Study of the Run-3 muon flux at the SND@LHC experiment

This study characterizes the long-range muon background at the SND@LHC experiment during LHC Run-3 by validating Monte Carlo simulations against measurements, identifying a persistent flux increase due to ATLAS horizontal crossing and diffractive losses, and confirming that upcoming detector upgrades will maintain efficiency despite projected rate rises in the High-Luminosity LHC era.

The Gist

The Big Picture: A Neutrino Detective in a Storm

Imagine the SND@LHC experiment as a high-tech detective agency located deep underground, about half a kilometer away from the main crime scene (the ATLAS collision point at the Large Hadron Collider).

The detectives' job is to catch neutrinos. Neutrinos are like "ghosts"—tiny, invisible particles that rarely interact with anything. To catch them, the detectives set up a trap (a detector) and wait for a ghost to bump into something.

The Problem: The area is incredibly noisy. Every time the main collider smashes protons together, it creates a massive storm of other particles, including muons. Muons are like "hooligans" compared to the ghostly neutrinos. They are heavy, fast, and they love to crash into things, creating fake signals that look exactly like the ghosts the detectives are trying to find.

This paper is essentially a report card on how well the team managed this "muon storm" during the 2022–2025 period (Run-3) and how they plan to handle an even bigger storm coming in the future.

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1. The Setup: Building a Shield

The detector is built like a fortress.

• The Veto (The Gatekeeper): Before the main trap, there's a gate made of scintillating bars (like glowing flashlights). If a muon tries to sneak in, the gate sees it and says, "Stop! That's a hooligan, not a ghost!"
• The Target (The Trap): Inside, there are layers of special film (emulsion) and tungsten plates. This is where the ghosts are supposed to leave a trace.
• The Muon System (The Net): At the back, there are more sensors to catch any muons that managed to punch through the front layers.

2. The Rollercoaster of 2022–2025

The LHC isn't a static machine; they constantly tweak how the proton beams are steered (the "optics") to get more collisions. Think of it like adjusting the lanes on a highway to prevent traffic jams.

• 2022–2023 (The Calm Days): The highway was set to "Standard Mode." The muon storm was manageable. The team's computer simulations (a digital twin of the experiment) predicted the noise level pretty well, within about 10–15% of reality.
• 2024 (The Chaos Year): They tried a new setting called "Reverse Polarity" to protect the machine's magnets from getting too hot. Result: Disaster. The muon noise doubled. It was like opening a floodgate. The simulations initially underestimated this, but once they tweaked the model, they realized the new settings were sending more high-energy muons straight toward the detector.
• 2025 (The Partial Fix): They went back to "Standard Mode," but they also changed the angle at which the proton beams cross each other (from vertical to horizontal).
  • The Surprise: They expected the noise to go back to 2022 levels. It didn't. It was still too loud.
  • The Detective Work: Using their super-computer simulations, they found the culprit. By crossing the beams horizontally, they accidentally kicked more "diffractive protons" (stray particles) into a specific region of the tunnel called the Dispersion Suppressor. These protons hit magnets, creating a new wave of muons that came in at a weird, sharp angle, bypassing some of the safety checks.

3. The "Aha!" Moment: Fixing the Simulation

The team noticed their computer model was missing a whole group of muons coming from a specific spot (half-cell 11).

• The Analogy: Imagine you are trying to count raindrops hitting your roof, but your sensor is only placed on the left side. You miss all the rain hitting the right side.
• The Fix: They moved their virtual "sensor" (the interface plane in the simulation) closer to the detector and widened it to catch those sneaky, high-angle muons. Suddenly, the computer predictions matched the real-world data perfectly.

4. The Band-Aid Solution: Orbit Bumps

Once they knew where the muons were coming from (the "half-cell 11" magnets), they tried to move the source.

• The Analogy: Imagine a sprinkler is spraying water on your garden. You can't turn off the sprinkler, but you can move the garden hose slightly so the water hits the dirt instead of your flowers.
• The Action: They created a tiny "orbit bump," shifting the proton beam just a few millimeters. This moved the source of the muon storm away from the detector.
• The Result: It worked! They reduced the noise by 15–20%. However, they decided not to keep this fix permanently because it was tricky to maintain and the machine was running at lower power anyway.

5. The Future: The HL-LHC (High-Luminosity LHC)

The machine is getting a massive upgrade for the 2030s.

• The Challenge: The "High-Luminosity" upgrade means the proton beams will be much brighter and the magnets will have bigger holes (apertures) to let more particles through. This means the muon storm will be four times worse than it is today.
• The Good News: The detector is getting an upgrade too. They are swapping out the old, slow "emulsion film" (which is like photographic film that needs to be developed) for Silicon Vertex Detectors (like super-fast digital cameras).
• The Verdict: Even though the muon storm will be a hurricane, the new digital detectors are fast and tough enough to handle it. They will still be able to catch the "ghosts" (neutrinos) without getting overwhelmed by the "hooligans" (muons).

Summary

This paper is a story of adaptation.

1. The machine changed, and the noise got worse.
2. The team used advanced computer simulations to figure out why the noise got worse (it was a specific type of muon coming from a specific angle).
3. They fixed their simulations to match reality.
4. They tested a small physical tweak to reduce the noise.
5. They confirmed that their future upgrades will be strong enough to survive an even louder future.

It's a great example of how scientists use math, physics, and clever engineering to keep their experiments running smoothly in a chaotic environment.

