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

The SND@LHC Collaboration

Abstract

Long-range muons produced in proton-proton collisions at the ATLAS interaction point constitute the primary background for neutrino interaction searches at the SND@LHC experiment. This work presents a comprehensive characterization of the muon flux throughout LHC Run-3, benchmarking Monte Carlo simulations against experimental measurements. Measured and simulated muon rates agree within 10-15% across all Run-3 configurations. Following the substantial background increase in 2024 as a result of a beam optics change, the reversion to nominal optics in 2025 did not restore the 2022-2023 levels due to the unprecedented adoption of horizontal crossing in ATLAS. As enlightened by simulation results, the latter enhanced the contribution of high-angle muons originating from diffractive proton losses in the LHC Dispersion Suppressor region. Their identification enabled the design of mitigation strategies that were experimentally validated. The simulation framework was also applied to the future High-Luminosity LHC configuration, resulting in a considerable muon rate rise, driven by both the planned luminosity increase and the enlarged magnet aperture. Nevertheless, the upgrade from emulsion films to silicon vertex detectors will preserve the efficiency of the experiment even in such a high-rate environment.

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

Abstract

Long-range muons produced in proton-proton collisions at the ATLAS interaction point constitute the primary background for neutrino interaction searches at the SND@LHC experiment. This work presents a comprehensive characterization of the muon flux throughout LHC Run-3, benchmarking Monte Carlo simulations against experimental measurements. Measured and simulated muon rates agree within 10-15% across all Run-3 configurations. Following the substantial background increase in 2024 as a result of a beam optics change, the reversion to nominal optics in 2025 did not restore the 2022-2023 levels due to the unprecedented adoption of horizontal crossing in ATLAS. As enlightened by simulation results, the latter enhanced the contribution of high-angle muons originating from diffractive proton losses in the LHC Dispersion Suppressor region. Their identification enabled the design of mitigation strategies that were experimentally validated. The simulation framework was also applied to the future High-Luminosity LHC configuration, resulting in a considerable muon rate rise, driven by both the planned luminosity increase and the enlarged magnet aperture. Nevertheless, the upgrade from emulsion films to silicon vertex detectors will preserve the efficiency of the experiment even in such a high-rate environment.
Paper Structure (7 sections, 2 equations, 9 figures, 3 tables)

This paper contains 7 sections, 2 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. The detector
  3. Ruling factors and simulation framework
  4. Data evolution and MC interpretation
  5. Muon angular distribution
  6. The HL-LHC picture
  7. Conclusion

Figures (9)

  • Figure 1: Representative example of a passing-through muon event. The top panel shows a top-down view of the detector and the bottom panel shows a side view.
  • Figure 2: Top view of the FLUKA geometry model on the right side of ATLAS at beam height. The transverse-to-longitudinal (x:z) scale ratio is 1:20 (values are in cm).
  • Figure 3: Distribution of the origin of the muons reaching SND@LHC in the simulation for the three indicated Run-3 configurations: 2022--2023 in yellow, 2024 in green, and 2025--2026 in blue. The $z$-coordinate refers to the location of the last nuclear reaction preceding the muon generation. See Fig. \ref{['fig:LHC_tunnel']} for the correspondence with the machine elements. $L_0 = 10^{34}$ cm$^{-2}$ s$^{-1}$ is the LHC design instantaneous luminosity, which was routinely doubled during Run-3.
  • Figure 4: Simulated negative and positive muon spectra at SND@LHC for the 2022--2023 and 2024 configurations. $L_0 = 10^{34}$ cm$^{-2}$ s$^{-1}$ is the LHC design instantaneous luminosity, which was routinely doubled during Run-3.
  • Figure 5: Measured horizontal angular distribution of muons reaching SND@LHC in 2022--2023 (yellow) and 2025 (blue), compared to the MC prediction for 2025 (points).
  • ...and 4 more figures