghosts through.

Technical Summary

Technical Summary: Optimization of Muon Suppression Using Sweeper Magnets for the Forward Physics Facility at the HL-LHC

Problem Statement
The Forward Physics Facility (FPF) at the High-Luminosity LHC (HL-LHC) is designed to enable high-statistics measurements of TeV-scale neutrinos. However, the intense flux of forward muons poses a critical challenge for neutrino detectors, particularly emulsion-based detectors like FASER$\nu$2 and liquid-argon detectors like FLArE.

• For Emulsion Detectors: High muon flux leads to a high density of background tracks, causing frequent detector replacement (currently multiple times per year for FASER$\nu$) and degrading reconstruction performance due to incorrect track matching and secondary electron showers from bremsstrahlung. The study targets a total track density below $10^6$ tracks cm$^{-2}$, corresponding to a muon density of approximately $5 \times 10^5$ cm$^{-2}$.
• For Liquid-Argon Detectors: Muon backgrounds cause charge pile-up, leading to space-charge effects that distort the drift electric field and degrade performance.
• Baseline Flux: Without mitigation, the simulated muon flux at the FASER$\nu$2 location is $3.76 \times 10^3$ cm$^{-2}$ per fb$^{-1}$. The operational target for annual detector replacement is $2.0 \times 10^3$ cm$^{-2}$ per fb$^{-1}$.

Methodology
The authors developed a comprehensive simulation framework combining multiple stages to evaluate muon suppression via sweeper magnets:

1. Event Generation: Proton-proton collisions at $\sqrt{s} = 14$ TeV were generated using the SIBYLL 2.3d hadronic interaction model via the CRMC package.
2. Beam Transport: Particles were transported through the LHC beamline using BDSIM, which accounts for beam optics and interactions with surrounding materials. The input for downstream simulations was extracted at 370 m downstream of the interaction point (where the Line of Sight exits the beam pipe).
3. Particle Tracking: A dedicated Geant4 simulation modeled muon propagation through the downstream tunnel geometry (LHC tunnel, TI18 tunnel, and FPF region). Electron tracking was disabled to reduce computational load.
4. Magnetic Field Modeling: Realistic magnetic field maps were calculated using finite element analysis (Elmer) for various magnet configurations (permanent SmCo magnets with iron yokes) and implemented in Geant4.
5. Biasing Techniques: To handle the rarity of forward muons, cross-section biasing and resampling (splitting and Russian roulette) were applied to enhance statistical precision without distorting the physical distributions.
6. Evaluation Metrics: Performance was quantified by the residual ratio $R$ (flux with magnet / flux without magnet) and the absolute muon flux $\Phi$.

Key Contributions and Magnet Configurations
The study investigated three candidate installation locations for sweeper magnets, each with distinct geometric constraints:

1. LHC Tunnel (~370 m): The earliest deflection point. Due to radiation concerns, permanent SmCo magnets were proposed. Configurations varied in length ($\ell$ = 18–36 m) and transverse offset ($\Delta x$) relative to the Line of Sight (LoS).
2. TI18 Tunnel (~480 m): An intermediate location constrained by the SND@HL-LHC detector. Magnet length was limited to 1 m.
3. FPF Entrance (~627 m): The closest point to the detector (14 m upstream). Space constraints limited magnet length to 1 m, with transverse dimensions matching the detector acceptance (25 cm $\times$ 64 cm).

A conceptual toy Monte Carlo study highlighted the interplay between magnetic deflection and Multiple Coulomb Scattering (MCS). It revealed that while magnets deflect low-energy muons, MCS in the rock can cause scattered muons to re-enter the detector acceptance. This implies an energy-dependent optimal distance between the magnet and the detector.

Results

• Single-Stage (LHC Tunnel): Installing a magnet in the LHC tunnel alone significantly reduces the flux. The optimized configuration (LHC-B: 40 cm $\times$ 40 cm cross-section, 27 m length, offset by -50 cm) reduces the flux to $1.89 \times 10^3$ cm$^{-2}$ per fb$^{-1}$ ($R \approx 0.50$). This meets the target for annual detector replacement. The suppression is most effective for muons in the 100 GeV to 1 TeV range.
• Multi-Stage Configurations:
  • Adding a magnet in the TI18 tunnel (e.g., LHC-A + TI18-B) further reduces the flux to $1.63 \times 10^3$ cm$^{-2}$ per fb$^{-1}$ ($R \approx 0.43$).
  • Adding a magnet at the FPF entrance (LHC-A + FPF-A) reduces the flux to $1.69 \times 10^3$ cm$^{-2}$ per fb$^{-1}$ ($R \approx 0.45$).
  • Optimized Multi-Stage: Combining magnets at all three locations (LHC tunnel, TI18 tunnel, and FPF entrance) achieves the lowest flux of $1.54 \times 10^3$ cm$^{-2}$ per fb$^{-1}$ ($R \approx 0.41$).
• Energy Dependence: The upstream LHC tunnel magnet provides the dominant suppression effect. Downstream magnets offer moderate additional gains, primarily by suppressing low-energy muons that are more easily deflected over short distances.

Significance and Claims
The paper claims that a properly optimized multi-stage sweeper magnet system can significantly reduce the forward muon background at the FPF, making the operation of high-mass emulsion detectors like FASER$\nu$2 feasible with a replacement frequency of approximately once per year.

• The study demonstrates that the primary suppression is achieved by early deflection in the LHC tunnel, satisfying the target flux of $2 \times 10^3$ cm$^{-2}$ per fb$^{-1}$.
• It highlights the importance of realistic transport simulations, specifically the role of Multiple Coulomb scattering in "refilling" the detector acceptance, which limits the effectiveness of simplified magnetic configurations.
• The authors note that the results provide a realistic and quantitatively validated basis for the design of muon mitigation systems. However, they modestly acknowledge limitations, including statistical uncertainties due to the current muon sample size and the need for future studies to account for beam crossing angle shifts and alternative flux predictions (e.g., FLUKA) to ensure robustness.