Technical Summary

1. Problem Statement

The SND@LHC (Scattering and Neutrino Detector at the LHC) experiment is designed to detect high-energy neutrinos produced in proton-proton ($pp$) collisions at the ATLAS Interaction Point (IP1). Located approximately 480 meters downstream in the TI18 service tunnel, the detector is shielded by ~100 meters of rock and concrete. However, long-range muons produced at IP1 constitute the primary background for neutrino interaction searches. These muons can penetrate the shielding, enter the detector without being vetoed, and induce showers (via pair production, bremsstrahlung, or deep inelastic scattering) or produce secondary neutral hadrons that mimic neutrino signals.

The challenge lies in the fact that the muon flux is highly sensitive to LHC machine optics and beam crossing schemes. During LHC Run-3 (2022–2025), significant changes in optics and crossing angles led to unpredictable variations in the muon background rate, threatening the experiment's efficiency and the lifespan of its emulsion target system. A comprehensive characterization and predictive model were required to interpret these variations and plan for the High-Luminosity LHC (HL-LHC) era.

2. Methodology

The study employs a dual approach combining experimental measurements with a two-step Monte Carlo (MC) simulation framework:

• Experimental Measurements:

  • Muon rates were measured using the Scintillating Fibre (SciFi) tracker within the SND@LHC detector.
  • Track reconstruction utilized the Hough Transform for pattern recognition and the Kalman filter (via the GENFIT package) for 3D trajectory fitting.
  • Rates were normalized to a reference instantaneous luminosity of $L = 2 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$.

• Simulation Framework:

  • Step 1 (FLUKA): Simulates $pp$ collisions at $\sqrt{s} = 13.6$ TeV using the DPMJET-III event generator. It transports collision debris through a detailed FLUKA geometry of the LHC tunnel (from ATLAS IP1 to a "scoring plane" upstream of SND@LHC). Variance reduction techniques (e.g., artificially reducing decay lengths of pions/kaons) were used to enhance muon statistics.
  • Step 2 (Geant4): Takes the particle output from the FLUKA scoring plane and simulates transport through the rock, tunnel, and the SND@LHC detector volume.
  • Optimization: An initial benchmark revealed a ~45% underestimation of the 2025 muon flux. Analysis identified a missing population of negative muons originating from the Dispersion Suppressor (DS) region. The interface plane between FLUKA and Geant4 was optimized (moved ~30m upstream and expanded transversely) to capture these high-angle muons.

3. Key Contributions

• Comprehensive Run-3 Benchmarking: The paper provides the first complete characterization of the muon flux across all Run-3 configurations (2022–2023 nominal, 2024 reverse-polarity, and 2025 nominal with horizontal crossing).
• Identification of Background Sources: The study successfully isolated the specific origins of muon flux variations:
  • 2024 Surge: Attributed to the "Reverse-Polarity" (RP) optics (inverted FDF sequence) which favored energetic positive muons from primary ATLAS collisions.
  • 2025 Residuals: Attributed to the adoption of horizontal crossing in ATLAS. This maximized the loss of diffractive protons in the Dispersion Suppressor (DS) half-cell 11 (approx. 400m from IP1). These losses generated large-angle negative muons that were previously missed by the simulation interface.
• Mitigation Strategy Development: Based on simulation insights, the team designed and tested beam orbit bumps to displace the proton loss clusters in the DS.
• HL-LHC Projections: The validated simulation framework was applied to the HL-LHC geometry to predict background rates for Run-4.

4. Key Results

• Agreement between Data and Simulation: After optimizing the simulation interface, the MC estimates agreed with experimental measurements within 10–15% across all Run-3 configurations.
  • 2022–2023: 557 Hz (Exp) vs. 500 Hz (MC).
  • 2024: 1154 Hz (Exp) vs. 1008 Hz (MC).
  • 2025: 799 Hz (Exp) vs. 907 Hz (MC).
• Angular Distribution Analysis: The 2025 configuration showed a pronounced bump in the horizontal angular distribution ($\theta_{xz} > 10$ mrad) caused by negative muons from DS half-cell 11.
• Mitigation Effectiveness:
  • Implementing a 9 mm orbit bump (shifting losses to half-cells 13/15) reduced the measured muon background by 20%.
  • An 8 mm orbit bump (shifting losses to cell 8-9) reduced the background by 15%.
  • Note: These reductions were slightly overestimated by MC due to the initial overestimation of the half-cell 11 contribution.
• HL-LHC Predictions:
  • The muon rate is predicted to rise to ~3345 Hz (at the HL-LHC target luminosity of $5 \times 10^{34} \text{ cm}^{-2}\text{s}^{-1}$), a fourfold increase over 2025 levels.
  • This increase is driven by both the higher luminosity and the enlarged magnet apertures (Q1–D1 string aperture increased from 70 mm to 150 mm), which allow more particles to escape the beam pipe.
  • Despite the high rate, the transition from emulsion films to silicon vertex detectors is expected to maintain the experiment's efficiency.

5. Significance

• Operational Safety: The study provides critical data for managing the SND@LHC detector's exposure limits. Understanding the muon flux is essential for determining the replacement frequency of the emulsion target system.
• Physics Validation: The successful benchmarking of the simulation framework against complex, varying machine conditions validates the tools used to predict backgrounds for future high-luminosity operations.
• Future Readiness: The identification of the DS half-cell 11 as a dominant background source for the HL-LHC era allows for proactive mitigation strategies (such as orbit bumps) to be considered for Run-4.
• Detector Evolution: The paper confirms that while the HL-LHC environment will be significantly more hostile to the detector, the planned upgrade to silicon technology will ensure the SND@LHC experiment remains viable for neutrino physics in the high-luminosity era.